Ab initio investigation of water complexes of some atmospherically important acids: HONO, HNO3 and HO2NO2

Abstract

The 1:1 complexes of water with nitric acid, pernitric acid and nitrous acid as well as their dissociation products, OH, HO₂, NO, NO₂ and NO₃, were investigated at the DFT/6-311++G(3df,3dp) level of theory. The global minima all display strong hydrogen bonding interactions (5–8 kcal mol⁻¹), with the acid acting as the donor and water as the acceptor. The global energy minima for the nitric acid and cis-nitrous acid complexes have six-membered cyclic structures, with a weak second hydrogen bond between the water and the acid. Much more weakly bound hydrogen bonded structures (0.5–1 kcal mol⁻¹) are formed with the water molecule acting as the hydrogen bond donor. Complex formation does not significantly alter the available reaction pathways for acid photodissociation, nor does it cause large shifts in the electronic spectra. Large red-shifts in the OH stretching region of the infrared spectrum are predicted, however.