Analysis of torsional spectra of molecules with two internal C3v rotors: Part XXII. Barrier to internal rotation of dimethylselenide and dimethylselenide-d3 and vibrational assignment for dimethylselenide-d3

Abstract The low frequency vibrational spectra of gaseous dimethylselenide and dimethylselenide-d 3 have been investigated from 50 to 500 cm −1 . A number of torsional transitions have been resolved in the far infrared spectra for both molecules and their analyses have given a torsional potential function based on a semirigid model. For the light molecule the effective barrier to internal rotation was found to be 507 ± 17 cm −1 (1450 cal mol −1 ) and the cosine-cosine coupling term, V 33 was found to be 54 ± 12 cm −1 (154 cal mol −1 ), whereas the sine-sine coupling term, V 33 , has a value of 10 10 ± 4 cm −1 (29 cal mol −1 ). An interaction between the low frequency CSeC bending mode and the (2,0) - (1,0) torsional transition is proposed as a possible explanation for the assignment of the torsional transitions for dimethylselenide-d 3 . The effective barrier for the -d 3 molecule was found to have a value of 520 ± 6 cm −1 (1487 cal mol −1 ) with a sine-sine coupling term value of −35 ± 7 cm −1 (−100 cal mol −1 ). A complete vibrational assignment is given for the dimethylselenide-d 3 molecule and a normal coordinate calculation has been carried out using a modified valence force field. These results are compared to the similar quantities in some corresponding molecules.