Infrared cavity ringdown spectroscopy of methanol clusters: Single donor hydrogen bonding

Infrared cavity ringdown laser absorption spectroscopy has been used to stu dy the O-H stretching vibrations of jet-cooled methanol clusters in direct absorption. Rovibrational bands for (CH3OH)(2), (CH3OH)(3), and (CH3OH)(4) have been measured. Both bonded and free O-H stretches were measured for th e dimer, indicating that its structure is linear. Five bands were assigned to the methanol trimer, indicating the presence of a second cyclic isomer i n the molecular beam. A detailed study of the free O-H stretching region sh ows that methanol clusters larger than dimer must exist in cyclic ring conf igurations. In order to facilitate spectral assignment, harmonic frequencie s and infrared intensities were calculated for the methanol monomer, dimer, and trimer with second order Moller-Plesset perturbation theory. Using the theoretical infrared intensities and measured vibrational band absorptions , absolute cluster concentrations were calculated. Results agree with previ ous experimental and theoretical work. (C) 1999 American Institute of Physi cs. [S0021-9606(99)01809-7].