THE A-TYPE K=0 MICROWAVE-SPECTRUM OF THE METHANOL DIMER

The rotational spectrum of(CH3OH)(2) has been observed in the region 4 -22 GHz with pulsed-beam Fabry-Perot cavity Fourier-transform microwav e spectrometers at NIST and at the University of Kiel. Each a-type R(J ), K-a = 0 transition is split into 15 states by tunneling motions for (CH3OH)(2), ((CH3OH)-C-13)(2), (CH3OD)(2), (CD3OH)(2), and (CD3OH)(2) . The preliminary analysis of the methyl internal rotation presented h ere was guided by the previously developed multidimensional tunneling theory which predicts 16 tunneling components for each R(J) transition from 25 distinct tunneling motions. Several isotopically mixed dimers of methanol have also been measured, namely (CH3OH)-C-13, CH3OD, CD3O H, and CD3OD bound to (CH3OH)-C-12. Since the hydrogen bond interchang e motion (which converts a donor into an acceptor) would produce a new and less favorable conformation from an energy viewpoint, it does not occur and only 10 tunneling components are observed for these mixed d imers. The structure of the complex is similar to that of water dimer with a hydrogen bond distance of 2.035 Angstrom and a tilt of the acce ptor methanol of 84 degrees from the O-H-O axis. The effective barrier to internal rotation for the donor methyl group of (CH3OH)(2) is V-3 = 183.0 cm(-1) and is one-half of the value for the methanol monomer ( 370 cm(-1)), while the barrier to internal rotation of the acceptor me thyl group is 120 cm(-1). (C) 1995 Academic Press, Inc.