Methanol as a Flexible Model

Abstract

A method based on the semirigid bender and the flexible model approaches has been developed for internal rotation in methanol. For construction of the torsion Hamiltonian matrix aK-dependent trigonometric variational basis set adapted toC3vsymmetry was employed. It allowed easy and unambiguous labeling of the torsion–rotational energy levels and it was computationally efficient as well. It was found that when certain idealized cases of structure variation with torsional angle are considered, an unusual splitting of theA1andA2energy levels occurs in the extreme case of a free rotor. It was shown how the results of theab initiocalculations forC3v(M) molecules could be linked to the developed flexible model using the approximation of equality of the torsional angle ρ and the average of the three top-frame dihedral angles τeff. An analysis of the process of determining the methanol molecular geometry employing the developed model was performed. Two of the zero-order parameters were fixed at the values obtained from electron diffraction measurement and the remaining zero-order parameters and some of the parameters describing variation of the geometry during torsion as well as the potential energy function were determined for variousJmax. The fitted values for the bending semirigidity parameters were found to be broadly consistent with those from the MO calculations, while the fitted bond stretching parameters were generally in poor agreement.