Date of Award

1988

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Lucia M. Babcock

Abstract

Reactions of several dioxo- and dithio-ligand anions with BF$\sb3$ and SiF$\sb4$ have been carried out in the gas phase using the flowing afterglow technique. These reactions can be described by a generalized mechanism in which the anion first participates in a three-body addition step in which the coordination sphere of the Lewis acid is expanded. The close proximity of the second OH or SH group to the Lewis acid center presents the possibility of formation of a ring-closed species in an overall bimolecular substitution step accompanied by the ejection of neutral HF. Reactions of several of the oxo-ligand anions with BF$\sb3$ and SiF$\sb4$ were observed to yield this chelated species. Evidence indicates that those processes which form bimolecular products are occurring on a double-well potential energy surface in which the first well represents the monodentate species, and the second well represents the associated bimolecular products. The height of the barrier between the two potential minima is reflected in the branching ratio of the two product species. Branching ratios between the three-body product and the bimolecular product were measured in order to elucidate the nature of factors influencing ring-closure. Our results indicate that the geometry around the Lewis acid center and the size and flexibility of the ligand anion are factors which affect chelation. Inductive effects also appear to have a small effect on the branching ratios. In addition, calculations carried out on the simplest of these systems suggest that the mechanism of ring-closure proceeds via the formation of an intramolecular hydrogen bond between the OH proton and one of the fluorines on the Lewis acid. The reactions carried out for this research proceed at rates which are between 60-90% of the collision rates for these processes, and are invariant with respect to pressure over the pressure range accessible to us. The exception to this is the reaction of BF$\sb3$ with 1,2-benzendithiol. This reaction exhibits a definite pressure dependence. Furthermore, our results indicate the existence of a radiative stabilization pathway in this system, which yields the addition product.

Pages

394

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