A comparative study on vibrational, conformational and electronic structure of 1,2-dimethyl-5-nitroimidazole and 2-methyl-5-nitroimidazole.

The FTIR and FT-Raman spectra of 1,2-dimethyl-5-nitroimidazole and 2-methyl-5-nitroimidazole have been recorded in the regions 4000-400 and 4000-100 cm(-1), respectively. The conformational analyses were performed and the energies of the different possible conformers were determined. The geometry of different conformers of the compounds were optimised with B3LYP and B3PW91 methods using 6-311++G and aug-cc-pVTZ basis sets to characterise all stationary points as minima. The optimised structural parameters of the most stable conformer were used in the vibrational frequency calculations. The Raman activities were also determined with B3LYP method using 6-311++G basis sets. The force constants obtained from the B3LYP/6-311++G method have been utilised in the normal coordinate analysis. The temperature dependence of the thermodynamic properties heat capacity at constant pressure (C(p)), entropy (S) and enthalpy change (ΔH(0→T)) for both the compounds were also determined by B3LYP/6-311++G method. The total electron density and MESP surfaces of the molecules were constructed by NBO analysis using B3LYP/6-311++G method to display electrostatic potential (electron+nuclei) distribution, molecular shape, size, and dipole moments of the molecule. The electronic properties HOMO and LUMO energies were measured. The influences of nitro and methyl groups on the skeletal modes have been investigated.