Thermochemistry for enthalpies and reaction paths of nitrous acid isomers

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 closed-and open-shell reference molecules. Three high-level composite methods G3, CBS-QB3, and CBS-APNO are used for the computation of enthalpy. The enthalpy of formation, H o f (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 (HNO 2 ), which is significantly higher than values used in recent NO x combustion mechanisms. H-NO 2 is the weakest bond in isonitrous acid; but HNO 2 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.

Thermochemical properties for n‐propyl, iso‐propyl, and tert‐butyl nitroalkanes, alkyl nitrites, and their carbon‐centered radicals

“…These methods employ a variety of different geometries, frequency determinations, and higher order energy corrections (vide infra). The accuracy of these methods has been demonstrated in our previous studies on HONO isomers [2] and methyl and ethyl nitro and nitrite compounds [3].…”

“…The establishment of these basic values will also aid in the determination of the thermochemical properties of higher homologues via use of isodesmic reactions with group conservation (isogeitonic reactions). This study continues development and evaluation of thermochemical properties on simplest nitro-and nitrite compounds from our previous analysis [2,3].…”

“…In our previous studies [2,3], we calculated the thermochemistry of methyl and ethyl nitro and nitrite compounds and found that HONO (nitrous acid) was more stable than the corresponding HNO 2 isomers by ∼8.0 kcal mol −1 . In contrast, nitro methane (CH 3 NO 2 ) …”

“…61 and 40 kcal mol −1 , respectively [4][5][6][7][8]. The nitro alkanes and alkyl nitrites can release NO and NO 2 and can serve as catalysts in oxidation of hydrocarbons [9][10][11]. In this regard, nitro compounds have been widely studied by both experimental and theoretical methods [4][5][6][7][8]; nevertheless, the thermochemical data are surprisingly limited.…”

Heterogeneous reactions of HONO formation from NO2 and HNO3: a review

“…3) and comparison of them shows that (a) HNO 2 (hydrogen nitrile) and (b) (cis-or trans-) HONO have the lowest potential energy [36]. HONO is thermally more stable than HNO 2 by 8 kcal/mol.…”