Photochemical Generation and Characterization of C-Aminophenyl-Nitrilimines: Insights on Their Bond-Shift Isomers by Matrix-Isolation IR Spectroscopy and Density Functional Theory Calculations

The intriguing ability of C-phenyl-nitrilimine to co-exist as allenic and propargylic bond-shift isomers motivated us to investigate how substituents in the phenyl ring influence this behavior. Building on our previous work on the meta- and para-OH substitution, here we extended this investigation to explore the effect of the NH2 substitution. For this purpose, C-(4-aminophenyl)- and C-(3-aminophenyl)-nitrilimines were photogenerated in an argon matrix at 15 K by narrowband UV-light irradiation (λ = 230 nm) of 5-(4-aminophenyl)- and 5-(3-aminophenyl)-tetrazole, respectively. The produced nitrilimines were further photoisomerized to carbodiimides via 1H-diazirines by irradiations at longer wavelengths (λ = 380 or 330 nm). Combining IR spectroscopic measurements and DFT calculations, it was found that the para-NH2-substituted nitrilimine exists as a single isomeric structure with a predominant allenic character. In contrast, the meta-NH2-substituted nitrilimine co-exists as two bond-shift isomers characterized by propargylic and allenic structures. To gain further understanding of the effects of phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we compared geometric and charge distribution data derived from theoretical calculations performed for C-phenyl-nitrilimine with those performed for the derivatives resulting from NH2 (electron-donating group) and NO2 (electron-withdrawing group) phenyl substitutions.

A particularly puzzling behavior has been observed for C-phenyl-nitrilimine (R′ = Ph, R″ = H).This molecule was theoretically predicted to exhibit two minima on the potential energy surface: one characterized by an allenic-like structure and the other by a propargylic-like structure [16] (Scheme 2), representing genuine bond-shift isomers [23,24].The experimental demonstration of this theoretically predicted behavior was achieved in our laboratory through the spectroscopic identification of these two isomeric forms under matrix-isolation conditions [18].This finding prompted us to investigate how substituents on the phenyl ring affect the bond-shift isomerism of the nitrilimine fragment.Our subsequent study on C-hydroxyphenyl-nitrilimines showed that the OH phenyl substitution leads to different outcomes depending on its position [17].When the OH group is in the meta position, the nitrilimine has structural features similar to the parent compound, displaying two bond-shift isomers with propargylic and allenic characters.In contrast, when the OH group is in the para position, only a single isomer with a strong allenic character was observed.Inspired by these results, the present study investigates both experimentally and theoretically the effects of the NH₂ substitution at the meta and para positions on the bondshift isomeric properties of C-phenyl-nitrilimine.These investigations involved the use of tunable narrowband UV light to generate C-aminophenyl-nitrilimines, from matrix-isolated 5-aminophenyl-tetrazoles, and the exploration of their photochemistry.Infrared spectroscopy, supported by DFT theoretical calculations, was employed to identify and characterize the photoproduced species.In addition, to gain deeper insights into the effects of the phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we expanded the theoretical studies to compare the influence of both electron-donating (NH2) and electron-withdrawing (NO2) groups in ortho, meta, and para positions relative to the unsubstituted C-phenyl-nitrilimine.This comprehensive approach enabled us to investigate both positional effects and the inherent characteristics of substituents.A particularly puzzling behavior has been observed for C-phenyl-nitrilimine (R ′ = Ph, R ′′ = H).This molecule was theoretically predicted to exhibit two minima on the potential energy surface: one characterized by an allenic-like structure and the other by a propargylic-like structure [16] (Scheme 2), representing genuine bond-shift isomers [23,24].The experimental demonstration of this theoretically predicted behavior was achieved in our laboratory through the spectroscopic identification of these two isomeric forms under matrix-isolation conditions [18].This finding prompted us to investigate how substituents on the phenyl ring affect the bond-shift isomerism of the nitrilimine fragment.Our subsequent study on C-hydroxyphenyl-nitrilimines showed that the OH phenyl substitution leads to different outcomes depending on its position [17].When the OH group is in the meta position, the nitrilimine has structural features similar to the parent compound, displaying two bond-shift isomers with propargylic and allenic characters.In contrast, when the OH group is in the para position, only a single isomer with a strong allenic character was observed.
OH), and fluoro (R′ = F, R″ = H, or F) substitutions have been shown to possess a significant carbenic character [19,21,22] (Scheme 1).Experimental identification of nitrilimines with distinct structural characteristics can be accomplished by analyzing the infrared frequency of the antisymmetric CNN stretching vibration [3,16,19,21].Frequencies in the 2300-2150 cm −1 range indicate nitrilimines predominantly propargylic, possessing a CN triple bond, while frequencies below the 2150-2000 cm −1 range denote allenic-type nitrilimines with a cumulenic CNN moiety.Nitrilimines characterized by weak absorption peaks below 2000 cm −1 have a strong carbenic character.A particularly puzzling behavior has been observed for C-phenyl-nitrilimine (R′ = Ph, R″ = H).This molecule was theoretically predicted to exhibit two minima on the potential energy surface: one characterized by an allenic-like structure and the other by a propargylic-like structure [16] (Scheme 2), representing genuine bond-shift isomers [23,24].The experimental demonstration of this theoretically predicted behavior was achieved in our laboratory through the spectroscopic identification of these two isomeric forms under matrix-isolation conditions [18].This finding prompted us to investigate how substituents on the phenyl ring affect the bond-shift isomerism of the nitrilimine fragment.Our subsequent study on C-hydroxyphenyl-nitrilimines showed that the OH phenyl substitution leads to different outcomes depending on its position [17].When the OH group is in the meta position, the nitrilimine has structural features similar to the parent compound, displaying two bond-shift isomers with propargylic and allenic characters.In contrast, when the OH group is in the para position, only a single isomer with a strong allenic character was observed.Inspired by these results, the present study investigates both experimentally and theoretically the effects of the NH₂ substitution at the meta and para positions on the bondshift isomeric properties of C-phenyl-nitrilimine.These investigations involved the use of tunable narrowband UV light to generate C-aminophenyl-nitrilimines, from matrix-isolated 5-aminophenyl-tetrazoles, and the exploration of their photochemistry.Infrared spectroscopy, supported by DFT theoretical calculations, was employed to identify and characterize the photoproduced species.In addition, to gain deeper insights into the effects of the phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we expanded the theoretical studies to compare the influence of both electron-donating (NH2) and electron-withdrawing (NO2) groups in ortho, meta, and para positions relative to the unsubstituted C-phenyl-nitrilimine.This comprehensive approach enabled us to investigate both positional effects and the inherent characteristics of substituents.Inspired by these results, the present study investigates both experimentally and theoretically the effects of the NH 2 substitution at the meta and para positions on the bondshift isomeric properties of C-phenyl-nitrilimine.These investigations involved the use of tunable narrowband UV light to generate C-aminophenyl-nitrilimines, from matrixisolated 5-aminophenyl-tetrazoles, and the exploration of their photochemistry.Infrared spectroscopy, supported by DFT theoretical calculations, was employed to identify and characterize the photoproduced species.In addition, to gain deeper insights into the effects of the phenyl substitution on the bond-shift isomerism of the nitrilimine fragment, we expanded the theoretical studies to compare the influence of both electron-donating (NH 2 ) and electron-withdrawing (NO 2 ) groups in ortho, meta, and para positions relative to the unsubstituted C-phenyl-nitrilimine.This comprehensive approach enabled us to investigate both positional effects and the inherent characteristics of substituents.

