DYNAMICS OF GAS-PHASE ION-MOLECULE REACTION

Gas-phase ion-molecule reactions are currently making significant contributions in many areas of chemistry. In this thesis we explore the application of ion cyclotron resonance (ICR) spectrometry to several different types of chemical problems.;A general model for ion-molecule reaction potential surfaces and a rate-equilibrium relation, the Marcus equation, have been used in conjunction with experimentally measured rate constants to propose a model for intrinsic nucleophilicities and leaving group abilities in gas-phase S(,N)2 displacement reactions of anions at methyl centers. This treatment has several advantages over past efforts. Since we study the reactions in the gas phase, we can focus on intrinsic properties of reactants and transition states with no interference from solvent effects. Also, application of the Marcus equation allows separation of kinetic and thermodynamic contributions to the reactions' energy barriers. This leads to our definition of intrinsic nucleophilicity for a species X('-) in terms of the energy barrier to the degenerate reaction X('-) + CH(,3)X (--->) XCH(,3) + X('-). Within our model, nucleophilicity and leaving group ability become equivalent, thus eliminating any need to distinguish between these "modes" of reactivity. Finally, we observe an interesting correlation between intrinsic nucleophilicity (a purely kinetic property) and methyl cation affinity (a purely thermodynamic property). The implications of this model and correlation are discussed in terms of reactivity and transition state structure.;Phenylnitrene radical anion is an interesting and previously unknown species which was discovered in our laboratory during the course of this work. We have made some preliminary investigations into the gas-phase chemistry of phenylnitrene radical anion, resulting in some novel free-radical displacements in reactions with carbonyl compounds which are unprecedented in negative ion-molecule chemistry. Mechanistic ramifications of these reactions are discussed.;An excellent example of the utility of gas-phase ion-molecule methods in thermochemical studies is found in our work with azide ion. By coupling our measurement of this species' proton affinity with other thermochemical data from our laboratory, we have been able to deduce what we believe are reliable values for the heats of formation of gaseous azide ion and azide radical, and the hydrogen-nitrogen bond dissociation enthalpy in hydrazoic acid. Our values are in sharp disagreement with current literature values which have heretofore all been obtained by indirect methods involving thermochemical cycles. To our knowledge, our work represents the first direct determination of these quantities.