Abstract
Recent studies show that nitrous acid, HONO, a significant precursor of the hydroxyl radical in the atmosphere, is formed during the photolysis of nitrogen dioxide in soils. The term nitrous acid is largely used interchangeably in the atmospheric literature, and the analytical methods employed do not often distinguish between the HONO structure (nitrous acid) and HNO~2~ (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO~2~ isomer, which has not been determined experimentally, and to evaluate its thermal and atmospheric stability relative to HONO. The thermochemistry of these isomers is also needed for reference and internal consistency in the calculation of larger nitrite and nitryl systems. We review, evaluate, and compare the thermochemical properties of several small nitric oxide and hydrogen nitrogen oxide molecules. The enthalpies of HONO and HNO~2~ are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closedβ and openβshell reference molecules. Three highβlevel composite methods G3, CBSβQB3, and CBSβAPNO are used for the computation of enthalpy. The enthalpy of formation, Ξ__H__^o^~f~(298 K), for HONO is determined as β18.90 Β± 0.05 kcal mol^β1^ (β79.08 Β± 0.2 kJ mol^β1^) and as β10.90 Β± 0.05 kcal mol^β1^ (β45.61 Β± 0.2 kJ mol^β1^) for nitryl hydride (HNO~2~), which is significantly higher than values used in recent NO~x~ combustion mechanisms. HβNO~2~ is the weakest bond in isonitrous acid; but HNO~2~ will isomerize to HONO with a similar barrier to the HOο£ΏNO bond energy; thus, it also serves as a source of OH in atmospheric chemistry. Kinetics of the isomerization is determined; a potential energy diagram of H/N/O~2~ system is presented, and an analysis of the triplet surface is initiated. Β© 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 378β398, 2007