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

The 1:1 complexes of water with nitric acid, pernitric acid and nitrous aci d as well as their dissociation products, OH, HO2, NO, NO2 and NO3, were in vestigated 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 t he acid acting as the donor and water as the acceptor. The global energy mi nima for the nitric acid and cis-nitrous acid complexes have six-membered c yclic structures, with a weak second hydrogen bond between the water and th e 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. C omplex formation does not significantly alter the available reaction pathwa ys for acid photodissociation, nor does it cause large shifts in the electr onic spectra. Large red-shifts in the OH stretching region of the infrared spectrum are predicted, however.