Effects of Hydration on the Properties of Protonated-Water−Nitric Acid Clusters

The uptake of nitric acid by protonated water (hydronium cation, H3O+) clusters and the characteristic structures of the resulting mixed aggregates, H+·(H2O)n·(HNO3)m, have been studied theoretically. As baseline simulations, B3LYP/6-311++G(d,p) geometry optimizations and energy calculations for n = 1−9 and m = 0−1 have been employed. To establish the mechanism of nitric acid uptake by hydronium ion−water clusters, the energetics of the various reaction pathways involving H3O+·(H2O)n and HNO3 are evaluated. In the presence of hydronium ion−water clusters, protonated nitric acid forms by a charge exchange reaction involving H3O+ and HNO3. Successive hydration of H+·HNO3 is an energetically favorable phenomena resulting in the formation of the mixed clusters containing one nitric acid molecule. However, when the number of water ligands on the clusters is less than about five, water binds more strongly to protonated water aggregates than to the mixed clusters. Accordingly, water will preferentially react with hydrated hydronium ions. Attachment reactions of nitric acid to the hydronium ion−water clusters are also found to be exothermic across the spectrum of sizes considered. However, switching reactions, involving H2O and H+·(H2O)n·(HNO3) for n = 2−7, are found to reestablish the hydronium ion−water cluster series. Above five water molecules, the binding energy of water to the mixed clusters containing one nitric acid molecule becomes comparable to that of the equivalent water aggregates; hence, the concentration of the former is expected to grow. Similar behavior has been experimentally observed by Castleman and co-workers [Zhang, X.; Mereand, E. L.; Castleman, A. W., Jr. J. Phys. Chem. 1994, 98, 3554. Gilligan, J. J.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 5601].1,2 The structural arrangement of the optimized mixed clusters has also been investigated. The results show that isomeric configurations exist and that, depending on the degree of hydration of the ionic clusters, the most stable structures are the hydronium ion (H3O+), H5O2+ or the nitronium ion (NO2+). For low levels of hydration (i.e., n = 0, 1), the nitronium ion−water structures appear to be the most stable. These results agree with experimental investigations [Cao, Y.; Choi, J.-H.; Haas, B.-M.; Johnson, M. S.; Okumura, M. J. Chem. Phys. 1993, 99, 9307. Cao, Y.; Choi, J.-H.; Haas, B.-M.; Okumura, M. J. Phys. Chem. 1994, 98, 12176. Choi, J.-H.; Kuwata, K. T.; Cao, Y.-B.; Haas, B.-M.; Okumura, M. J. Phys. Chem. A 1997, 101, 6753].3-5 The geometry optimized structures also show that, depending on the cluster arrangement, the nitric acid molecule can dissociate into NO3- and H3O+ when the number of water ligands in the cluster is greater than about seven.