Theoretical and experimental study of torsional potentials, molecular structure (monomer and dimer), vibrational analysis and molecular characteristics of some dimethyl bipyridines

This study deals with the determination of torsional potentials, molecular geometry in monomer and dimer form and vibrational assignments of 4,4'-dimethyl-2,2'-bipyridine (4DB); 5,5'-dimethyl-2,2'-bipyridine (5DB); and 6,6'-dimethyl-2,2'-bipyridine (6DB) using quantum chemical calculations carried out by density functional theory (DFT) employing B3LYP functional in conjunction with 6-311++G(d,p) basis set. Existence of inter-molecular hydrogen bonds was predicted. Fourier Transform infrared (FTIR) and Fourier Transform Raman (FT-Raman) spectra were recorded and vibrational analysis of the molecules was made using potential energy distribution (PED) and eigen vectors obtained in the computations. Observed and calculated frequencies agreed with an rms error 9.20, 8.21, and 8.33 cm(-1) for 4DB, 5DB, and 6DB, respectively. H-1 and C-13 NMR spectra were simulated using time-dependent DFT (TD-DFT); compared with the recorded experimental spectra of the samples in Chloroform-d (CDCl3) solvent and observed that the chemical shifts agree well with their theoretical counterparts. Electronic transitions were analyzed using experimental and simulated UV-Vis spectra of the three molecules. Molecular characteristics like HOMO-LUMO; thermodynamic parameters; and molecular electrostatic surface potential (MESP) quantified with natural charges obtained by NBO analysis are also investigated. (C) 2019 Elsevier B.V. All rights reserved.