High-Pressure Photoinduced Reactivity of CH3OH and CD3OH

The room-temperature reactivity of liquid methanol induced by two- photon absorption of near UV photons (350 nm) was studied as a function of pressure. Different chemical reactions were triggered by the radical species produced through the population of the lowest electronic excited singlet state because of its dissociative character. Experiments were performed at room temperature between 0.1 and 1.8 GPa on CH3OH and between 0.2 and 1.5 GPa on CD3OH. Different irradiation cycles were performed at constant pressure conditions, and FTIR and Raman spectra were measured to monitor the reaction evolution. Methoxymethanol and methylformate were the main products and the only ones detected in all the experiments. Ethylene glycol formed only at low pressure (0.2-0.3 GPa), whereas small amounts of methane, water, and unsaturated (C=C) species were also detected independently of the reaction pressure. Only dissociation along the O-H and C-O coordinates was relevant in the investigated pressure range. Ethylene glycol, methoxymethanol, and methylformate derive from the dissociation channel involving the O-H bond cleavage, whereas methane and unsaturated species come from the dissociation along the C-O bond. The comparison of the results obtained for the two isotopomers at the different investigated pressures allowed the identification of three different reactive paths that, starting from the methoxy radical, lead to the formation of the main products. The important effect of pressure on the reaction evolution could suggest a modification of the potential energy surface of the lowest electronic excited state along the O-H coordinate on increasing pressure.