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p ′ /1m ′ or 1p ′′ /1m ′′ , respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p ′′ /1p ′ and 1m ′′ /1m ′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p ′′ or 1m ′′ isomers (2H tautomeric forms) compared to the 1p ′ or 1m ′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).Molecule 1p″ does not display conformers.However, molecule 1m″ exhibits two stable conformations distinguished by syn (s) and anti (a) orientations of the tetrazole NH group relative to the phenyl NH2 group (see Table 1 for details).According to the calculations, 1m″-a is the most stable conformer.The Gibbs energy of 1m″-s relative to 1m″-a is Molecules 2024, 29, x FOR PEER REVIEW 3 of 20

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).Molecule 1p″ does not display conformers.However, molecule 1m″ exhibits two stable conformations distinguished by syn (s) and anti (a) orientations of the tetrazole NH group relative to the phenyl NH2 group (see Table 1 for details).According to the calculations, 1m″-a is the most stable conformer.The Gibbs energy of 1m″-s relative to 1m″-a is Molecules 2024, 29, x FOR PEER REVIEW 3 of 20

Structure and Matrix-Isolated IR Spectra of 5-Aminophenyl-Tetrazoles
In this study, 5-(4-aminophenyl)-tetrazole will be denoted as 1p, and 5-(3-aminophenyl)-tetrazole as 1m, in accordance with the nomenclature previously used by us for the analogous hydroxy-substituted 5-phenyl-tetrazoles (letters p and m signify the para and meta positions of the substituent group in the phenyl ring relative to the tetrazole ring, respectively) [17].Both molecules can exist in the 1H or 2H tautomeric forms (see Table 1), abbreviated here as 1p′/1m′ or 1p″/1m″, respectively, which is also consistent with the terminology followed in our previous work [17].However, it has been established that direct access to nitrilimines is only viable through the 2H tautomer [16][17][18][19][20]25].Therefore, to theoretically evaluate the potential of compounds 1p and 1m as candidates for the photoproduction of the corresponding C-(4-aminophenyl)-nitrilimine 2p and C-(3-aminophenyl)-nitrilimine 2m, the former were energetically characterized at the B3LYP/6-311++G(d,p) level of theory and the obtained results are summarized in Table 1.Based on the Boltzmann populations estimated from the Gibbs energies at 380 K (temperature of the sublimated compounds before the matrix deposition, see Section 3), the 1p″/1p′ and 1m″/1m′ tautomeric ratios were determined to be 92/8 and 96/4, respectively, thus demonstrating the suitability of 1p and 1m as precursors of nitrilimines.The higher stability of the 1p″ or 1m″ isomers (2H tautomeric forms) compared to the 1p′ or 1m′ isomers (1H tautomeric forms) can be attributed to the increased electron delocalization within the tetrazole ring in the formers [26], which is reflected by the longer formal N=N and C=N double bonds and shorter formal N-N single bond (see caption of Table 1).Molecule 1p″ does not display conformers.However, molecule 1m″ exhibits two stable conformations distinguished by syn (s) and anti (a) orientations of the tetrazole NH group relative to the phenyl NH2 group (see Table 1 for details).According to the calculations, 1m″-a is the most stable conformer.The Gibbs energy of 1m″-s relative to 1m″-a is Molecule 1p ′′ does not display conformers.However, molecule 1m ′′ exhibits two stable conformations distinguished by syn (s) and anti (a) orientations of the tetrazole NH group relative to the phenyl NH 2 group (see Table 1 for details).According to the calculations, 1m ′′ -a is the most stable conformer.The Gibbs energy of 1m ′′ -s relative to 1m ′′ -a is less than 1 kJ mol −1 , resulting in a computed 1m ′′ -a/1m ′′ -s ratio of 57/39 at 380 K (Table 1).Consequently, a mixture of 1m ′′ -a and 1m ′′ -s conformers should dominate the vapor phase prior to the Ar matrix deposition.Besides the gas-phase populations, evaluation of the energy barrier for the conversion of the higher (1m ′′ -s) into the lower energy (1m ′′ -a) conformer is also relevant for predicting the composition of the deposited Ar matrix [27][28][29][30].The energy barrier for the conformational relaxation, obtained from the B3LYP potential energy profile (PES) for the internal rotation around the inter-rings C−C bond, was calculated to be 20 kJ mol −1 (Figure S1).This value is sufficiently large to allow efficient trapping of both conformers in an Ar matrix at 15 K [27][28][29][30].
Figures 1a and 2a depict the experimental IR spectra of 1p and 1m, respectively, isolated in an Ar matrix at 15 K, accompanied by the theoretical IR spectra of 1p ′′ and 1m ′′ , shown in Figures 1b and 2b.For 1m ′′ , the theoretical spectrum was derived by co-adding the B3LYP spectra calculated for the two predicted conformers (Figure 2c) with the intensities multiplied by their estimated gas-phase Boltzmann populations.Overall, there is a good correspondence between the experimental spectra of matrix-isolated 1p and 1m and the theoretical spectra computed for 1p ′′ and 1m ′′ , respectively.On the contrary, the experimental spectra are not well reproduced by the spectra computed for 1p ′ or 1m ′ (Figures S2 and S3).This is consistent with the theoretical predictions and demonstrates that there is an overwhelming predominance of the 2H tautomeric forms in the as-deposited Ar matrices.
less than 1 kJ mol −1 , resulting in a computed 1m″-a/1m″-s ratio of 57/39 at 380 K (Table 1).Consequently, a mixture of 1m″-a and 1m″-s conformers should dominate the vapor phase prior to the Ar matrix deposition.Besides the gas-phase populations, evaluation of the energy barrier for the conversion of the higher (1m″-s) into the lower energy (1m″-a) conformer is also relevant for predicting the composition of the deposited Ar matrix [27][28][29][30].The energy barrier for the conformational relaxation, obtained from the B3LYP potential energy profile (PES) for the internal rotation around the inter-rings C−C bond, was calculated to be 20 kJ mol −1 (Figure S1).This value is sufficiently large to allow efficient trapping of both conformers in an Ar matrix at 15 K [27][28][29][30].
Figures 1a and 2a depict the experimental IR spectra of 1p and 1m, respectively, isolated in an Ar matrix at 15 K, accompanied by the theoretical IR spectra of 1p″ and 1m″, shown in Figures 1b and 2b.For 1m″, the theoretical spectrum was derived by co-adding the B3LYP spectra calculated for the two predicted conformers (Figure 2c) with the intensities multiplied by their estimated gas-phase Boltzmann populations.Overall, there is a good correspondence between the experimental spectra of matrix-isolated 1p and 1m and the theoretical spectra computed for 1p″ and 1m″, respectively.On the contrary, the experimental spectra are not well reproduced by the spectra computed for 1p′ or 1m′ (Fig- ures S2 and S3).This is consistent with the theoretical predictions and demonstrates that there is an overwhelming predominance of the 2H tautomeric forms in the as-deposited Ar matrices.Tables S1 and S2 present the proposed IR band assignments for 1p and 1m, respectively, which were based on the comparison of the experimental spectra with the computed vibrational data.The higher frequency region of the two experimental mid-IR spectra is characterized by a very strong band at 3469−3467 cm −1 , which is characteristic of the tetrazole ν(NH) mode [26], together with two weaker bands at 3513 and 3421 cm −1 for 1p, or at 3507 and 3417 cm −1 for 1m, corresponding to the νas(NH2) and νs(NH2) modes, respectively.In the fingerprint region, the spectrum of 1p is dominated by the bands at 1625 [δ(NH2)], 1498/1483 [δ(CNN); δ(NCN); δ(NH); ν(CN)tz], 1449 [ν(CC)ph-tz; δ(NCN)], and 1294 [ν(CCNH 2 )] cm −1 , while the spectrum of 1m is dominated by the bands at 1621 [δ(NH2)], 1498 [δ(CNN); ν(CN)tz; δ(NH)], 763 [γ(C)tz], and 714 [γ(N)tz] cm −1 .Even though the spectra calculated for the two 1m″ conformers are very similar (as shown in Figure 2c), a detailed spectral analysis allows the identification of five experimental bands specific to each of them.Spectral features of 1m″-a are observed at 1498, 1445, 1437, 1315, and 1253 cm −1 (highlighted by blue circles in Figure 2a), while spectral signatures of 1m″-s are found at 1476, 1469, 1428, 1277, and 1272 cm −1 (indicated by red squares in Figure 2a).

