Proton Transfer Reactions between Methanol and Formic Acid Deposited on Free Ar Nanoparticles

We have sequentially picked up two astrochemically relevant Brønsted acids (methanol and formic acid) on the surface of argon nanoparticles acting as a cold support. Photoionization and electron ionization yield (HCOOH)H⁺, (CH₃OH)H⁺, and mixed protonated clusters. Experiments with perdeuterated methanol CD₃OD demonstrate notable proton transfer (PT) to formic acid acting as a proton acceptor in addition to the PT from formic acid which is, perhaps, a more intuitive one. We, therefore, for the first time observed reactions between two different complex molecules adsorbed individually on argon nanoparticles. The experimental results are compared with state-of-the-art quantum chemistry calculations, showing that both CH₃OH^•+ and HCOOH^•+ radical cations resulting from ionization can act as efficient proton donors and neutral CH₃OH and HCOOH as proton acceptors. According to the theoretical calculations, the C-H bond cleavage in the radical cation should be more favorable than the O-H bond cleavage. Both channels are observed and distinguished in the experiments with CD₃OH and CH₃OD. Our detailed mechanism of formation of the CH₃O^• and CH₂OH^• radicals contributes to understanding of astrochemistry in the protostellar medium.