Theoretical analysis of the CH stretching overtone vibration of 1,2-dichloroethylene

The intensity of the CH stretching overtone spectra of liquid cis-dichloroethylene is known to be greater than that of the trans isomer, even though the transition energies are almost the same. To obtain theoretical insight on this feature, we performed a vibrational calculation under the local mode model, which is preferred for describing vibration of light-heavy bonds, using the grid method and the potential energy surface (PES) and the dipole moment function (DMF) calculated by hybrid density functional theory method. It was determined that the DMF, in the direction perpendicular to the CC bond, was significantly distorted from linearity as a function of the CH bond length. This distortion, which is regarded as the electric anharmonicity, was greater for the cis isomer, thus giving a stronger overtone absorption intensity for this isomer. In addition, the numerical accuracy in representing the PES and DMF was discussed for the overtone vibrational calculation.