Ab Initio Aqueous Thermochemistry: Application to the Oxidation of Hydroxylamine in Nitric Acid Solution

Ab initio molecular orbital calculations were performed and thermochemical parameters estimated for 46
species involved in the oxidation of hydroxylamine in aqueous nitric acid solution. Solution-phase properties
were estimated using the several levels of theory in Gaussian03 and using COSMOtherm. The use of
computational chemistry calculations for the estimation of physical properties and constants in solution is
addressed. The connection between the pseudochemical potential of Ben-Naim and the traditional standard
state-based thermochemistry is shown, and the connection of these ideas to computational chemistry results
is established. This theoretical framework provides a basis for the practical use of the solution-phase
computational chemistry estimates for real systems, without the implicit assumptions that often hide the nuances
of solution-phase thermochemistry. The effect of nonidealities and a method to account for them is also
discussed. A method is presented for estimating the solvation enthalpy and entropy for dilute aqueous solutions
based on the solvation free energy from the ab initio calculations. The accuracy of the estimated thermochemical
parameters was determined through comparison with (i) enthalpies of formation in the gas phase and in
solution, (ii) Henry's law data for aqueous solutions, and (iii) various reaction equilibria in aqueous solution.
Typical mean absolute deviations (MAD) for the solvation free energy in room-temperature water appear to
be ∼1.5 kcal/mol for most methods investigated. The MAD for computed enthalpies of formation in solution
was 1.5−3 kcal/mol, depending on the methodology employed and the type of species (ion, radical, closed-shell) being computed. This work provides a relatively simple and unambiguous approach that can be used
to estimate the thermochemical parameters needed to build detailed ab initio kinetic models of systems in
aqueous solution. Technical challenges that limit the accuracy of the estimates are highlighted.