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.

A detailed chemical kinetic model for gas-phase synthesis of iron nanoparticles is presented in this work. The thermochemical data for Fe n clusters (n g 2), iron carbonyls, and iron-cluster complexes with CO were computed using density functional theory at the B3PW91/6-311+G(d) level of theory. Chemically activated and fall-off reaction rates were estimated by the QRRK method and three-body reaction theory. Kinetic models were developed for two pressures (0.3 and 1.2 atm) and validated against literature shock-tube measurements of Fe concentrations and averaged nanoparticle diameters. The new model indicates that the nanoparticle formation chemistry is much more complex than that assumed in earlier studies. For the important temperature range near 800 K in a CO atmosphere, the Fe atom formation and consumption are largely controlled by the chemistry of Fe(CO) 2 , especially the reactions Fe(CO) 2 h FeCO + CO, Fe + Fe(CO) 2 h Fe 2 CO + CO, and Fe(CO) 2 + Fe(CO) 2 h Fe 2 (CO) 3 + CO. The decomposition of Fe(CO) 5 is restricted by the rate of the spinforbidden reaction, Fe(CO) 5 h Fe(CO) 4 + CO. This model facilitates the understanding of how the reaction conditions affect the yield and size distribution of iron nanoparticles, which will be a crucial aspect in the gas-phase synthesis of carbon nanotubes.

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