Photogeneration and Spectroscopic Characterization of C-(4-Aminophenyl)-Nitrilimine
Phototransformations in 1p isolated in solid Ar at 15 K were triggered by performing in situ irradiations with narrowband light at λ = 230 nm.This wavelength was chosen because of its demonstrated effectiveness in inducing photoreactions in analogous matrix-isolated molecules [17][18][19] and due to its proximity to the wavelength of maximum absorption in the UV spectrum of 5-phenyl-tetrazole dissolved in ethanol (λ max = 229.4nm) [31].The spectral changes originated by the UV irradiations are depicted in the difference spectrum presented in Figure 3a for the 2300-1100 cm −1 range and in Figure S4a for the 3600-3200 cm −1 range.The negative bands indicate the consumption of 1p, while the positive ones denote the appearance of photoproducts.Focusing on the bands emerging in the 2300-1700 cm −1 region, the doublet at 2098/2059 cm −1 appears shortly after 10 s of irradiation but ceases to grow for longer irradiation times.In contrast, the other two bands, observed at 2150 cm −1 and 1781 cm −1 , continue to grow (see Figure S5 for details).These findings suggest that the first band should correspond to a primary photoproduct, whereas the remaining two should be associated with secondary photoproducts.
Considering the typical photochemical behavior reported for 2H-tetrazoles [11,32,33], it is highly plausible that the primary photoproduct would correspond to C-(4-aminophenyl)nitrilimine 2p, which is produced by extrusion of molecular nitrogen from 5-(4-aminophenyl)tetrazole 1p.To confirm this hypothesis, we have conducted subsequent irradiations at a longer wavelength (λ = 380 nm) with the aim of promoting the consumption of putative 2p without affecting 1p.The identification of 2p was definitively confirmed by the excellent agreement of the negative part of the difference IR spectrum obtained by subtracting the spectrum recorded before the irradiation at λ = 380 nm from that recorded after the irradiation at this wavelength, with the B3LYP/6-311++G(d,p) theoretical spectrum of this species, as depicted in Figure 3b,c (and additionally in Figure S4).A detailed assignment of the IR spectrum of 2p is provided in Table 2. Its most characteristic spectral feature is the doublet band at 2098/2059 cm −1 , which is theoretically predicted at 2078 cm −1 and is ascribed to the antisymmetric stretching vibration of the CNN fragment [ν as (CNN)].The placement of this band within the range of 2100-2000 cm −1 strongly suggests that the photogenerated nitrilimine adopts an allenic-type structure [3,16].Further elaboration on this topic will be provided later.
[νas(CNN)].The placement of this band within the range of 2100-2000 cm −1 st gests that the photogenerated nitrilimine adopts an allenic-type structure [3, elaboration on this topic will be provided later.S3 for details Concomitantly with the decrease in the bands assigned to 2p, other emerged upon the excitations at λ = 230 nm, including those at 2150 cm −1 and are intensified during the irradiations at λ = 380 nm (see Figure 3b).This demonstrates that the corresponding photoproducts are generated with the in of 2p.Building upon existing knowledge of the photochemistry of nitrilimines toproducts are most likely 4-aminophenyl-1H-diazirine 3p and 4-amino bodiimide 4p [3,[17][18][19]34,35].Both species could be easily identified through th of additional irradiations carried out at λ = 330 nm (after the consumption comparison of the difference spectrum resulting from these irradiations (Figu the calculated difference spectrum 4p-3p (Figure 4b) demonstrates clearly th tive bands are assigned to 3p while the positive ones are assigned to 4p, i.e., arranges to 4p [16,20].The main absorption of 3p is that identified at 1781 cm − at 1808 cm −1 ), which is particularly characteristic of 1H-azirines [17,18,20,36 signed to the three-membered ring ν(C=N) vibration.Regarding 4p, its most band is identified at 2150 cm −1 (predicted at 2115 cm −1 ), which is particularly ch of carbodiimides and is assigned to the νas(NCN) vibration [16][17][18].In summ sults reveal that once 2p is generated from 1p by N2 elimination, it photois yield 4p via 3p (see Scheme 3).This reasoning agrees with the mechanism o that has been established in previous photochemical studies carried out for tems [17,18].S3 for details).
Concomitantly with the decrease in the bands assigned to 2p, other bands that emerged upon the excitations at λ = 230 nm, including those at 2150 cm −1 and 1781 cm −1 , are intensified during the irradiations at λ = 380 nm (see Figure 3b).This observation demonstrates that the corresponding photoproducts are generated with the intermediacy of 2p.Building upon existing knowledge of the photochemistry of nitrilimines, these photoproducts are most likely 4-aminophenyl-1H-diazirine 3p and 4-aminophenyl-carbodiimide 4p [3,[17][18][19]34,35].Both species could be easily identified through the outcomes of additional irradiations carried out at λ = 330 nm (after the consumption of 2p).The comparison of the difference spectrum resulting from these irradiations (Figure 4a) with the calculated difference spectrum 4p-3p (Figure 4b) demonstrates clearly that the negative bands are assigned to 3p while the positive ones are assigned to 4p, i.e., 3p photorearranges to 4p [16,20].The main absorption of 3p is that identified at 1781 cm −1 (predicted at 1808 cm −1 ), which is particularly characteristic of 1H-azirines [17,18,20,36] and is assigned to the three-membered ring ν(C=N) vibration.Regarding 4p, its most prominent band is identified at 2150 cm −1 (predicted at 2115 cm −1 ), which is particularly characteristic of carbodiimides and is assigned to the ν as (NCN) vibration [16][17][18].In summary, our results reveal that once 2p is generated from 1p by N 2 elimination, it photoisomerizes to yield 4p via 3p (see Scheme 3).This reasoning agrees with the mechanism of reactivity that has been established in previous photochemical studies carried out for similar systems [17,18].

