Ab initio modeling of complex aqueous and gaseous systems containing nitrogen

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.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Chemical Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• Ashcraft, Robert Wilson

Advisor dc:contributor.advisor

• William H. Green.

• Chemical Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/45918

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/45918