X-Ray , IR , NMR , UV-visible spectra and DFT analysis of 5-aryloxy-( 1 H )-tetrazoles , structure , conformation and tautomerism

Faculty of Chemistry, Urmia University, 57159, Urmia, Iran International University of Chabahar (IUC), Chabahar, Iran Department of Physical Chemistry, Faculty of Chemistry, University of Kashan, Kashan, Iran. Department of Chemistry, Faculty of Science, Tehran University, Tehran, Iran Department of Chemistry, Faculty of Science, Atatürk University, 25240, Erzurum, Turkey Department of Chemistry, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia C H R O N I C L E A B S T R A C T


Introduction
5-Substituted tetrazoles are reported to possess antibacterial 1 , antifungal 2 , antiviral 3 , analgesic 4,5 , anti-inflammatory 6 , antiulcer 7 and antihypertensive 8 activities.The tetrazole function is metabolically stable 9 .The similarities between the acidic character of the tetrazole group and carboxylic acid group 10 have inspired medicinal chemists to synthesize substituted tetrazoles as potential medicinal agents.Tetrazoles are an important functionality, not only as precursors to a variety of nitrogencontaining heterocycles 11 but also as materials with applications in explosives 12 and even as increasive lubricants 13 .Several works were reported about tetrazole tautomerization [14][15][16] and isomerization 17 .
In this work, characteristics of geometrical parameters, harmonic frequencies and NMR that exist in complexes are completely investigated by a DFT (B3LYP) approach.Based on these concepts, as part of investigation of 5-(2,6-dimethyl-(1) and 5-(2,6-diisopropylphenoxy)-(1H)tetrazole (2), herein we report the optimized structures of 1 and 2 in the gas and solid phases and also the corresponding experimental and optimized IR, 1 H NMR, 13 C NMR and UV-visible spectra in the solution and gas phases, respectively.

Results and Discussion
This paper presents results on the tautomeric behavior, rotation barrier and the comparison of the experimental and theoretical IR, 1 H, 13  In the compound 1, the crystal structure indicated that the tetrazole and phenyl rings are nearly perpendicular to each other, forming a dihedral angle of 95.5° (versus 92.08° from calcd.B3LYP/6-311G(d,p)).Because of the conjugation of O1 with tetrazole ring, the bond distance C1-O1 [1.322 Å] is slightly shorter than O1-C2 [1.399 Å].These bond distances for C1-O1 and O1-C2 were obtained 1.330 and 1.419 Å with calculation by B3LYP/6-311G(d,p) method, respectively.These data are in good agreement with experimental results (Table 2).Similarly, in the compound 2, the crystal structure indicated that the tetrazole and phenyl rings are nearly perpendicular to each other, forming a dihedral angle of 85.91° (versus 107.3° from calcd.B3LYP/6-311G(d,p)).Because of the conjugation of O1 with tetrazole ring, the bond distance C2-O1 [1.327 Å] is slightly shorter than O1-C7 [1.426 Å].These bond distances for C2-O1 and O1-C7 were obtained 1.329 and 1.422 Å with calculation by B3LYP/6-311G(d,p) method, respectively and are in good agreement with experimental results.The torsion angles between phenyl ring and each of methyl units on two isopropyl groups are -110.70°,124.18° and -80.2° and 154.12°, respectively (Table 2).The selected parameters of bond lengths, angles and torsion angles of 1 and 2 derived by experimental and calculated results are shown in Table 2.The crystal packing diagram of 1 exhibits an intermolecular N1-H1••••N4 hydrogen bonds and compared with the calculated at B3LYP/6-311G(d,p) method (Table 3).The crystal structure indicated that the bond distance value between donor -hydrogen (N1-H1) and hydrogen-acceptor (H1••••N4) were found in results 0.861 and 1.959 Å, respectively.For instance, these bond distances were also found in results 1.015 for (N1-H1) and 1.863 Å for (H1••••N4) by calculated at B3LYP/6-311G(d,p) method.The donor-acceptor distance value (N1••••N4) was obtained 2.804 by experimental method.This parameter was found 2.869 Å by B3LYP/6-311G(d,p) method.The angle of N1-H1••••N4 was found 166.9 and 170.7° by experimental and calculated at B3LYP/6-31G(d), respectively.The results of calculated method are in good agreement with experimental results (Table 3).The crystal packing diagram of 2 also exhibits an intermolecular N3-H31••••N6 hydrogen bonds with the calculated by B3LYP/6-311G(d,p) method (Table 3).The crystal structure indicated that the bond distance value between donor -hydrogen (N3-H31) and hydrogen acceptor (H31••••N6) were found in results 0.926 and 1.919 Å, respectively.For instance, these bond distances were also found in results 1.009 for (N3-H31) and 1.938 Å for (H31••••N6) by calculated at B3LYP/6-311G(d,p) method.The donor-acceptor distance value (N3••••N6) was obtained 2.835 by experimental method.This parameter was found 2.940 Å by calculated methods B3LYP/6-311G(d,p) level.The angle of N3-H31••••N6 was found 169.1 and 171.46° found by experimental and calculated at B3LYP/6-311G(d,p) method, respectively.The results of calculated method are in good agreement with experimental results (Table 3).The calculated structures for 1 and 2 having intermolecular H-bond are shown in Fig. 6.Fig. 6.Optimized structure and intermolecular H-bond in 1 (a) and 2 (b).Calculated at B3LYP/6-311 G(d,p) basis sets IR spectra of 1 and 2 were derived from experimental and calculated results with B3LYP/6-311G(d,p) are shown in Fig. 7 and Fig. 8, respectively.These data indicated the good agreement together between the experimental and calculated result (Fig. 8). 1 H and 13 C NMR spectra of 1 and 2 calculated at B3LYP/6-311G(d,p) method are also in good agreement with experimental results (Figs. 9-10).The UV-visible spectra of compounds 1 and 2 were measured in EtOH and the corresponding λ max were obtained 335 nm for 1 and 297 and 354 nm for 2, respectively (Fig. 11).UV-visible spectra of 1 and 2 were also calculated at B3LYP/6-311G(d,p) method and is shown in Fig. 12.The molecular geometry of compounds 1 and 2 were optimized by the calculation at DFT (B3LYP) at 6-311G(d,p) basis sets.These have been identified to correspond to local minima with all positive values of vibrational frequencies (NIMAG=0) and are shown in Figs.13-16.In 1, the rotational energy barrier for dihedral angles (φ) to rotate 360° around the bond between O1-C1 and/or O1-C2 were calculated by B3LYP/6-311G(d,p) method and equal to 7.0 kcal/mol and is shown in Fig. 13.And the rotational energy barrier for dihedral angles (φ) to rotate 360° around the bond between C3-C9 and/or C7-C8 were calculated by B3LYP/6-311G(d,p) method and equal to 0.9 kcal/mol and is shown in Fig. 14.In 2, the rotational energy barrier for dihedral angles (φ) to rotate 360° around the bond between O1-C2 and/or O1-C7 were calculated by B3LYP/6-311G(d,p) method and equal to 13.1 kcal/mol and is shown in Fig. 15.And the rotational energy barrier for dihedral angles (φ) to rotate 360° around the bond between C8-C9 and/or C15-C16 were calculated by B3LYP/6-311G(d,p) method and equal to 8.5 kcal/mol and is shown in Fig. 16.The corresponding maximum and minimum energies derived from rotational barrier between tetrazole -phenyl rings in 1 and 2, between methyl -phenyl in 1 and isopropyl -phenyl ring in 2 were calculated, respectively and are summarized in Table 4.

