THEORETICAL MOLECULAR STRUCTURE, VIBRATIONAL FREQUENCIES AND NMR INVESTIGATIONS OF 2-[(1E)-2-AZA-2-(5-METHYL(2-PYRIDYL)ETHENYL)]-4-BROMOBENZEN-1-OL

The normal mode frequencies and corresponding vibrational assignments, H and C NMR chemical shifts and structural parameters (bond lengths, bond and dihedral angles) of 2-[(1E)-2-aza-2-(5methyl(2-pyridyl)ethenyl)]-4-bromobenzen-1-ol (2mpe-4bb) Schiff base compound have been theoretically examined by means of Hartree-Fock (HF) and Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31G(d) and 6-311++G(d,p) basis sets. Furthermore, reliable vibrational assignments have made on the basis of potential energy distribution (PED) calculated and the thermodynamics functions, highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO) of 2mpe-4bb have been predicted. Theoretical results have been successfully compared with available experimental data in the literature. Regarding the calculations, 2mpe4bb prefers enol-imine form and DFT method is superior to HF approach except for predicting bond lengths.


INTRODUCTION
Compounds with the structure XC=NY are known as Schiff bases, which are usually synthesized from the condensation of primary amines and active carbonyl groups. Some Schiff bases are used as starting materials in the reactions of important drugs, such as antibiotics, antiallergic and antitumor substances [1,2]. Schiff bases have been widely used as ligand in the coordination chemistry [3] and they also show antibacterial [4,5], antifungal [6] and herbicidal [7] activities. In these compounds, two types of intermolecular hydrogen bonds (either O-H…N or O…H-N) between the OH group and the imine nitrogen can exist [8,9]. According to the formation of hydrogen bonds, tautomeric equilibrium prefers phenol-imine (O-H…N) or ketoamine (O…H-N) form.
In the previous publication, 2mpe-4bb Schiff base compound (C 13 H 11 BrN 2 O) was synthesized by Dal and coworkers [9]. They also reported some selected geometric parameters and vibrational frequencies of 2mpe-4bb together with its full 1 H and 13 C NMR chemical shifts in the same study. Even though, many Schiff base compounds have wide applications in the medical and pharmaceutical industry, there are few theoretical data for their spectroscopic spectra in literature. A detailed quantum chemical study will aid in making definitive assignments to the fundamental normal modes and chemical shifts and in clarifying the obtained experimental data for the title compound. Furthermore, the presented data as theoretically may be helpful in context of the further studies of 2mpe-4bb. For the above goals, we have calculated structural parameters, thermodynamics functions, vibrational frequencies together with PED, HOMO and LUMO data, 1 H and 13 C NMR chemical shifts of 2mpe-4bb using HF and DFT methods with 6-31G(d) and 6-311++G(d,p) basis sets and compared with available experimental values.

CALCULATIONS
For the calculations, the title compound was first optimized by HF and DFT methods with 6-31G(d) basis set in the gas phase together with the keyword volume ( Figure 1). The energy of this compound with the latter method was found lower than the former (Table 1). After optimization, the vibrational frequencies of 2mpe-4bb were calculated using the same methods and the basis set with the keyword freq and pop, and then scaled by 0.8929 for HF and 0.9613 for DFT [11][12][13][14] to generate the corrected frequencies. PED calculations; which show the relative contributions of the redundant internal coordinates to each normal vibrational mode of the molecule and thus make it possible to describe the character of each mode numerically; were carried out by the VEDA 4 (Vibrational Energy Distribution Analysis) program [18].
For the NMR calculations, the title compound was first fully optimized at HF and DFT methods using 6-31G(d) basis set in chloroform with the IEFPCM method [14][15][16][17] (Table 1). After optimization, 1 H and 13 C NMR chemical shifts (δ H and δ C ) of 2mpe-4bb were calculated using the GIAO method [14][15][16][17] for the solvent given above at the HF/6-311++G(d,p) and DFT/6-311++G(d,p) levels under the keyword nmr. Relative chemical shifts were then estimated using the corresponding tetramethylsilan (TMS) shieldings calculated in advance at the same theoretical levels as the reference. Calculated 1 H and 13 C isotropic chemical shieldings for the reference at the DFT/6-311++G(d,p)//6-31G(d) and HF/6-311++G(d,p)// 6-31G(d) levels are given in Table 2 together with the experimental values [19,20]. All the calculations were performed using Gaussian 03 program on a personal computer and GaussView program was used for visualization of the structure [21,22].

