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, an d the infrared and overtone Raman intensities by ab initio methods usi ng extended basis sets. The conclusions that were drawn from a correla tion to the observed spectra are the following. (i) The modest basis s et 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 thi s 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 th e polarizability and the dipole moment. (ii) The technique of full rel axation of the coordinates of the top and the frame was unable to acco unt 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 magni tude of the sine.sine gearing term was found to display complex behavi our and was positive in the region of the eclipsed-eclipsed equilibriu m conformation and negative for the staggered-staggered conformation.