Microwave rotation-tunneling spectroscopy of the water–methanol dimer: Direct structural proof for the strongest bound conformation

Rotation-tunneling a-type spectra of CH3OH⋯H2O and CH3OD⋯D2O were recorded between 18 and 60 GHz using direct absorption microwave spectroscopy, and for CH3OH⋯H2O, CH313OH⋯H2O, CH3OH⋯DOH, CD3OH⋯H2O, and CH3OD⋯D2O between 7 and 24 GHz using a Fourier-transform microwave spectrometer. Because CH3OH and H2O are capable of both accepting and donating hydrogen bonds, there exists some question as to which donor–acceptor pairing of the molecules is the lowest energy form. This question is further emphasized by the ambiguity and variety present in previous experimental and computational results. Transitions arising from the methyl torsional A state were assigned in each of the studied isotopomers, and for the A and E states in CH3OH⋯H2O. While the measured components of the dipole moment for the parent (H,12C,16O) isotopomer—μa=7.956±0.03×10−30 C m (2.385±0.008 D), μb=3.636±0.02×10−30 C m (1.090±0.006 D), μc=0.43±0.47×10−30 C m (0.13±0.14 D), where the errors correspond to 1σ uncertainties—are consistent with either conformation, the fit of the structure to the rotational constants demonstrates unambiguously that the lower-energy conformation formed in supersonically cooled molecular beams corresponds to a water–donor, methanol–acceptor complex. The results and implications for future work are also discussed in terms of the permutation-inversion theory presented by Hougen and Ohashi [J. Mol. Spectros. 159, 363 (1993)].