Unimolecular decomposition of the isomers of [HNO2]+ and [HNO2]− systems: a DFT study Original Research Article

The potential energy surface (PES) of [HNO2]+ and [HNO2]− systems in their 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+ (1c+), HNO2+ (2+), cyc-HNO2+ (3+) and HNOO+ (4+), with a relative energetic ordering 1t+<1c+<2+<3+<4+ have been identified on the cationic surface. The anionic surface includes all corresponding isomers except the cyclic form of HNO2−. The calculations indicate that in the cationic and neutral [HNO2] systems, the trans isomer is more stable than the cis isomer, while in the anionic system both isomers are equally stable. The present study suggests that NO+ and hydroxide radical react to yield HONO+ as the dominant product at thermal energies. The HONO+ potential well is stabilised 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 collision-induced dissociation of HONO+ into HO+NO+. Unimolecular dissociation of HNO2+ at energies of 30 kcal/mol and above would lead to the competitive H+NO2+ and OH+NO+ product formation. The reaction of hydroxide ion (OH−) with nitric oxide is found to proceed through the associated HONO− system followed by electron detachment. However, the appreciable difference in the geometry of cis-HONO and cis-HONO− results in a weak Frank–Condon wavefunction overlap and thereby an inefficient associative electron detachment process.