THE METHANOL HCL COMPLEX - STRUCTURE AND METHYL-GROUP INTERNAL-ROTATION BARRIER

The rotational spectra of the methanol . HCl complex and several of it s isotopomers were reinvestigated by Fourier transform-microwave spect roscopy. 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 inte rnal 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 m ethanol . 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 to rsional barrier does not change upon complexation, an estimate for the hydroxyl torsional barrier of 155(5) cm(-1) was obtained. The rotatio nal 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.