Prediction of Vibrational Spectra by Computational Chemistry –Ab initio Molecular Orbital Calculation of Vibrational Frequencies and Infrared Intensities of Gas Phase Dichloromethane Molecules CH₂Cl₂, CHDCl₂ and CD₂Cl₂–

Abstract

Vibrational frequencies and infrared intensities of dichloromethane molecules CH₂Cl₂, CHDCl₂, CD₂Cl₂ in the gas phase have been calculated using the ab initio molecular orbital method with many combinations of basis set functions and electron correlation correction methods, and compared with experimental values. The dependence of calculated frequencies and intensities on basis set function and electron correlation correction method was made clear, and as a recommended combination of basis set function and electron correlation correction method, MP2/6-311G(3df, 3pd) was proposed and gave an average error of 0.93% for vibrational frequency calculation, and MP2/D95(3df, 3pd) was proposed to give an average error of 22.9% for infrared intensity calculation. However, there are numerous difficulties in the theoretical analysis of vibrational spectra when using computational chemistry methods.