Photogeneration and Spectroscopic Characterization of C-(3-Aminophenyl)-Nitrilimine
Exposing matrix-isolated 1m to narrowband UV light tuned at λ = 230 nm resulted in the appearance of new bands at 2067 (triplet), 1787, and 2142/2106 cm −1 within the 2300-1700 cm −1 region (see Figure 5a and Figure S5).These spectral positions closely match those observed during the photolysis of 1p, indicating the formation of analogous photoproducts: C-(3-aminophenyl)-nitrilimine 2m, 3-aminophenyl-1H-diazirine 3m, and 4-aminophenyl-carbodiimine 4m.The only significative difference from the photochemical behavior of 1p is the emergence of an additional band at 2239 cm −1 , which displays an identical growth pattern to the band at ~2067 cm −1 (Figure S5), suggesting that it is also assigned to a primary photoproduct.This IR band falls within a spectral region where propargylic nitrilimines typically absorb [3,[16][17][18].Hence, it is most likely that both allenic and propargylic forms of 2m, designated as 2mA and 2mP, respectively, are generated from 1m, similar to the photochemistry observed for the 5-phenyl-tetrazole [18] and 5-(3hydroxyphenyl)-tetrazole [17].This interpretation is also supported by the computed PES as a function of the CNN bond angle, which reveals the existence of two separate energy minima for 2m but only a single minimum for 2p (Figure 6).
Experimental evidence confirming the co-existence of the two bond-shift isomers was obtained through subsequent irradiations at λ = 330 nm.In the first 20 min of irradiation, several bands that had appeared after the initial irradiation of the sample at λ = 230 nm decreased in intensity (Figure 5b).After an additional 10 min of irradiation, some of these bands were completely depleted, while others continued to diminish (Figure 5c).This allowed the distinction of two species based on their different kinetics of consumption.From the comparison of the negative parts of the two obtained experimental difference IR spectra (Figure 5b,c) with the calculated IR spectra of 2mA and 2mP (Figure 5d), the following can be concluded: (i) the bands centered at 2238 and 1221 cm −1 , corresponding to the species that experienced a more rapid consumption, match the ν as (CNN) and δ(NH) modes predicted for 2mP at 2236 and 1220 cm −1 , respectively; (ii) the bands at 2067 and 1246/1236 cm −1 , ascribed to the species that experienced a slower consumption, are well reproduced by the ν as (CNN) and δ(NH) modes theoretically predicted for 2mA at 2091 and 1240 cm −1 .These results clearly confirm the in situ generation of both propargylicand allenic-type isomers of 2m upon UV irradiation of matrix-isolated 1m ′′ (Scheme 4).An assignment of the IR spectra of 2mA and 2mP is given in Table 3.The faster kinetics of consumption of 2mP (k 1 )compared to 2mA (k 2 ) under UV irradiation at λ = 330 nm is consistent with the significantly higher absorptivity of the former relative to the latter, as deduced from the comparison of the TD-DFT UV spectra for the two isomers (Figure S6).The set of IR bands that increase upon the irradiation at λ = 330 nm, due to the consumption of 2mP and 2mA, are attributed to 3m and 4m, with the most characteristic spectral features being identified at 1787 cm −1 for 3m [ν(C=N)] and at 2067 cm −1 for 4m [ν as (NCN)].
is the emergence of an additional band at 2239 cm −1 , which displays an identical growth pattern to the band at ~2067 cm −1 (Figure S5), suggesting that it is also assigned to a primary photoproduct.This IR band falls within a spectral region where propargylic nitrilimines typically absorb [3,[16][17][18].Hence, it is most likely that both allenic and propargylic forms of 2m, designated as 2mA and 2mP, respectively, are generated from 1m, similar to the photochemistry observed for the 5-phenyl-tetrazole [18] and 5-(3-hydroxyphenyl)-tetrazole [17].This interpretation is also supported by the computed PES as a function of the CNN bond angle, which reveals the existence of two separate energy minima for 2m but only a single minimum for 2p (Figure 6).S3 for details).S3 for details).Experimental evidence confirming the co-existence of the two bond-shift isomers was obtained through subsequent irradiations at λ = 330 nm.In the first 20 min of irradiation, several bands that had appeared after the initial irradiation of the sample at λ = 230 nm decreased in intensity (Figure 5b).After an additional 10 min of irradiation, some of these bands were completely depleted, while others continued to diminish (Figure 5c).This al- spectively.Calculated harmonic wavenumbers were multiplied by 0.950 (above 2000 cm −1 ) or 0.980 (below 2000 cm −1 ).Vibrations predicted with very low intensities and lacking counterparts in the experimental spectrum were omitted.Vibrational assignment was based on the results provided by the "vibAnalysis" software, supported by ChemCraft animation of the vibrations.c Abbreviations: ν, stretching; δ, in-plane bending; ph, phenyl ring; s, symmetric; as, antisymmetric; CN refers to the carbon atom of the CNN moiety and NA to the nitrogen atom of the amino group.Sign "−" designates a combination of vibrations occurring in the opposite phase.

