The acidities of HNOx (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 kcalmol-1 at 0K, based on extrapolations through the aug-cc-pVQZ basis set, are: ΔHf(NO)=22.07 (calculated) vs. 21.46±0.04 (experimental); ΔHf(HNO)=26.39 (calculated) vs. 26.3±1 (experimental); ΔHf(NO2)=10.12 (calculated) vs. 8.59±0.2 (experimental); ΔHf(HONO2)=-29.87 (calculated) vs. -29.75±0.1 (experimental); ΔHf(HONO)=-15.79 (calculated) vs. -17.37±0.32 (experimental); ΔHf(HOONO)=-0.89 (calculated); ΔHf(NO2-)=-41.95 (calculated) vs. -43.8±0.2 (experimental); ΔHf(NO3-)=-70.76 (calculated) vs. -71.7±0.30 (experimental); and ΔHf(ONOO-)=-16.74 (calculated). The electron affinity of NO is calculated to be 0.62kcalmol-1, based on extrapolations up through the aug-cc-pV5Z basis set, and falls within 0.02kcalmol-1 of the experimental value. The corresponding heats of formation at 0K inkcalmol-1 of the anions are: ΔHf(HNO)=26.93 (calculated); ΔHf(NO)=22.28 (calculated); ΔHf(NO-)=21.66 (calculated) vs. 20.86±0.16 (experimental). The calculated gas phase acidities of HNO, HONO, HOONO, and HONO2 are found to be in excellent agreement with experiment to within 1kcalmol-1. Solvation calculations show that the free energy change for Ka in aqueous solution for HONO2, HONO, and HNO can be calculated within ∼2, 3, and 5kcalmol-1 of experiment, respectively. However, the agreement for HCN (with an established pKa) 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 HONO2, HONO, or HNO in terms of the solution phase acidity. Contrary to the view that HNO is an acid in aqueous solution, the pKa 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.
|Number of pages||18|
|Journal||International Journal of Mass Spectrometry|
|State||Published - Jul 1 2003|
Bibliographical noteFunding Information:
We thank Profs. Dale Margerum (Purdue), Ken Houk (UCLA), Kirk Peterson (WSU-Tri Cities), and John Watts (Jackson State) for helpful discussions and for sharing results with us before publication. The work at Pacific Northwest National Laboratory (PNNL) was supported in part by the U.S. Department of Energy, Offices of Basic Energy Sciences, Division of Chemical Sciences and Biological and Environmental Research, under Contract No. DE-AC06-76RLO 1830 for PNNL and in part by a subcontract to Battelle Pacific Northwest Division from Oregon Health Sciences University under the auspices of an National Institute of Environmental Health Sciences Superfund Basic Research Center award. Part of this research was performed in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL) at PNNL using the Molecular Sciences Computing Facility. The EMSL is a national user facility funded by the Office of Biological and Environmental Research in the U.S. Department of Energy. PNNL is a multiprogram national laboratory operated by Battelle Memorial Institute for the U.S. Department of Energy.
- Ab initio calculations
- Gas phase acidity
- Solvation effects
ASJC Scopus subject areas
- Condensed Matter Physics
- Physical and Theoretical Chemistry