An ab initio three-dimensional torsion-torsion-wagging analysis of the far infrared spectra of dimethylamine

Abstract

The far infrared transitions of dimethylamine (DMA) corresponding to the double methyl rotation and the amine hydrogen wagging, are determined in three dimensions using ab initio calculation (3D). For this purpose, the potential energy function for the nuclear motion was first calculated using the MP2 and the MP4 methods with the 6-31G(d, p) basis set. The equation for the nuclear motion was solved variationally by expanding the solutions on a set of symmetry eigenvectors. Finally, the frequencies were deduced. The results are compared with previous studies that were performed on DMA for the wagging in one dimension, the wagging and CNC bending in two dimensions, the double methyl torsion in two dimensions, and the double torsion and CNC bending in three dimensions, as well as with the experimental data. The theoretical results agree reasonably well with the experiments. In addition, it is found that the explicit consideration of the CNC bending or amine hydrogen wagging in each 3D calculation has an opposite shifting effect on the frequencies and both vibration modes have to be considered explicitly with the torsional modes in a 4D calculation in order to accurately reproduce the far infrared spectrum.