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

The frequencies of the three large amplitude modes of dimethylamine (D MA) were analyzed using a three-dimensional (3D) model, the independen t variables of which are the two torsional angles and the CNC symmetri c bending angle. For this purpose, the potential energy surface and th e kinetic parameters of the vibrational Hamiltonian were determined us ing fully optimized ab initio calculations performed at the MP2/6-311G (d,p) and MP4/6-311G(d,p) levels on 150 nuclear conformations. The pos itions of the two first hot bands were also calculated. The three fund amentals, 255.4, 216.9, and 409.8 cm(-1), as determined with the MP4/6 -311G(d,p) approximation, are in a good agreement with the experimenta l frequencies of 256.3, 219.4, and 383 cm(-1), respectively. Torsional frequencies were compared with those obtained from a previously publi shed two-dimensional model. The calculations confirm that the 3D model is indispensable for obtaining accurate band separations for the two different torsional modes.