METHANOL AS A FLEXIBLE MODEL

A method based on the semirigid bender and the flexible model approach es has been developed for internal rotation in methanol. For construct ion of the torsion Hamiltonian matrix a K-dependent trigonometric vari ational basis set adapted to C-3v symmetry was employed. It allowed ea sy and unambiguous labeling of the torsion-rotational energy levels an d it was computationally efficient as well. II was found that when cer tain idealized cases of structure variation with torsional angle are c onsidered, an unusual splitting of the A(1) and A(2) energy levels occ urs in the extreme case of a free rotor. It was shown how the results of the ab initio calculations for C-3v(M) molecules could be linked to the developed flexible model using the approximation of equality of t he torsional angle rho and the average of the three top-frame dihedral angles tau(eff). An analysis of the process of determining the methan ol molecular geometry employing the developed model was performed, Two of the zero-order parameters were fixed at the values obtained from e lectron diffraction measurement and the remaining zero-order parameter s and some of the parameters describing variation of the geometry duri ng torsion as well as the potential energy function were determined fo r various J(max). The fitted values for the bending semirigidity param eters were found to be broadly consistent with those from the MO calcu lations, while the fitted bond stretching parameters were generally in poor agreement. (C) 1996 Academic Press, Inc.