Ab initio modeling of complex aqueous and gaseous systems containing nitrogen

Abstract

Nitrogen chemistry is ubiquitous in everyday life, from biological processes at ambient conditions to atmospheric chemistry at low pressures and temperatures to high-temperature combustion. Understanding the chemical behavior of nitrogen-containing species under a variety of conditions and in multiple phases is critical to accurately modeling system behavior. The further ability to model system behavior based solely on a first principles approach would be a boon to researchers attempting to design and understand technologies utilizing complex systems. This work attempts to further these abilities for both solution-phase and gas-phase predictions from ab initio calculations. An overview of solvation thermodynamics is given that relates computational chemistry to phenomenological thermodynamics for common equilibrium expressions. Special attention is paid to fully understanding the role of activity coefficients, standard states, and reference states and how these affect the subsequent expressions. A procedure is outlined for estimating the thermochemical properties of small molecules in aqueous solution based on computational chemistry calculations utilizing continuum solvation models. The partitioning of the entropic and enthalpic contributions is of the utmost importance if one is to accurately estimate the enthalpy of formation and entropy in solution. Procedures for rate coefficient estimation via solution-phase transition state theory, simple electron transfer theory, and dissociative isomerizations within a solvent cage are also discussed. The oxidation of hydroxylamine in aqueous nitric acid was chosen as a test system. A detailed chemical mechanism was constructed and thermochemical and rate parameters from computational chemistry calculations were used to model the behavior of the system. Using current continuum solvation models, it does not appear possible to build reliable predictive models of complex aqueous systems, particular those with a high ionic strength. However, the present semi-quantitative models may be helpful in focusing attention on the key unknowns.

(cont.) Group additivity values were estimated for more than 50 new functional groups containing nitrogen based on high-level computational chemistry estimates of the thermochemical parameters of 105 non-cyclic C/H/N/O species. The thermochemical and kinetics databases of the group's Reaction Mechanism Generator software were restructured to be more extensible and to explicitly include nitrogen chemistry. This allows new chemistry to be added to the software more easily and will allow predictions for gas-phase nitrogencontaining systems in the very near future.