Stability of the Bond-Shift Isomers and Their Attempted Interconversion
As shown in Figure 6, the energy barrier separating 2mP and 2mA is very small (less than 1 kJ mol −1 ).Additionally, the energy difference between the two isomers is also very small (Table S3).According to these data, it can be hypothesized that the co-existence of these two isomers in the low-temperature Ar matrix is due to the fact that they are in equilibrium.However, our experimental observations do not support this hypothesis.In fact, after subjecting the sample to 20 min of UV irradiation at λ = 330 nm, resulting in the selective depletion of 2mP, we monitored the sample for several hours (13 h) in dark conditions (a metal plate was placed between the source of the spectrometer and the sample) to assess whether this isomer could be regenerated at the expense of 2mA.However, no  a Experimental wavenumbers are given in cm −1 .Only the bands in the 3600-3200 cm −1 and 2300-1200 cm −1 regions are included.Abbreviations: n.o.= not observed.b Data extracted from the vibrational calculations carried out for the B3LYP/6-311++G(d,p) lowest energy conformers of 2mA (2mA-a) and 2mP (2mP-a); see Table S3 for details.Values before and after the slash refer to 2mA and 2mP, respectively.Calculated harmonic wavenumbers were multiplied by 0.950 (above 2000 cm −1 ) or 0.980 (below 2000 cm −1 ).Vibrations predicted with very low intensities and lacking counterparts in the experimental spectrum were omitted.Vibrational assignment was based on the results provided by the "vibAnalysis" software, supported by ChemCraft animation of the vibrations.c Abbreviations: ν, stretching; δ, in-plane bending; ph, phenyl ring; s, symmetric; as, antisymmetric; C N refers to the carbon atom of the CNN moiety and N A to the nitrogen atom of the amino group.Sign "−" designates a combination of vibrations occurring in the opposite phase.

Stability of the Bond-Shift Isomers and Their Attempted Interconversion
As shown in Figure 6, the energy barrier separating 2mP and 2mA is very small (less than 1 kJ mol −1 ).Additionally, the energy difference between the two isomers is also very small (Table S3).According to these data, it can be hypothesized that the co-existence of these two isomers in the low-temperature Ar matrix is due to the fact that they are in equilibrium.However, our experimental observations do not support this hypothesis.In fact, after subjecting the sample to 20 min of UV irradiation at λ = 330 nm, resulting in the selective depletion of 2mP, we monitored the sample for several hours (13 h) in dark conditions (a metal plate was placed between the source of the spectrometer and the sample) to assess whether this isomer could be regenerated at the expense of 2mA.However, no such regeneration was observed, definitively showing that the two isomers exist independently in the cryogenic matrix.
Having established that 2mA and 2mP are not in equilibrium in the Ar matrix, we attempted their interconversion by vibrational excitation [36][37][38] with narrowband NIR light produced by a laser-OPO source or with broadband NIR/mid-IR light emitted from the IR spectrometer source.To determine the specific frequencies of vibrational modes available for performing selective NIR vibrational excitation, two spectra were recorded in the 7500-4000 cm −1 region: one immediately after the deposition of 1m in the Ar matrix and another after a series of UV irradiations at λ = 230 nm, followed by irradiations at λ = 330 nm to deplete 2mP while retaining 2mA in the matrix.The resulting NIR difference spectrum was compared with B3LYP/6-311++G(d,p) anharmonic vibrational calculations for the precursor and photoproducts (see Figure 7 and Table S6).
Molecules 2024, 29, x FOR PEER REVIEW 12 of 20 such regeneration was observed, definitively showing that the two isomers exist independently in the cryogenic matrix.
Having established that 2mA and 2mP are not in equilibrium in the Ar matrix, we attempted their interconversion by vibrational excitation [36][37][38] with narrowband NIR light produced by a laser-OPO source or with broadband NIR/mid-IR light emitted from the IR spectrometer source.To determine the specific frequencies of vibrational modes available for performing selective NIR vibrational excitation, two spectra were recorded in the 7500-4000 cm −1 region: one immediately after the deposition of 1m in the Ar matrix and another after a series of UV irradiations at λ = 230 nm, followed by irradiations at λ = 330 nm to deplete 2mP while retaining 2mA in the matrix.The resulting NIR difference spectrum was compared with B3LYP/6-311++G(d,p) anharmonic vibrational calculations for the precursor and photoproducts (see Figure 7 and Table S6).This spectral comparison allowed us to unequivocally identify the band corresponding to the first overtone of the ν(NH) stretching vibration of 2mA at 6307 cm −1 .We also tried to identify the 2νas(NH2) and 2νs(NH2) overtones, as well as the νas(NH2) + νs(NH2) combination band corresponding to this species.However, as shown in Figure 7, these  S6 for details).
This spectral comparison allowed us to unequivocally identify the band corresponding to the first overtone of the ν(NH) stretching vibration of 2mA at 6307 cm −1 .We also tried to identify the 2ν as (NH 2 ) and 2ν s (NH 2 ) overtones, as well as the ν as (NH 2 ) + ν s (NH 2 ) combination band corresponding to this species.However, as shown in Figure 7, these bands overlap with those of the precursor and other photoproducts, making it difficult to pinpoint the exact frequencies suitable for laser irradiations.Irradiating the matrix for 20 min with the laser tuned to 6307 cm −1 did not produce any spectral changes indicating the 2mA → 2mP conversion.The exposure of the sample to the broadband NIR/mid-IR radiation emitted by the spectrometer source for approximately 50 h also did not result in any detectable interconversion.Although unexpected, considering the theoretical data, these results are in line with previous findings for C-phenyl-nitrilimine [18] and C-(3hydroxyphenyl)-nitrilimine [17].For both molecules, two bond-shift isomers (allenic and propargylic) were also found to co-exist in an Ar matrix after photolysis of the corresponding tetrazole precursors, and in both cases, their interconversion was not achieved either by annealing experiments (for C-phenyl-nitrilimine) or by selective NIR excitation of the OH stretching overtones (for C-(3-hydroxyphenyl)-nitrilimine).Two factors may help to explain this observed behavior in the bond-shift isomers of the three phenyl-nitrilimines.Firstly, intrinsic limitations of theoretical methods that might hinder accurate calculation of the PES for the interconversion between the two bond-shift isomers.Secondly, the matrixenvironment may lead to an increase in the energy barrier separating the two isomers, making their interconversion more difficult, as it has been reported in some other cryogenic matrix-isolation experiments involving systems with conformational isomers separated by very small energy barriers [24,[39][40][41][42][43]].

