AN AB-INITIO 3-DIMENSIONAL TORSION-TORSION-WAGGING ANALYSIS OF THE FAR-INFRARED SPECTRA OF DIMETHYLAMINE

The far infrared transitions of dimethylamine (DMA) corresponding to t he double methyl rotation and the amine hydrogen wagging, are determin ed in three dimensions using ab initio calculation (3D). For this purp ose, the potential energy function for the nuclear motion was first ca lculated 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 e xpanding the solutions on a set of symmetry eigenvectors. Finally, the frequencies were deduced. The results are compared with previous stud ies that were performed on DMA for the wagging in one dimension, the w agging and CNC bending in two dimensions, the double methyl torsion in two dimensions, and the double torsion and CNC bending in three dimen sions, as well as with the experimental data. The theoretical results agree reasonably well with the experiments. In addition, it is found t hat the explicit consideration of the CNC bending or amine hydrogen wa gging in each 3D calculation has an opposite shifting effect on the fr equencies and both vibration modes have to be considered explicitly wi th the torsional modes in a 4D calculation in order to accurately repr oduce the far infrared spectrum.