Acidities of nitrous and nitric acids

The energies of nitrous and nitric acid and their anions were calculated at the MP2(FC)/6-311(++)G(2d,2p) level. Their acidity was related to the acidity of hydroxylamine derivatives using isodesmic reactions. This procedure reveals that the molecules of HNO2 and HNO3 are strongly stabilized (by 119 or 206 kJ.mol(-1), respectively) but their anions are stabilized even more (by 282 or 428 kJ-mol(-1)). The acidity of these acids is thus caused by the low energy of their anions and not by the high energy of the acid molecules, similarly to the case of carboxylic acids. This stability may be partly due to resonance but the contribution of this effect can only be estimated. Correlation of the acidities of various acids with the inductive and polarization parameters led to the following estimate: in the case of nitrous acid 46% of the acidity enhancement is due to resonance while in the case of nitric acid it is 25%. (c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).