RESULTS AND DISCUSSION
2mpe-4bb Schiff base has either enol-imine (O-H…N) or keto-amine (O…H-N) conformation. Dal and coworkers determined the crystal structure of 2mpe-4bb and showed that it belongs to the enol-imine form [9]. The optimized molecular structure and numbering of the atoms for 2mpe-4bb are given in Figure 1. Several thermodynamics parameters, capacity, zero point energy, entropy etc., calculated by HF and DFT with 6-31G(d) and 6-31++G(d,p) basis sets are presented in Table 1. The variation in the zero point vibrational energy seems to be insignificant for the same method in different medium. The change in entropy of 2mpe-4bb is at room temperature. The dipole moment is expected to be larger in solution than the corresponding dipole moment in the gas phase. This situation is clearly observed in Table 1 and the dipole moment increases gradually from lower to higher dielectric. The increases on going from gas to chloroform as non-polar solvent are between 13% and 18%. The optimized structural parameters (bond lengths and angles) by HF and DFT with 6-31G(d) basis set are listed in Table 3 and compared with available experimental crystal geometry of 2mpe-4bb [9]. It can be easily seen that the skeleton of 2mpe-4bb is planar since the experimental dihedral angle between the phenyl and pyridine rings is 4.1 o [9]. Related theoretical dihedral angles are found as 3.  [23]. This bond length has been calculated as 1.328 Å for HF and 1.339 Å for DFT (Table 3). Considering the general compliance between the experimental and calculated structural parameters, it can be concluded that 2mpe-4bb compound prefers enol-imine tautomeric conformation.  It can be seen that HF method correlates well for the bond lengths while DFT shows well correlation for the bond angles compared with the other method. We note that the experimental and calculated results belong to solid and gaseous phases, respectively, for the structural parameters and vibrational frequencies.
2mpe-4bb consists of 28 atoms, so it has 78 normal mode frequencies and belongs to the C 1 point group with the only identity (E) symmetry element or operation. Some calculated and corrected vibrational wavenumbers and corresponding vibrational assignments with PED data for 2mpe-4bb at HF and DFT with 6-31G(d) basis set are given in Table 4, together with the available experimental data, for comparison. The correction factors (0.8929 for HF and 0.9613 for DFT), widely used in the literature [11][12][13][14], are applied to generate the corrected wavenumbers ( Table 4). The OH stretching band of 2mpe-4bb was observed at 3426 cm -1 and this implies that the H atom has intermolecular hydrogen bonding [9]. The corrected wavenumber of the OH stretching mode for HF is 3439 cm -1 whereas this mode is 3428 cm -1 for DFT. The CN stretching band observed at 1610 cm -1 [9], in agreement with values reported in the literature for pyridine derived Schiff base, is theoretically predicted at 1663 cm -1 and 1613 cm -1 for HF and DFT, respectively. 2mpe-4bb compound with strong band at 1280 cm -1 possesses highest percentage of enol-imine form due to the stabilization of phenolic CO bond [9]. The CO mode of 2mpe-4bb has been computed at 1283 cm -1 for HF and 1284 cm -1 for DFT.  [9]. b Raw calculated frequencies multiplied by 0.8929 (HF) and 0.9613 (B3LYP) correction factors [11][12][13][14]. c Units of IR intensity are km/mol. d Units of Raman scattering activity are Å 4 /amu. PED data are taken from VEDA4.
The HOMO and LUMO orbitals are main orbital take part in chemical stability. The HOMO describes the ability to donate an electron and LUMO as an electron acceptor. The absorption of electronic transition is defined from the ground to the first excited state. In other words, the transitions can be described from HOMO to LUMO. The HOMO is located over all carbon, N4 and Br17 atoms in 2mpe-4bb while the LUMO is dominated for N8 and O16 atoms. The atomic compositions of frontier molecular orbital and their orbital energies are shown in Figure 2. We have also calculated the 1 H and 13 C NMR chemical shifts of the title compound. Then, we have compared the theoretical and experimental chemical shifts of 2mpe-4bb. The results are shown in Table 5. According to the results, the calculated chemical shifts are in compliance with the experimental results. The largest difference between the experimental and calculated 1 H -13 C chemical shifts is 0.68-7.8 ppm for HF and 0.46-7.0 ppm for DFT with the 6-311++G(d,p) basis set. The correlation values for proton -carbon chemical shifts are found to be 0.98102 (δ calc. = 0.930δ exp. + 0.986) -0.99803 (δ calc. = 1.048δ exp. -0.779) for HF/6-311++G(d,p) and 0.99379 (δ calc. = 0.977δ exp. + 0.420) -0.99857 (δ calc. = 1.041δ exp. -0.825) for DFT/6-311++G(d,p). It can be seen that the results of DFT method are better than HF for predicting NMR properties.

CONCLUSIONS
The theoretical investigations of 2mpe-4bb Schiff base compound are successfully performed by using quantum chemical calculations such as HF and DFT. In conclusion, following results can be summarized: (i) calculated bond lengths are, in general, slightly bigger than experimental ones which are possibly due to the shortening of the bond lengths of the title molecule during the experimental measurements conducted at very low temperature. However, similar generalizations are not possible for bond angles; (ii) any differences observed between the experimental and calculated wavenumbers could be due to the fact that the calculations are performed for single molecule in the gaseous state contrary to the experimental values recorded in the presence of intermolecular interactions; (iii) for the structural bond lengths, HF method is better than DFT whereas DFT method with B3LYP functional and 6-31G(d) or 6-311++G(d,p) basis sets is better than HF for the bond angles, vibrational wavenumbers and NMR chemical shifts; and (iv) all the theoretical results have supported the conclusion obtained in the experimental studies that 2mpe-4bb is in enol-imine isomer.