FT-IR, FT-raman and UV spectra and ab initio HF and DFT study of conformational analysis, molecular structure and properties of ortho- meta– and para–chlorophenylboronic acid isomers

Abstract

In this study, the FT-IR, FT-Raman, and UV–Vis spectroscopic properties of three monosubstituted phenylboronic acid derivatives: ortho-chlorophenylboronic acid (o-ClPhBA), meta-chlorophenylboronic acid (m-ClPhBA) and para-chlorophenylboronic acid (p-ClPhBA) molecules are investigated both experimentally and theoretically using Density Functional Theory (B3LYP) and Hartree Fock (HF). In order to find the stable possible conformations of the compounds, the conformational analysis was carried out by running potential energy surface (PES) scan by means of rotation of two structural parameters, the dihedral angles indicated as φ2 (C6–B–O1–H1A) and φ3 (C6–B–O2–H2A), varying from −180° to 180° with an increment of 10° using B3LYP/6-31G level of theory. Also, to determinate the most stable conformer for all the molecules, potential energy curve (PEC) the stable possible conformations on PES scan were investigated as a function of φ1 (C1-C6–B–O1) dihedral angle from 0° up to 180° with an increment of 10° using B3LYP/6–311++G(d,p) and HF/6–311++G(d,p) level of theory. For all the studied compounds, two conformational structures (conformer anti-anti, syn-syn) that did not have imaginary frequency values outside the equilibrium state (conformer anti-syn) were detected theoretically at the both methods.Due to their conformational flexibility, the relative stabilities of the anti-syn, anti-anti, and syn-syn conformers of o-ClPhBA, m-ClPhBA, and p-ClPhBA are 0.0, 4.66, and 6.76 kcal/mol, respectively, at the B3LYP/6–311++G(d,p) level of theory. For the HF/6–311++G(d,p) level of theory, the relative stabilities are 0.00, 4.54, and 6.11 kcal/mol for o-ClPhBA; 0.00, 3.98, and 1.51 kcal/mol for m-ClPhBA; and 0.00, 4.10, and 1.44 kcal/mol for p-ClPhBA, respectively. Some of the determined stable conformations of these molecules are different in symmetry groups. It was observed that the increase in the symmetry was effective in the of molecular properties, especially for vibrational frequencies. The structural parameter, dipole moments (μ), vibrational frequencies, polarizability (α), hyperpolarizability (β), the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) of the stable conformers were calculated by using Ab initio HF/6–311++G(d,p) and DFT/B3LYP/6–311++G(d,p) level of theory. The assignments of fundamental vibrational modes of the studied molecule were performed based on total energy distribution (TED) analysis.