Theoretical studies of cooperativity effects in the ternary complexes of nitrous acid with ammonia and water

Theoretical MP2 and B3LYP studies of the structures, energetics and vibrational spectra of the ternary complexes of nitrous acid with the ammonia and water molecules are presented. Six stable structures for the 1:1:1 HONO/NH3/H2O complexes were found. The lowest energy complexes formed by the trans- and cis-HONO isomers have cyclic seven- and eight-membered structures in which the mediating water molecule is strongly bound to the OH group of trans-HONO and to the nitrogen atom of NH3. In the other eight-membered cyclic structure (formed by cis-HONO), two six-membered structures (formed by trans- and cis-HONO) and the chain structure (formed by trans-HONO) the main contribution to the complex stabilization comes form the interaction between the OH group of HONO and the nitrogen atom of ammonia, however, the other two weak bonds formed by the H2O molecule with NH3 and HONO give also nonegligible contributions. For all stable complexes the attractive three-body interaction energy terms, (ΔE3), are calculated. The strongest cooperativity effects, as determined by ΔE3 values, occur for the seven and eight-membered cyclic complexes, in the six-membered structures the cooperativity is reduced due to the strong distortion of the hydrogen bonds in the smaller rings. The analysis of the cooperative effects have been also performed for the ternary HONO/H2O/H2O complexes and the results are compared with those obtained for the HONO/H2O/H2O ones.