Li Xiaolong, Fan Wenbin, Shao Xin, Fang Wei, Zhang Dong H, Zhou Mingfei, Francisco Joseph S, Zeng Xiaoqing
Department of Chemistry, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan University, Shanghai 200433, China.
State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China.
J Phys Chem Lett. 2025 Feb 6;16(5):1320-1326. doi: 10.1021/acs.jpclett.4c03483. Epub 2025 Jan 28.
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.
亚硝酸（HONO）的光化学在大气化学中起着至关重要的作用，因为它是大气中羟基自由基（OH）的关键来源；然而，我们对HONO光化学的潜在机制，尤其是在有水存在的情况下的理解还远远不够完整，因为光反应中的瞬态中间体尚未被观测到。在此，我们报道了在低温氮基质中，水对微溶剂化HONO的光化学作用。具体而言，通过难以捉摸的过氧异构体HNOO的水络合物中间体，经由亚氨基（NH-HO）的水络合物的逐步光解O氧化，在基质中表面制备了HONO的1:1氢键水络合物。在193 nm处光解时，由于基质笼效应，基质隔离的HONO-HO络合物分解，生成OH和NO的三元水络合物。利用D、N和O同位素标记以及理论水平为（U）CCSD/AVTZ的量子化学计算，借助基质隔离红外光谱和紫外-可见光谱对这种罕见的水分离自由基对（OH-HO-NO）进行了鉴定，其最稳定结构分别通过OH···OH和ON···HOH接触，展示了OH和NO自由基与HO的单独氢键相互作用。这种三元络合物极其不稳定，因为它在4 - 12 K的温度范围内通过量子力学隧穿自发地进行自由基重组，重新形成HONO-HO络合物，其中O、H/D、N动力学同位素效应分别为1.43、2.33和0.91。在温度从15 K升高到21 K时，重组主要通过克服估计高度为0.12(1) kcal/mol的活化势垒进行。
