Ab Initio Molecular Orbital Calculations on NO⁺(H₂O)_n Cluster Ions. 2. Thermodynamic Values for Stepwise Hydration and Nitrous Acid Formation

Abstract

Ab initio molecular orbital calculations have been used to compute thermodynamic constants (ΔH°, ΔS°, ΔG°) for the stepwise hydration reactions of NO+(H2O) n and for the competing rearrangement reaction which produces HONO and H+(H2O) n (for n ≤ 4). Geometry optimizations and harmonic frequency calculations were performed at the MP2/6-311++G(2d,p) level, and relative energies were computed at the MP2/aug-cc-pVTZ level with MP2/6-311++G(2d,p) optimized geometries. The geometry changes and energetics of these competing solvation and rearrangement reactions have been studied, and reasons are proposed to explain why NO+(H2O) n +1 formation is the dominant process for n = 1 and n = 2 but HONO + H+(H2O) n formation contributes for n = 3 and becomes more important for n = 4.

Ab initio molecular dynamics simulation of aqueous solution of nitric oxide in different formal oxidation states

“…The formation of nitrous acid from the reaction of nitrosonium cation with water is also an important upper atmosphere reaction [61]. Experimental measurements [62] and gas phase ab initio calculations [63] have shown that this reaction is dominant when at least four water molecules is involved. The formation of nitrous acid in aqueous solution within 440 fs time scale shows that this reaction is essentially diffusion controlled with negligible energy barrier.…”

Quantum chemical study of the structure, spectroscopy and reactivity of NO + .(H 2 O) n =1−5 clusters

“…The structure and vibrational frequencies of NO + . (H 2 O) n clusters for n in the range 2-5 have been studied by several groups at the MP2, MP4 or coupled cluster levels of theory [19][20][21][22]. For the n = 2 cluster, MP2 calculations predict that the lowest energy structure has both water molecules bonding to NO + .…”

Ab initio electron correlated studies on the intracluster reaction of NO+(H2O)n → H3O+(H2O)n−2 (HONO) (n = 4 and 5)

“…While optimized structures have also been theoretically investigated 2,[10][11][12][13][14][15] for n = 1-4, the intracluster reaction mechanism of each hydration number has not been fully analyzed. For example, Hammam et al 11,12 performed ab initio molecular orbital (MO) calculations on n = 1-4 clusters and predicted the hydration sequence of NO + (H 2 O) n (n = 1-4) producing the HONOÁH + (H 2 O) 3 adduct. Due to requirements of heavy computational resources, no transition state (TS) structures have so far been found.…”

“…However, ab initio MO-MC or MD simulations are difficult to apply for NO + (H 2 O) n clusters since the second-order Møller-Plesset (MP2) perturbation theory with a large basis set is required to obtain reliable structures and energies as discussed below. Thus, we have optimized structures by extending hydrogen bonding networks based on both optimized structures reported by Hamman et al 11,12 and stable structures of small water clusters. 21 The geometries of NO + (H 2 O) n (n = 4, 5) clusters were fully optimized by using the Gaussian03 package.…”