Ab initio calculations and analysis of the torsional spectra of dimethylamine and dimethylphosphine.

In the present paper, ab initio calculations at MP2/RHF level are performed with different basis sets 6-31G(d,p), 6-311G(d,p), and 6-311(df ,p) to determine the potential energy functions, the kinetic parameters, and the dipole moment components as a function of the double methyl rotation in dimethylamine (DMA) and dimethylphosphine (DMP). From the potential energy and kinetic parameters, the torsional energy levels and torsional functions are determined, and from the dipole moment variations, the far infrared spectra are synthesized by calculating both the frequencies and the intensities. The results are in relatively good agreement with experimental spectra. Calculations confirm the assignments performed with the experimental potentials fitted with only five terms. The calculations, however, allow to reassign the observed band at 239.8 cm[SUP-1] in DMA and at 177.2 cm[SUP-1] in DMP to the superimposition of two different transitions: the 03→04 third sequence and an 10→11 vibrationally excited fundamental. [ABSTRACT FROM AUTHOR]