Linestrengths of torsion-rotation transitions of methanol for J≤22,K≤14, and vt≤2 from Hamiltonian-based calculations

Linestrengths have been obtained for methanol based on matrix elements of the dipole moment operator evaluated in a set of torsion-relation eigenfunctions. The latter were obtained from the parameters and effective Hamiltonian giving the best spectral fit available for a data set containing J less than or equal to 20, K less than or equal to 14, and upsilon(t) less than or equal to 1 microwave and far infrared transitions. The dipole moment function was represented as a Fourier expansion in the torsional angle gamma. Values for the permanent components mu(a) and mu(b) were obtained from experiment; values for their cos 3 gamma variation and for the sin 3 gamma variation of mu(c) were obtained from quantum chemistry molecular orbital calculations. The approach described allows calculation of the linestrength for any transition which satisfies the general selection rule A(1) <-> A(2) or E <-> E. A complete set of such linestrengths for all methanol torsion-rotation transitions involving levels with J less than or equal to 22, K less than or equal to 14, and upsilon(t) less than or equal to 2, and with intensities and frequencies above the rather small cutoff limits of 10(-6) D-2 and 1 kHz, respectively, together with individual contributions from the permanent and gamma-varying dipole moment components, have been tabulated and are available on request (ftp.monash.edu.au/pub/chem/1998a). (C) 1999 Academic Press.