The theoretical determination of rate constants and thermochemical properties of small boron and nitrogen compounds.

摘要

In this dissertation, we first report ab initio calculations of features of the potential energy surfaces for three reactions of potential importance for the combustion of boron in fluorine-containing environments: HOBO + HF → FBO + H2, OBBO + HF → FBO + HBO, and BF + H2O → FBO + H2. The first two reactions proceed through four-center transition states and produce the major high temperature combustion product, FBO. For both these reactions, transition state theory calculations, based on high level multi-reference configuration interaction calculations, predict reaction rates to be orders of magnitude slower than those used in recent combustion modeling efforts. The third reaction produces the major product, FBO, via the reaction sequence BF + H2O → HBFOH → FBO + H2. The first step involves a three-center transition state forming the stable HBFOH molecule which then falls apart through a four-center transition state yielding the product, FBO. Results from high-level multi-reference configuration interaction calculations indicate that the initial barrier is too large for significant product to form.;Another reaction studied, NH2 + H ↔ 3NH + H2, has recently been suggested as a possible alternative NH 2 loss mechanism in efforts to stop product branching in the NH 2 + NO reaction. We report high level ab initio calculations that characterize the triplet potential energy surface for the NH2 + H ↔ 3NH + H2 direct hydrogen abstraction reaction. Variational transition state theory calculations incorporate tunneling in the zero-curvature approximation through the ground state vibrationally adiabatic potential energy curve and incorporate anharmonic contributions to the vibrational partition functions through an independent normal mode approximation. The general agreement between the calculated and measured high-temperature rates for both the forward and reverse reactions indicate this reaction proceeds as a direct abstraction under these conditions, contrary to the predictions of other recent ab initio calculations.;Finally, we present a comprehensive ab initio study of small neutral and cationic boron oxides (BnOm, BnO m+, n 4, m 5) motivated by a need for more accurate and reliable structural and thermodynamic information. Theoretical dissociation energy thresholds for the cations are then compared with dissociation thresholds generated from recent collision-induced dissociation experiments. The theoretical and experimental results are largely consistent, although there are some unresolved issues.