New assignments in the torsional spectrum of CH2DOH

Torsional subbands of CH2DOH are assigned in the 20-750cm-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 CH3 or CD3 group. In the case of CH2DOH, 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 1cm-1.