Effect of the Position and Nature of the Phenyl Substitution on the Structure of C-Phenyl-Nitrilimines
According to the experimental results, the structure of photogenerated C-aminophenylnitrilimines varies with the positioning of the NH 2 group relative to the CNN fragment.For the para-NH 2 substitution (2p), only one isomeric form is identified.Conversely, for the meta-NH 2 substitution (2m), two distinct isomeric forms are observed: 2mA and 2mP.This is consistent with the PES calculated for the two nitrilimines at different levels, as depicted in Figure 6.Specifically, a single minimum is predicted for 2p, while two minima are predicted for 2m.Based on the wavenumbers corresponding to the ν as (CNN) band-2098/2057 cm −1 for 2p, 2067 cm −1 for 2mA, and 2239 cm −1 for 2mP-it can be inferred that 2p and 2mA are predominantly allenic, while 2mP is predominantly propargylic.This is corroborated by the geometric data extracted from the B3LYP/cc-pVTZ fully optimized geometries, as detailed in Table 4.Both 2p and 2mA have a non-planar structure (C R C N N H H dihedral = 98-99º) and a bent CCN framework (C R C N N C angle = 142-143º).Conversely, 2mP has a more planar structure (C R C N N H H dihedral = 161º), together with a much straighter CCN framework (C R C N N C angle = 178º).Additionally, the CN bond is shorter and the NN bond is longer in 2mP compared to 2mA and 2p.The experimental and theoretical results obtained for 2p and 2m align with the results of previous studies on analogous OH-substituted phenyl nitrilimines [17].From these findings, a general conclusion can be drawn: electron-donor substituents located at the para position relative to the CNN fragment promote an allenictype structure, whereas meta substitution stabilizes both allenic and propargylic structures.
To deepen our understanding of how phenyl substitution affects the structure of the CNN fragment, we extended our theoretical study to include a derivative with an electronwithdrawing substituent (NO 2 ) and also considered the ortho position for both NH 2 and NO 2 substitutions.The PES computed for these newly explored molecules are depicted in Figure S7, and the geometric data extracted from the B3LYP/cc-pVTZ fully optimized geometries of the respective minima are provided in Table 4.The results for the meta-NO 2 substitution closely resemble those obtained for the meta-NH 2 substitution, where both the allenic and propargylic structures are potentially stable minima, though the corresponding PES is even flatter.Moreover, the corresponding geometric parameters exhibit similar trends comparing the meta-NO 2 with the meta-NH 2 substitution.Regarding the ortho and para substitutions, there is a notable difference between the two substituents.For NH 2 , both substitutions stabilize allenic-type isomers exhibiting identical geometric parameters, whereas for NO 2 , they stabilize propargylic-type isomers that also share similar geometric features (Figure S7 and Table 4).Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH 2 and NO 2 phenyl-substituted derivatives a .

Allenic-Type Structures
ring carbon atom attached to the CNN group (CR) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in CR is also partly delocalized to CN, which also becomes negatively charged.Such an increase in the negative charge at CN favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO2 derivatives, CR becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the CN atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.
Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH2 and NO2 phenyl-substituted derivatives a .ring carbon atom attached to the CNN group (CR) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in CR is also partly delocalized to CN, which also becomes negatively charged.Such an increase in the negative charge at CN favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO2 derivatives, CR becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the CN atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.

Allenic-Type Structures
Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH2 and NO2 phenyl-substituted derivatives a .ring carbon atom attached to the CNN group (CR) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in CR is also partly delocalized to CN, which also becomes negatively charged.Such an increase in the negative charge at CN favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO2 derivatives, CR becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the CN atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.

Allenic-Type Structures
Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH2 and NO2 phenyl-substituted derivatives a .ring carbon atom attached to the CNN group (CR) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in CR is also partly delocalized to CN, which also becomes negatively charged.Such an increase in the negative charge at CN favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO2 derivatives, CR becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the CN atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.

Allenic-Type Structures
Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH2 and NO2 phenyl-substituted derivatives a .ring carbon atom attached to the CNN group (CR) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in CR is also partly delocalized to CN, which also becomes negatively charged.Such an increase in the negative charge at CN favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO2 derivatives, CR becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the CN atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.

Allenic-Type Structures
Table 4. Geometric parameters and Hirshfeld atomic charges computed at the B3LYP/cc-pVTZ level for C-phenyl-nitrilimine and its ortho-, meta-, and para-NH2 and NO2 phenyl-substituted derivatives a .S3 and S5).Atom labeling is given in the allenictype structure of C-phenyl-nitrilimine.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using  S3 and S5).Atom labeling is given in the allenictype structure of C-phenyl-nitrilimine.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using  S3 and S5).Atom labeling is given in the allenictype structure of C-phenyl-nitrilimine.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using  S3 and S5).Atom labeling is given in the allenictype structure of C-phenyl-nitrilimine.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using  S3 and S5).Atom labeling is given in the allenictype structure of C-phenyl-nitrilimine.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using  S3 and S5).Atom labeling is given in the allenic-type structure of C-phenyl-nitrilimine.
To better comprehend the influence of the phenyl substitution on the structure of the nitrilimine fragment, we have calculated the charge distributions in the NH 2 -and NO 2 -substituted C-phenyl-nitrilimines, as well as in the unsubstituted molecule (used as reference), and the obtained results are included in Table 4.The Hirshfeld population analysis [44] was used for this purpose because of its proven accuracy in estimating atomic charges of organic molecules [45] and its ability to provide a comprehensive description of both reactivity and regioselectivity in electrophilic aromatic substitutions [46][47][48].For C-phenyl-nitrilimine, the interplay of resonance and inductive effects originated by the phenyl ring leads to the stabilization of both allenic and propargylic forms [18].The allenic form is characterized by a negative charge at the nitrilimine carbon atom (C N ) and a positive charge at the adjacent nitrogen atom (N C ), whereas in the propargylic form, N C carries a positive charge while the terminal nitrogen (N H ) bears a negative charge (see Scheme 1).The meta substitution reveals that regardless of the substituent's nature, the charge distribution within the nitrile imine fragment is not much affected with respect to that of the analogous unsubstituted isomers.This explains why both allenic and propargylic forms remain minima on the respective PESs.Rather different results are obtained for the ortho and para substitutions, depending on the substituent nature.For the NH 2 derivatives, the ring carbon atom attached to the CNN group (C R ) becomes significantly more negatively charged compared to the unsubstituted molecule.This is consistent with the electron-donating nature of this substituent, which increases the electron density at the ring carbons in the ortho and para positions.According to the results presented in Table 4, the increased electron charge in C R is also partly delocalized to C N , which also becomes negatively charged.Such an increase in the negative charge at C N favors the allenic-type structure and consequently disfavors the propargylic one.Regarding the NO 2 derivatives, C R becomes significantly more positively charged as compared to the unsubstituted molecule.This is also consistent with the electron-withdrawing nature of this group, which decreases the electron charge of the ring carbon in ortho or para positions with respect to the substituent.This electron withdrawal is also extended to the C N atom, which results in the destabilization of the allenic form and consequent stabilization of the propargylic one.

