Photochemistry of Microsolvated Nitrous Acid: Observation of the Water-Separated Complex of Nitric Oxide and Hydroxyl Radical

The photochemistry of nitrous acid (HONO) plays a crucial role in atmospheric chemistry as it serves as a key source of hydroxyl radicals (OH) in the atmosphere; however, our comprehension of the underlying mechanism for the photochemistry of HONO especially in the presence of water is far from being complete as the transient intermediates in the photoreactions have not been observed. Herein, we report the photochemistry of microsolvated HONO by water in a cryogenic N matrix. Specifically, the 1:1 hydrogen-bonded water complex of HONO was facially prepared in the matrix through stepwise photolytic O oxidation of the water complex of imidogen (NH-HO) via the intermediacy of the elusive water complex of peroxyl isomer HNOO. Upon photolysis at 193 nm, the matrix-isolated HONO-HO complex decomposes by yielding the ternary water complex of OH and NO due to the matrix cage effect. The identification of this rare water-separated radical pair (OH-HO-NO) with matrix-isolation infrared and ultraviolet-visible spectroscopy is aided by D, N, and O isotope labeling and quantum chemical calculations at the (U)CCSD/AVTZ level of theory, and its most stable structure exhibits separate hydrogen bonding interactions of the OH and NO radicals with HO via OH···OH and ON···HOH contacts, respectively. This ternary complex is extremely unstable, as it undergoes spontaneous radical recombination to reform the HONO-HO complex in the temperature range of 4-12 K through quantum-mechanical tunneling with O, H/D, N kinetic isotopic effects of 1.43, 2.33, and 0.91, respectively. At increased temperatures from 15 to 21 K, the recombination proceeds predominantly by overcoming the activation barrier with an estimated height of 0.12(1) kcal/mol.