Observation of HOCO generation in [CO-(H2O)n ]+ (n = 2 and 3) clusters by infrared photodissociation spectroscopy

Abstract

The generation of the HOCO radical, a notable intermediate in atmospheric chemistry and astrochemistry, in the [CO-(H₂O)_n]⁺ (n = 2 and 3) clusters was experimentally confirmed by gas-phase infrared photodissociation (IRPD) spectroscopy. For the n = 2 cluster, only the unreacted proton-transferred isomer, a less stable structure than the most stable HOCO-containing isomer, was observed in the IRPD spectrum, despite monitoring the HOCO elimination channel. Reaction path search calculations indicated that the proton-transferred isomer photoisomerizes to the HOCO-containing isomer upon IR absorption, providing indirect evidence for formation of HOCO in [CO-(H₂O)₂]⁺. In contrast, for the n = 3 cluster, the IRPD spectrum of a HOCO-containing structure was directly obtained, which is consistent with predictions from quantum chemical simulations.