Experimental Methods
Commercial samples of 5-(4-aminophenyl)-tetrazole (1p, Alfa Aesar, 97% purity) and 5-(3-aminophenyl)-tetrazole (1m, Alfa Aesar, 97% purity) were sublimated at ~380 K inside a miniature glass oven connected to the vacuum chamber of a closed-cycle helium cryostat (APD Cryogenics, with a DE-202A expander), through a stainless steel needle valve.Before cooling the cryostat, a freeze-pump-thaw procedure was performed to purify the sample from volatile impurities.The matrices were prepared by co-deposition of vapors resulting from the sublimation of the compounds with an excess of argon (Air Liquid, N60, purity 99.999%) onto a CsI optical substrate, cooled down to 15 K.The temperature of the CsI window was measured directly at the sample holder using a silicon diode sensor connected to a digital controller (Scientific Instruments, West Palm Beach, FL, USA, model 9650-1).Spectra in the mid-IR region (4000-400 cm −1 ) were recorded with a resolution of 0.5 cm −1 employing a Thermo Nicolet 6700 FTIR spectrometer, equipped with a cadmium telluride (MCT-B) detector and a potassium bromide (KBr) beam splitter.Spectra in the NIR region (7500-4000 cm −1 ) were also recorded with a resolution of 1 cm −1 , using the same spectrometer and an Indium Gallium Arsenide (InGaAs) detector with a CaF 2 beam splitter.The spectrometer underwent continuous purging with a stream of dry, CO 2 -filtered air to eliminate atmospheric water and CO 2 vapors.The prepared matrixes were then irradiated through the outer quartz window of the cryostat by applying narrowband UV radiation (fwhm = 0.2 cm −1 ) provided by the frequency-doubled signal beam of a Quanta-Ray MOPO-SL optical parametric oscillator (OPO), pumped with a pulsed Nd:YAG laser pro 230 from Spectra-Physics (repetition rate = 10 Hz, pulse energy ~1-3 mJ, duration = 10 ns).The UV irradiations were performed at λ = 230, 308, 330, and 380 nm, with different exposure times.

Computational Methods
All calculations were carried out using the Gaussian 16 (Revision B.01) program package [49].The geometries of the experimentally studied molecules were fully optimized at the B3LYP/6-311++G(d,p) [50][51][52] level of theory, followed by harmonic vibrational calculations at the same level.The B3LYP/6-311++G(d,p) vibrational data were then used to assist the spectral interpretation.To account for the incomplete treatment of electron correlation, basis set limitations, and neglected anharmonic effects, the harmonic vibrational frequencies were multiplied by 0.980 for the region below 2000 cm −1 and 0.950 for the region above 2000 cm −1 [17].Theoretical IR spectra were obtained from the vibrational data by convoluting the peaks with Lorentzian functions (fwhm = 2 cm −1 ) centered at the scaled wavenumbers and peak heights matching the calculated IR intensities.An approximate description of the vibrational modes was based on the results provided by the "vibAnalysis" software (version 1.2.2) [53], supported by ChemCraft animation of the vibrations [54].Anharmonic vibrational calculations were also carried out at B3LYP/6-311++G(d,p) level using the fully automated second-order vibrational perturbative approach developed by Barone and co-workers [55,56].For some of the studied molecules, theoretical absorption UV-spectra was simulated from time-dependent density functional theory calculations (TD-DFT) [57,58] performed at the B3LYP/6-311++G(d,p) level.The Cartesian coordinates extracted from full geometry optimizations carried out for all experimentally studied molecules are provided as Supplementary Materials.
Additional calculations were performed for C-phenyl-nitrilimine, as well as for the NH 2 -and NO 2 -substituted C-phenyl-nitrilimines in ortho, meta, and para positions.This included relaxed potential energy scans as a function of the nitrilimine CNN angle at the B3LYP/6-311++G(d,p), PBE0 [59]/6-311++G(d,p), and PW6B95 [60]/6-311++G(d,p) levels (only for the substituted C-phenyl-nitrilimines) and full geometry optimizations of the identified minima (plus vibrational calculations) at the B3LYP/cc-pVTZ level.This basis set was chosen for consistency with calculations previously performed on analogous molecules [17].The outcomes of the optimizations at this level served to obtain geometric and charge distribution data for nitrilimines.

