The gas and solution phase acidities of HNO, HOONO, HONO, and HONO 2

Abstract

The acidities of HNO x (where x=1–3) have been calculated in the gas phase and in solution by using high level ab initio molecular orbital theory. Coupled cluster (CCSD(T)) energies, extrapolated to the complete basis set (CBS) limit, were used to determine thermodynamic properties of the species in the gas phase, and the fully polarizable continuum model (FPCM) was used to calculate the effect of solvent on the energy changes. The calculated gas phase heats of formation in kcal mol −1 at 0 K, based on extrapolations through the aug-cc-pVQZ basis set, are: Δ H f(NO)=22.07 (calculated) vs. 21.46±0.04 (experimental); Δ H f(HNO)=26.39 (calculated) vs. 26.3±1 (experimental); Δ H f(NO 2)=10.12 (calculated) vs. 8.59±0.2 (experimental); Δ H f(HONO 2)=−29.87 (calculated) vs. −29.75±0.1 (experimental); Δ H f(HONO)=−15.79 (calculated) vs. −17.37±0.32 (experimental); Δ H f(HOONO)=−0.89 (calculated); Δ H f(NO 2 −)=−41.95 (calculated) vs. −43.8±0.2 (experimental); Δ H f(NO 3 −)=−70.76 (calculated) vs. −71.7±0.30 (experimental); and Δ H f(ONOO −)=−16.74 (calculated). The electron affinity of NO is calculated to be 0.62 kcal mol −1, based on extrapolations up through the aug-cc-pV5Z basis set, and falls within 0.02 kcal mol −1 of the experimental value. The corresponding heats of formation at 0 K in kcal mol −1 of the anions are: Δ H f(HNO)=26.93 (calculated); Δ H f(NO)=22.28 (calculated); Δ H f(NO −)=21.66 (calculated) vs. 20.86±0.16 (experimental). The calculated gas phase acidities of HNO, HONO, HOONO, and HONO 2 are found to be in excellent agreement with experiment to within 1 kcal mol −1. Solvation calculations show that the free energy change for K a in aqueous solution for HONO 2, HONO, and HNO can be calculated within ∼2, 3, and 5 kcal mol −1 of experiment, respectively. However, the agreement for HCN (with an established p K a) and HOONO is much worse and the calculated results suggest that the effect of the directly interacting solvent shells around HCN and HOONO are quite different from those around HONO 2, HONO, or HNO in terms of the solution phase acidity. Contrary to the view that HNO is an acid in aqueous solution, the p K a is estimated to be between 10 and 13 for HNO consistent with the latest experimental result. This suggests that the behavior of HNO in biological systems warrants further investigation as it is not an acid as has previously been accepted.