DFT and AIM Study of the Protonation of Nitrous Acid and the pKaof Nitrous Acidium Ion

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Abstract
The gas phase and aqueous thermochemistry, NMR chemical shifts, and the topology of chemical bonding of nitrous acid (HONO) and nitrous acidium ion (H2ONO+) have been investigated by ab initio methods using density functional theory. By the same methods, the dissociation of H2ONO+ to give the nitrosonium ion (NO+) and water has also been investigated. We have used Becke’s hybrid functional (B3LYP), and geometry optimizations were performed with the 6–311++G(d,p) basis set. In addition, highly accurate ab initio composite methods (G3 and CBS-Q) were used. Solvation energies were calculated using the conductor-like polarizable continuum model, CPCM, at the B3LYP/6–311++G(d,p) level of theory, with the UAKS cavity model. The pKa value of H2ONO+ was calculated using two different schemes: the direct method and the proton exchange method. The calculated pKa values at different levels of theory range from −9.4 to −15.6, showing that H2ONO+ is a strong acid (i.e., HONO is only a weak base). The equilibrium constant, KR, for protonation of nitrous acid followed by dissociation to give NO+ and H2O has also been calculated using the same methodologies. The pKR value calculated by the G3 and CBS-QB3 methods is in best (and satisfactory) agreement with experimental results, which allows us to narrow down the likely value of the pKa of H2ONO+ to about −10, a value appreciably more acidic than literature values.

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