Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

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 HNO2 (nitryl hydride or isonitrous acid). The objective of this study is to determine the thermochemistry of the HNO2 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 HNO2 are calculated using computational chemistry with the following methods of analysis for the atomization, isomerization, and work reactions using closed- and open-shell reference molecules. Three high-level composite methods G3, CBS-OB3, and CBS-APNO are used for the computation of enthalpy. The enthalpy of formation, Delta H-f(o) (298 K), for HONO is determined as -18.90 +/- 0.05 kcal mol(-1) (-79.08 +/- 0.2 kJ mol(-1)) and as -10.90 +/- 0.05 kcal mol(-1) (-45.61 +/- 0.2 kJ mol(-1)) for nitryl hydride (HNO2), which is significantly higher than values used in recent NOx combustion mechanisms. H-NO2 is the weakest bond in isonitrous acid; but HNO2 Will isomerize to HONO with a similar barrier to the HO-NO 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. (C) 2007 Wiley Periodicals, Inc.