The Microwave Spectrum of the Methanol Dimer for K = 0 and 1 States

The rotational spectrum of (CH3OH)2 has been observed in the 8 to 24 GHz region with a pulsed-beam Fabry-Perot cavity Fourier-transform microwave spectrometer. Previously we demonstrated that each transition of the a-type R(J), Ka = 0 is split into 15 states of the 16 theoretically expected states by tunneling motions. Here we show that the K = 1 states are split into the 16 expected states through the assignment of the Ka = 1 a-type transitions and DeltaKa = 1 b-type transitions. The internal-rotation analysis of the two inequivalent methyl groups presented here was guided by the previous experimental observations and theory for multidimensional tunneling, which predicts 16 tunneling components for each R(J) transition from 25 distinct tunneling motions. The effective barrier to internal rotation for the donor methyl group of (CH3OH)2 is V3 = 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 methyl group is 120 cm-1, one-third of the methanol monomer. The structure of the methanol dimer complex is similar to that of water dimer with a hydrogen bond distance of 1.96(2) A and tilt of the acceptor methanol of 77(2)degrees from the O-H-O axis (one standard deviation uncertainty). This structure shows good agreement with the angular orientation of the methyl groups derived in the internal-rotation analysis. Copyright 1997 Academic Press. Copyright 1997Academic Press