Conclusions
The effect of substitution in the phenyl ring on the structure of C-phenyl-nitrilimines has been explored in this study by considering the paraand meta-NH 2 -substituted compounds (C-(4-aminophenyl)-and C-(3-aminophenyl)-nitrilimines, respectively), which were experimentally studied by matrix-isolation spectroscopy and quantum chemical calculations.The comprehensive results obtained for these compounds, which bear a π electrondonating substituent, were complemented by computational studies performed on the ortho-NH 2 derivative, as well as on the ortho-, meta-, and para-NO 2 -substituted analogous C-phenyl-nitrilimines, which bear a π electron-withdrawing substituent.Our previous data for the parent compound and the OH-substituted C-phenyl-nitrilimines [16][17][18] were also considered for the elucidation of the effects of the nature of the substituents on the structure of the C-phenyl-nitrilimines, in particular the existence of different bond-shift isomers.
The combined results of the experiments and computations allowed us to conclude that, while the para-NH 2 substitution stabilizes a single isomeric structure of the predominant allenic type, the meta-NH 2 substitution leads to the co-existence of both propargylicand allenic-type bond-shift isomers, a trend that we have also observed before for the analogous OH-substituted C-phenyl-nitrilimines [17].
The calculations performed on the whole set of molecules mentioned above, in particular the obtained equilibrium geometries, potential energy surfaces landscapes, and Hirshfeld charges, permitted attaining a comprehensive understanding of the effects of phenyl substitution on the bond-shift isomerism in C-phenyl nitrilimines.Parent C-phenylnitrilimine exists in both allenic and propargylic forms [18].Regardless of the substituent's nature, meta substitution (either electron donating or withdrawing) does not affect substantially the electronic features within the nitrilimine fragment, so meta substitution preserves the co-existence of the allenic and propargylic forms observed for the parent compound.On the other hand, ortho and para substitutions considerably change the electronic structure of the nitrilimine moiety.For electron donating substitutions (as in the case of the OH and NH 2 derivatives), an increase in the negative charge at C N takes place, which favors the allenic-type isomer that becomes the single stable isomeric form, while in the case of electron-withdrawing substitutions (as for the NO 2 derivatives), the C N atom becomes

Figure 4 .
Figure 4. (a) Experimental difference IR spectrum showing changes after 15 min of irradiation at λ = 330 nm subsequently to the consumption of 2p at 380 nm (Figure 3).The negative bands are due to the consumed 4-aminophenyl-1H-diazirine 3p while the positive ones are assigned to the photogenerated 4-aminophenyl-carbodiimide 4p.(b) B3LYP/6-311++G(d,p) theoretical difference spectrum obtained as the spectrum of the most stable conformer of 4p minus the spectrum of the most stable conformer of 3p (see TableS4for details).

Figure 4 .
Figure 4. (a) Experimental difference IR spectrum showing changes after 15 min of irradiation at λ = 330 nm subsequently to the consumption of 2p at 380 nm (Figure 3).The negative bands are due to the consumed 4-aminophenyl-1H-diazirine 3p while the positive ones are assigned to the photogenerated 4-aminophenyl-carbodiimide 4p.(b) B3LYP/6-311++G(d,p) theoretical difference spectrum obtained as the spectrum of the most stable conformer of 4p minus the spectrum of the most stable conformer of 3p (see TableS4for details).

Figure 4 .Scheme 3 .
Figure 4. (a) Experimental difference IR spectrum showing changes after 15 min of irradiation at λ = 330 nm subsequently to the consumption of 2p at 380 nm (Figure 3).The negative bands are due to the consumed 4-aminophenyl-1H-diazirine 3p while the positive ones are assigned to the photogenerated 4-aminophenyl-carbodiimide 4p.(b) B3LYP/6-311++G(d,p) theoretical difference spectrum obtained as the spectrum of the most stable conformer of 4p minus the spectrum of the most stable conformer of 3p (see TableS4for details).

Molecules 2024 , 20 Figure 6 .
Figure 6.Relaxed potential energy scan as function of the CCN angle for (a) C-(4-aminophenyl)nitrilimine 2p and (b) C-(3-aminophenyl)-nitrilimine 2m.The scans were computed using the B3LYP (blue), PW6B95 (green), and PBE0 (orange) DFT functionals with the 6-311++G(d,p) basis set.Along the scans, the value of the CCN angle was incrementally fixed in steps of 2.5 degrees, and all other geometrical parameters were fully optimized.

Figure 6 .
Figure 6.Relaxed potential energy scan as function of the CCN angle for (a) C-(4-aminophenyl)nitrilimine 2p and (b) C-(3-aminophenyl)-nitrilimine 2m.The scans were computed using the B3LYP (blue), PW6B95 (green), and PBE0 (orange) DFT functionals with the 6-311++G(d,p) basis set.Along the scans, the value of the CCN angle was incrementally fixed in steps of 2.5 degrees, and all other geometrical parameters were fully optimized.

Table 1 .
Structures, relative Gibbs energies at 380 K (∆G, in kJ mol

the isomers designations, 1p′/1m′ or 1p″/1m″ correspond to the 1H or 2H tautomeric forms, respectively. In the case of 1m, the letter after the hyphen denotes the relative orientation of the NH and
Mean bond lengths in the tetrazole ring extracted from the full geometry optimizations carried out for NH2 groups (a = anti; s = syn).

the isomers designations, 1p′/1m′ or 1p″/1m″ correspond to the 1H or 2H tautomeric forms
, respectively.In the case of 1m, the letter after the hyphen denotes the relative orientation of the NH and NH2 groups (a = anti; s = syn).Mean bond lengths in the tetrazole ring extracted from the full geometry optimizations carried out for

the isomers designations, 1p′/1m′ or 1p″/1m″ correspond to the 1H or 2H tautomeric forms
, respectively.In the case of 1m, the letter after the hyphen denotes the relative orientation of the NH and NH2 groups (a = anti; s = syn).Mean bond lengths in the tetrazole ring extracted from the full geometry optimizations carried out for
a Experimental wavenumbers are given in cm −1 .Only the bands in the 3600-3200 cm −1 and 2300-800 cm −1 regions are included.Abbreviations: n.o.= not observed.bDataextracted from the vibrational calculations carried out for the B3LYP/6-311++G(d,p) lowest energy conformer of 2p (2p-a); see TableS3for details.Calculated harmonic wavenumbers were multiplied by 0.950 (above 2000 cm −1 ) or 0.980 (below 2000 cm −1 ).Vibrations predicted with very low intensities and lacking counterparts in the experimental spectrum were omitted.c Vibrational assignment was based on the results provided by the "vibAnalysis" software, supported by ChemCraft animation of the vibrations.Abbreviations: ν, stretching; δ, in-plane bending; γ, out-of-plane bending; ρ, rocking; ph, phenyl ring; s, symmetric; as, antisymmetric; C N refers to the carbon atom of the CNN moiety and N A to the nitrogen atom of the amino group.Signs "+" and "−" designate combination of vibrations occurring in the same phase and in the opposite phase, respectively.
The data refer to the CNN fragment and were extracted from the optimized geometries of the lowest energy conformers of each molecule (see Tables a The data refer to the CNN fragment and were extracted from the optimized geometries of the lowest energy conformers of each molecule (see Tables a The data refer to the CNN fragment and were extracted from the optimized geometries of the lowest energy conformers of each molecule (see Tables a The data refer to the CNN fragment and were extracted from the optimized geometries of the lowest energy conformers of each molecule (see Tables a The data refer to the CNN fragment and were extracted from the optimized geometries of the lowest energy conformers of each molecule (see Tables a