Date of Award

Spring 1-1-2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

Veronica M. Bierbaum

Second Advisor

W. Carl Lineberger

Third Advisor

G. Barney Ellison

Abstract

Numerous experimental and theoretical studies have probed reactions using gas phase ion chemistry to develop an intrinsic understanding of kinetics, mechanisms and structure-energy relationships. The fundamental knowledge gained from these studies allows predictive tools to be developed for understanding chemical systems (earth's atmosphere, biological, etc.). This thesis describes the reactive characteristics and competitive processes within several organic ion molecule reactions. An overview of the basic principles of gas phase ion-molecule reactions and the experimental methods employed in our studies are given in Chapters 1-2. In Chapter 3, comparisons of the reactivity and mechanistic pathways between the reactions of alkyl iodides with CN&hibar; in the gas phase and several solvents are reported. The mechanistic results are strikingly similar; however, a tighter gas phase transition state is suggested. In Chapter 4, competition between the substitution (SN2) and elimination (E2) reactions of alkyl iodides with Cl-, CN-, and HS- is evaluated. Contrary to previous studies, our results reveal competition between the SN2 and E2 pathways. Discussions cover reaction efficiencies, kinetic isotope effects, linear basicity-reactivity relationships, electrostatic models, and transition state looseness parameters. In Chapter 5, our investigation of the alpha-effect in the gas phase shows enhanced nucleophilicity for HOO- relative to "normal" alkoxides in three separate reaction series validating an intrinsic origin of the effect. Variations in electron affinities and bond strengths between the normal and alpha-anions indicate that HOO- has distinctive thermochemical properties. In Chapter 6, we apply Marcus theory in an attempt to resolve discrepancies between experimental and computational studies on the existence of the alpha-effect in SN2 reactions with CH3Cl. Marcus theory indicates that the intrinsic differences between normal and alpha-nucleophiles are small and can be easily masked by thermodynamic driving forces. In Chapter 7, we explore the intrinsic behavior of alpha-nucleophiles in competitive reaction mechanisms. The alpha-effect is not seen in the E2 mechanism, but is reported for nucleophilic attack at both sp22 and sp3 carbon sites. This is rationalized by "soft" base behavior. In Chapter 8, the reactivity of 1,3,5-triazine is investigated. Significant hydride acceptor properties are observed. Anion-arene binding modes and their influence on reaction pathways are discussed.

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Chemistry Commons

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