THE METHANOL CENTER-DOT AR COMPLEX - APPARENT REDUCTION OF THE METHYL-GROUP INTERNAL-ROTATION BARRIER

The rotational spectra of the methanol Ar complex and five of its isot opomers were reinvestigated by Fourier transform microwave spectroscop y. New E internal rotation state transitions were measured and fit to determine the potential barrier that hinders the methyl group internal rotation for all the species. A dramatic reduction of this torsional barrier height determined using the conventional internal rotation Ham iltonian(from 373 cm(-1) in bare methanol to about 68.5 cm(-1) in CH3O H . Ar) was observed. It was proposed that this apparent reduction ari ses from neglect of the large amplitude hydroxyl hydrogen wagging moti on (or its hindered internal rotation) relative to argon. Assuming tha t the methyl group torsional barrier does not change upon complexation , an estimate For the hydroxyl torsional barrier of about 130 cm(-1) w as obtained. The rotational constants obtained for various isotopomers of the complex were also used to obtain an improved structure of the complex. The structure and potential function calculations suggest tha t the hydroxyl hydrogen is oriented toward the argon as if weakly hydr ogen bonding to the electron-rich rare gas atom. (C) 1995 Academic Pre ss, Inc.