3.1. Molecular geometry

The geometry optimization of MEPCL2, see Fig. 1, was carried out at the HF/6-311++G(d,p) and B3LYP//6-311++G(d,p) levels of theory.

Fig. 1 Optimized molecular structure of MEPCL2 with atom numbering. 

No imaginary frequencies were obtained, which ensures us that each gradient optimization located corresponds to true minimum energy on the energy potential surface. The selected optimized structural parameters, bond lengths and bond angles of the title compound are reported in Table 1. From this table, it is possible to observe that the bond lengths and bond angles values calculated at the HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels of theory compare favorably well with those reported in the literature. A linear fit of the optimized data, see Fig. 2 suggests that the bond lengths values calculated at the HF/6-311++G(d,p) level is slightly better than the geometry obtained at the B3LYP/6-311++G(d,p) level. However, this difference is in the order of the error associated with the resolution of the wavefunction when numerical algorithms are employed. Thus, no significant differences were obtained, which is indicative that the electronic correlation has a little effect on the molecular geometry of the MEPCL2.

Fig. 2 Linear fit bond length and bond angle. 

3.2. Vibrational analysis

The number of potential active fundamental vibrations of a non-linear molecule which contains N atoms is equal to (3N-6), apart from three translational and three rotational degrees of freedom. Therefore, MEPCL2 molecule has 33 atoms with 93 normal modes of vibrations. In the present work, we have done a detailed vibrational assignment of the experimental wavenumbers reported in a previous work, through the comparison with theoretically scaled wavenumbers evaluated at the HF/6-311++G(d,p) and B3LYP/6-311++G(d,p) levels of theory. In order to fit the theoretical wavenumbers to the experimental data, the scaling factors have been introduced by using a least square optimization of the computed to the experimental data. Vibrational frequencies are scaled as 0.9067 for HF and 0.961 for B3LYP¹⁰ to take into account the systematic errors caused by basis set incompleteness, neglect of electron correlation, in the case of HF calculations, and vibrational anharmonicity. The assignments of the compound are calculated by using VEDA program¹⁰. The calculated experimental and scaled frequencies using HF and DFT (B3LYP) with the 6-311++G(d,p) basis set are listed in Table 2.

3.2.4. P-Cl and P-O vibrations

The experimental P-Cl stretching vibrations are observed in the interval 587-435 cm⁻¹. Also, a band at 447 cm⁻¹ is assigned as P-Cl vibration. The calculated P-Cl in-plane and out-of-plane bending vibrations are observed at 387, 233 and 34 cm⁻¹. The P-O phenyl linkage gives rise to two bands. A strong band at 1260-1160 cm⁻¹ is mainly due to the stretching of the C-O bond of the phenyl group. Also, the band at 992 cm⁻¹ is related to a C-O stretching vibration.

The simulated infrared spectra of MEPCL2 obtained at the HF/ 6-311G++(d,p) and B3LYP/6-311G++(d,p) levels of theory are shown in Fig. 3.

Fig. 3 Computed FT-IR spectra for MEPCL2 

3.3. Frontier molecular orbitals (FMOs)

The highest occupied molecular orbital (HOMO) and the lowest-lying unoccupied molecular orbital (LUMO) may describe the electronic transition, non-linear optic properties, and UV-Vis spectra of a molecular system¹⁶. Also, the energy gap between HOMO and LUMO determines the kinetic stability, chemical reactivity and, optical polarizability and chemical hardness- softness of a molecule^17,18. The hard molecules are less polarizable than the soft ones because they need big energy to excitation. The electronic calculated through the TD-DFT method. In order to evaluate the energetic behavior of MEPCL2 in the solvent, we carried out calculations considering acetonitrile, water, chloroform and gas phases. The energies of the four molecular orbitals of MEPCL2: the second highest and highest occupied MO's (HOMO and HOMO-1), the lowest and the second lowest unoccupied MO's (LUMO and LUMO+1) were calculated at the TD-DFT/B3LYP/6-311++G(d,p) level of theory and they are reported in Table 3. Also, the 3D plots of the HOMO- 1, HOMO, LUMO and LUMO+1 orbitals computed at TD-DFT/B3LYP/6-311++G(d,p) level of theory for MEPCL2 molecule are depicted in Fig. 4. It is clear from this figure that, while the HOMO is localized on almost the whole molecule, LUMO is especially localized on the ring. Also, note that both, the HOMOs and the LUMOs are mostly anti-bonding type orbitals.

Fig. 4 3D view of HOMO and LUMO diagram with energy gaps. 

The calculated energy values of the HOMO and LUMO energy gaps are 6.3017, 6.3008, 6.3085 and 6.3183 eV in acetonitrile, water, chloroform and gas phases, respectively. Thus, it is clear that the highest energy gap is obtained when chloroform solvent is employed which suggest that MEPCL2 is more chemically stable in such solvent. In view of calculated absorption spectra, the maximum absorption wavelength corresponds to the electronic transition from the HOMO to LUMO with 92% and from the HOMO to LUMO+1 with 9% contribution, see Table 4. The other wavelength, excitation energies, oscillator strength and calculated counterparts with major contributions are listed in Table 4.

