Degree Name

Master of Science (MS)

Semester of Degree Completion

2001

Thesis Director

Carol A. Deakyne

Abstract

This thesis describes two separate computational chemistry projects. The first project is a study of constant bond dissociation enthalpy ratios, and the second project is a study of the rearrangement and fragmentation processes on the singlet potential energy surface of the [H3, B, C, F]+ system.

G2 and G2(MP2) enthalpies have been calculated for isoelectronic AB2 systems with singlet-linear, singlet-nonlinear, triplet-linear, and triplet-nonlinear electronic states and geometries, and for isoelectronic AB systems with singlet and triplet electronic states. For these systems, A = B-, Al-, Ga-, C, Si, Ge, N+, P+, or As+, and B = O, S, or Se. The calculational results show that the ground states of both the AB2 and AB systems are singlets. The G2 and G2(MP2) procedures give essentially the same bond dissociation enthalpy ratios D0 m(AB2) I (2 D0 m(AB)). The bond dissociation enthalpy ratios follow the order: cation (average 0.66) < neutral (average 0.76) < anion (average 0.86). There is also good agreement between the calculated and experimental results for the neutral systems.

The singlet potential energy surface of the cation [H3, B, C, F]+ has been investigated in detail using MP2/aug-cc-pVDZ, QCISD/aug-cc-pVDZ and QCISD(T)/aug-cc-pVTZ methods. Ten minima have been located, including two hydrogen-bonded structures and six structures with a non-terminal fluorine atom. Some of these isomers may have double bond character. Most of the possible fragments have been examined using MP2/aug-cc-pVDZ and QCISD(T)/aug-cc-pVTZ methods, and the products CH3+ + BF were found to be most stable.

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