Crystallographic data
For the crystal structure determination, the single-crystal of the compound 1 was used for data collection on a four-circle Rigaku R-AXIS RAPID-S diffractometer (equipped with a two dimensional area IP detector).The graphite-monochromatized MoKα radiation (λ = 0.71073 Å) and oscillation scans technique with Δω=5˚ for one image were used for data collection.The lattice parameters were determined by the least-squares methods on the basis of all reflections with F2>2σ(F2).Integration of the intensities, correction for Lorentz and polarization effects and cell refinement was performed using Crystal Clear (Rigaku/MSC Inc., 2005) software 24 .The structures were solved by direct methods using SHELXS-97 25 and refined by a full-matrix least-squares procedure using the program SHELXL-97 25 .The crystal structures of 1 and 2 and their crystal packing diagrams are shown in Fig. 1 and Fig. 2, respectively.Some of the crystallographic data of 1 and 2 are given in Table 1.The selected bond lengths, angles and torsion angles with their calculated data for 1 and 2 are shown in Table 2.For the crystal structure determinations, single-crystals of 2 were used for data collection on an Oxford Diffraction Gemini E diffractometer.The computing details; Data collection: Gemini, (Oxford Diffraction, 2006) 26 ; cell refinement: CrysAlis RED, (Oxford Diffraction, 2006)  26 ; data reduction: CrysAlis RED, (Oxford Diffraction, 2002) 26 ; program(s) used to solve structure: SIR92 27 ; program(s) used to refine structure: CRYSTALS 28 ; molecular graphics: CAMERON 29 ; software used to prepare material for publication: CRYSTALS 28 .The crystallographic data for structures 1 (entry no.CCDC-838541) and 2 (entry no.CCDC-819010) were deposited to the Cambridge Crystallographic Data Center and are available free of charge upon request to CCDC, 12 Union Road, Cambridge, UK (Fax: +44-1223-336033, e-mail: deposit@ccdc.cam.ac.uk).

Conclusion
In summary, the structures of 5-(2,6-dimethylphenoxy)-(1H)-tetrazole 1 and 5-(2,6diisopropylphenoxy)-(1H)-tetrazole 2 were elucidated by X-ray crystallography.The crystal packing diagrams of these compounds exhibits an intermolecular N1-H1••••N4 and N3-H31••••N6 hydrogen bonds, respectively and compared with those calculated by B3LYP/6-311G(d,p) method.These structures were also analyzed at B3LYP/6-311G(d,p) method in the gas phase.The N1-H1 and N3-H31 form of tetrazoles were found to be more stable in both solid and gas phases.IR, 1 H, 13 C NMR and UV-visible spectra were calculated at B3LYP/6-311G(d,p) method and were in good agreement with the corresponding experimental results.The rotational barrier around between phenyl and tetrazole rings and also between phenyl ring and isopropyl group in 2 is higher than that of rotational barrier in 1.

Table 1 .
Summary of crystallographic data for 1 and 2

Table 4 .
Calculated maximum and minimum energies derived from rotational barrier between tetrazole and phenyl rings in 1 and 2 (B3LYP/6-311G(d,p)) a Maximum b Minimum