Note that the calculations of the molecular orbital show that the visible absorption maxima of MEPCL2 corresponds to the electron transition between frontier orbitals such as translation from HOMO to LUMO, see Table 4. The calculated absorption spectra showed five bands at 227.8, 220.9, 211.5, 208.9 and 201.6 nm for acetonitrile and at 227.2 nm in the experimental UV spectrum with maximum absorbance in the same solvent. In chloroform, water and gas phases, the theoretical maximum absorption bands are predicted at 228.2, 227.8 and 226.2 nm, respectively. All the maximum absorption bands are coming from HOMO to LUMO transition with energy contribution about 96 to 97 %. The next maximum peaks are predicated on HOMO-1 to LUMO+1 in all UV spectra with 80 to 90% around 200 nm. In Fig. 5 are shown the theoretical UV spectra obtained at the TD-DFT/6-311++G(d,p) level of theory, in all cases, it is possible to observe that the maximum absorbance is in the range 226-228 nm.

Fig. 5 Computed UV-Vis graphs in different solvent for MEPCL2. 

3.4. Ή and ¹³C NMR spectra from quantum calculations

The theoretical values for ¹H and ¹³C NMR of MEPCL2 are given in Table 5. The theoretical ¹H and ¹³C NMR chemical shifts of MEPCL2 have been compared with the experimental data measured in water and CDCl₃ solvents.

A comparison, between the experimental and computed ¹³C NMR spectra of MEPCL2, indicates an increase in the value of the chemical shifts of the carbon atoms C1 and C2 of cyclohexane, due to heavy substitutions, which is caused by the electronic charge distribution around of these carbon atoms.

The hydrogen peaks in the cyclohexane are observed experimentally from 2.228 to 1.331 ppm, while that the evaluated at the B3LYP/6-311+G (d,p) level of theory are in the range 2.238 – 1.174 ppm.

The methyl hydrogen atom H22 peak identified at 0.809 ppm is the lowest chemical shift among the entire hydrogen atoms. Probably, it is because of its electronic interaction with other atoms is lesser. The correlation graphs of the experimental and theoretical ¹H, and ¹³C NMR chemical shift are presented as supplementary material in Fig 6. A good correlation between predicted and observed ¹³C and ¹H NMR chemical shifts is found. Moreover, the slope and intercept of the least-square, correlation lines were used to scale the GIAO isotropic absolute shielding constants. From Fig. 6 it is clear that the solvent water shows more deviation that the observed in the CDCl₃ solvent. The relation is usually linear and described by the following equations (1-4):

Fig. 6 Correlation Proton and Carbon NMR between experimental and computed data. 

Proton NMR

Carbon NMR:

These results indicate that the calculations performed at the B3LYP/6-311++G(d,p) predicted adequately the experimental behavior of the Ή and ¹³C NMR spectra of MEPCL2.

3.5. Global Chemical Reactivity of MEPCL2

The reactivity of a molecular system can be analyzed employing the global reactivity descriptors derived from the DFT theory, which are evaluated through the total energies of the neutral, anionic and cationic systems. Thus, the ionization potential is determined from the energy difference between the energy of the compound derived from electron-transfer (radical cation) and the respective neutral compound; IP = E_cation- E_n; while the electron affinity is evaluated as the energy difference between the neutral molecule and the anion molecule: EA = E_n - E_anion. According to the Koopmans’ theorem, in HF calculations, ionization potentials (IP) and electron affinities (EA) may be approximated to the HOMO and LUMO's energies respectively. On the other hand, the validity of the Koopmans’ theorem within the DFT approximation is controversial, nonetheless, it has been shown that the Khon-Sham orbitals produce DFT reactivity descriptors that correlate quite well with the reactivity descriptors obtained from Hartree-Fock calculations¹⁹. Thus, in the present work, we decided to employ the second approximation. Additionally, from IP and EA values it is possible to evaluate the electronegativity (χ), hardness (η), softness (ζ), and electrophilicity index (ψ) of the molecular system, through the equations (5-8)²⁰:

The values of electronegativity, chemical hardness, softness, electrophilicity index and dipolar moment are reported in Table 6, in the solvents acetonitrile, water, chloroform and gas phases.

From these values, observe that the values of χ, η, ζ, and ψ are similar, this means that MEPCL2 is showing the same global chemical behavior in the different solvents, but it is clear that the dipolar moment has different values which depend on the solvent in where the molecule is immersed. The above mentioned is indicative of the presence of different intramolecular interactions between the solvent and MEPCL2.

3.6. Local reactivity of MEPCL2 from Molecular Electrostatic Potential

In the present study, the molecular electrostatic potential (MESP) of MEPCL2 is plotted in order to analyze its local reactivity, see Fig. 7. The MESP is a plot of the electrostatic potential mapped onto an electron density isosurface and is useful to identify electrophilic and nucleophilic regions around the molecule. In the majority of the MESP, while the maximum negative region, which preferred site for electrophilic attack indications as red color, the maximum positive region which preferred site for nucleophilic attack symptoms as blue color. The electrophilic attack is more around phosphorous and carbon atoms. It is extended around chlorine atoms. Similarly, nucleophilic attacks are shown in hydrogen atoms and more in methyl group substitutions. The different values of the electrostatic potential at the MESP surface are represented by different colors; red, blue and green represent the regions of most negative, most positive and zero electrostatic potential, respectively. The color code of these maps is in the range between −0.02335 e. (deepest red) to 0.02335 e (deepest blue) in the compound, where blue indicates the strongest attraction and red indicates the strongest repulsion^21–26.

Fig. 7 Molecular electrostatic potential (MESP) of MEPCL2