INFRARED EXCITATION AND DISSOCIATION OF METHANOL DIMERS AND TRIMERS

Excitation by means of one CO2-laser photon without dissociation has been observed for methanol dimers and trimers with low internal energy. Methanol dimers with relatively high internal energy do dissociate after absorption of one CO2-laser photon. Experiments with two CO2 lasers demonstrate that for dimers with low internal energy absorption of a second photon is necessary to produce dissociation over a broad spectral range. Methanol trimers can absorb a second CO2-laser photon without dissociating. For optimum absorption the frequency of the second photon is shifted 4 cm-1 to the red with respect to the first one. After correction for temperature-dependent contributions, the dissociation energy of the dimer at 0 K, D0, is obtained as 3.2 ± 0.1 kcal/ mol. Ab initio calculations on the dimer at the correlated SCF+MP2 level of theory yield D0=3.5±0.3 kcal/mol, as well as the vibrationless binding energy, ΔE, of -5.3±0.2 kcal/mol at a predicted O⋯O distance of 2.91 Å. Additional calculations assign the observed dimer peaks at 1026.5 and 1051.6 cm-1 to absorption by the CO modes of the hydrogen-bond acceptor and donor molecules, respectively. © 1990.