AN AB-INITIO AND SPECTROSCOPIC STUDY OF DIMETHYL ETHER - AN ANALYSIS OF THE INFRARED AND RAMAN-SPECTRA

We have calculated the potential surface with respect to the torsional angles of the methyl groups, the torsional energy level structure, and the infrared and overtone Raman intensities by ab initio methods using extended basis sets. The conclusions that were drawn from a correlation to the observed spectra are the following. (i) The modest basis set MP2/6-31G(d,p) yields a value of 238.32 cm(-1) for the via gearing mode in reasonable agreement with the observed value of 241.0 cm(-1). Extension of the basis set to MP2/6-311G(df,p) marginally improves this value to 239.36 cm(-1). A somewhat larger basis set, RHF/6-311+G(3df,3p) was needed for a satisfactory description of the components of the polarizability and the dipole moment. (ii) The technique of full relaxation of the coordinates of the top and the frame was unable to account for the strong resonance between the 2v(12) overtone gearing level and the v(7) COC in-plane bending mode. (iii) The asymmetry along the diagonals of the potential surface that determines the sign and magnitude of the sine.sine gearing term was found to display complex behaviour and was positive in the region of the eclipsed-eclipsed equilibrium conformation and negative for the staggered-staggered conformation.