The methanol·HCl complex: structure and methyl group internal rotation barrier

Abstract

The rotational spectra of the methanol·HCl complex and several of its isotopomers were reinvestigated by Fourier transform-microwave spectroscopy. An ambiguity in the assignment of the ground state rotational spectrum was clarified. New E state transitions were measured and fit to determine the potential barrier that hinders the methyl group internal rotation for all species. A dramatic reduction in this torsional barrier height was determined using the conventional internal rotation Hamiltonian (from 373 cm −1 in bare methanol to about 74 cm −1 in methanol·HCl). This apparent barrier reduction arises from neglect of the large amplitude hydroxyl hydrogen wagging motion (or its hindered internal rotation) relative to HCl. Assuming that the methyl group torsional barrier does not change upon complexation, an estimate for the hydroxyl torsional barrier of 155(5) cm −1 was obtained. The rotational constants obtained for various isotopomers of the complex as well as the Cl quadrupole hyperfine coupling constants were used to obtain an improved structure for the complex.