New assignments in the torsional spectrum of [formula omitted

Abstract

Torsional subbands of CH 2 DOH are assigned in the 20– 750 cm - 1 region using the results of a theoretical calculation accounting for the fact that this molecule displays a large amplitude internal rotation of a methyl group without threefold symmetry. Ab initio calculations and a harmonic expansion along a minimum energy path allow us to obtain a mass dependent effective potential energy function. This potential energy function is used to retrieve rotation–torsion energy levels making use of a flexible model and deriving a four dimensional Hamiltonian. Its matrix is set up evaluating numerically torsional matrix elements with weights and nodes appropriate for periodic functions. The rotation–torsion energy levels thus obtained are in good agreement with experimental ones in the case of the isotopic species with a symmetrical CH 3 or CD 3 group. In the case of CH 2 DOH , a synthetic spectrum is calculated and allows us to assign 35 torsional subbands in the recorded far infrared and mid infrared spectra. The discrepancies between observed and calculated Q-branch origins are on the order of 1 cm - 1 .