Unimolecular decomposition of the isomers of [HNO2](+) and [HNO2](-) systems: a DFT study

The potential energy surface (PES) of [HNO2](+) and [HNO2](-) systems in th eir electronic ground state has been investigated using density functional calculations employing the B3LYP functional and a basis set of 6-311 + + G( 3df, 2pd) quality. Five stable isomers, viz., trans-HONO+ (1t +), cis-HONO (Ic +), HNO2+ (2 +), cyc-HNO2+ (3 +) and HNOO+ (4 +), with a relative ener getic ordering 1t + < 1c + < 2 + < 3 + < 4 + have been identified on the ca tionic surface. The anionic surface includes all corresponding isomers exce pt the cyclic form of HNO;. The calculations indicate that in the cationic and neutral [HNO,] systems, the trans isomer is more stable than the cis is omer, while in the anionic system both isomers are equally stable. The pres ent study suggests that NO+ and hydroxide radical react to yield HONO+ as t he dominant product at thermal energies. The HONO+ potential well is stabil ised by only 9.4 kcal/mol while the vertical electron detachment energy for HONO+ is calculated to be 22.1 kcal/mol. Consequently charge stripping of HONO+ to the superelectrophile, HONO2+, is ineffective compared to the coll ision-induced dissociation of HONO+ into HO + NO+. Unimolecular dissociatio n of HNO2+ at energies of 30 kcal/mol and above would lead to the competiti ve H + NO2+ and OH + NO+ product formation. The reaction of hydroxide ion ( OH-) with nitric oxide is found to proceed through the associated HONO- sys tem followed by electron detachment. However, the appreciable difference in the geometry of cis-HONO and cis-HONO- results in a weak Frank-Condon wave function overlap and thereby an inefficient associative electron detachment process. (C) 1999 Elsevier Science B.V. All rights reserved.