Photochemistryof Microsolvated Nitrous Acid: Observationof 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 N₂ matrix. Specifically, the 1:1 hydrogen-bonded water complex of HONO was facially prepared in the matrix through stepwise photolytic O₂ oxidation of the water complex of imidogen (NH–H₂O) via the intermediacy of the elusive water complex of peroxyl isomer HNOO. Upon photolysis at 193 nm, the matrix-isolated HONO–H₂O 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–H₂O–NO) with matrix-isolation infrared and ultraviolet–visible spectroscopy is aided by D, ¹⁵N, and ¹⁸O 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 H₂O via OH···OH₂ and ON···HOH contacts, respectively. This ternary complex is extremely unstable, as it undergoes spontaneous radical recombination to reform the HONO–H₂O complex in the temperature range of 4–12 K through quantum-mechanical tunneling with ^16/18O, H/D, ^14/15N 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.