AB INITIO THEORETICAL STUDY OF THE METHYL AND PHOSPHINE MODES IN ETHYLPHOSPHINE

Abstract

The far infrared (FIR) methyl and phosphine torsional frequencies and intensities are determined theoretically in ethylphosphine from ab initio calculations. For this purpose, the potential energy function for the double rotation of the methyl and phosphine groups of ethylphosphine in its electronic ground state is determined by using the MP2/RHF approximation and the 6-31G (3df,p) basis set with full optimization of the geometry with respect to the remaining coordinates. The kinetic parameters were deduced from the optimal geometries for each conformation. The numerical results are fitted to a symmetry adapted analytical form and introduced into the Hamiltonian operator. The Schr""{o}dinger equation for these two motions is solved by developing the solutions on the basis of products of trigonometric functions. From the energy levels, the torsional functions and the dipole moment variations the FIR spectrum is synthesized. The calculated frequencies in the region from 200 to $140 cm^{-1}$ agree reasonably well with the experimental data. A new assignement, however, is suggested for some trans-trans and the gauche-gauche transitions between the phosphine levels. Additional transitions between the methyl levels are also proposed.