A nanosecond laser flash photolysis study on substituent effects and carbocation reactivity in heterogeneous and homogeneous media.

摘要

The first project of this thesis investigates the effects of various aryl substituents on the reactivity of zeolite-encapsulated carbocations. In this work, a series of diarylmethyl carbocations were generated within the cavities of non-protic alkali metal cation-exchanged zeolites through simple one laser (266 nm) or two-laser irradiation (266 nm/308 nm) of the corresponding diarylacetic acid precursors. Rapid loss of CO2, followed by photochemical oxidation of the diarylmethyl radical yielded the corresponding carbocations. The reactivity of the diarylmethyl carbocations was found to be highly dependant on the alkali metal counterion and on the Si/Al ratio of the zeolite framework. Hammett-type plots constructed for the decay of the diarylmethyl carbocations in non-protic zeolites resulted in rho + values that were substantially more shallow than the corresponding values in solution (1:2 CH3CN/H2O, TFE). The effects of the aryl substituents were also investigated at low temperatures, where rho + was found to increase with decreasing temperature of the intrazeolite environment. Examination of the low temperature data through the use of Eyring, enthalpy-entropy compensation and Exner plots, as well as statistical calculations concluded that an isokinetic relationship existed and was responsible for the leveling effect on carbocation reactivity in non-protic zeolites. Isokinetic temperatures for the decay of zeolite-encapsulated diarylmethyl carbocations were found to be between 363 and 518 K, significantly lower than the isokinetic temperatures for the reaction of the same diarylmethyl carbocations in solution (> 1000 K). Furthermore, the low IKT temperatures for the decay of the diarylmethyl carbocations in alkali metal cation-exchanged X and Y zeolites were attributed to the formation of a framework-bound alkoxy adduct during the decay of these carbocations when confined within the zeolite framework.;In the second project, a series of alpha-vinyl arylmethyl cations were generated and studied using nanosecond laser flash photolysis. Rate constants for the decay of the substituted alpha-vinyl arylmethyl cations were determined in pure solvents and solvent mixtures of HFIP and TFE. The absolute reactivity of the carbocations with added nucleophiles was obtained. The reactivities of the alpha-vinyl arylmethyl cations were then compared to the reactivities of the corresponding alpha-methyl, alpha-phenyl, and alpha-cyclopropyl arylmethyl carbocations as a function of substituent.