ON THE STRUCTURE OF THE METHANOL WATER DIMER

Molecular-beam depletion spectroscopy has been employed to study the dissociation of mixed methanol-water clusters upon excitation of the C-O stretch vibration of the methanol constituent at 9.7-mu-m. Using the scattering selection technique, we were able to measure a methanol-water dimer dissociation spectrum which was not disturbed by fragmentation artifacts of larger clusters. In contrast to the pure methanol dimer spectrum which shows two peaks at 1026.5 and 1051.6 cm-1 corresponding to the excitation of the proton acceptor and donor in the complex, the dissociation spectrum of the methanol-water heterodimer features only one single peak at 1027.8 cm-1. This indicates that, in the mixed complex, the methanol molecule occupies the proton-acceptor position whereas the water molecule acts as the proton donor. Heterodimers with their constituents in the reversed position could not be observed under the given experimental conditions.