Photoelectron Spectroscopy of Alkyl and Alkenyl Free Radicals

Author: Schultz, Jocelyn Chupka

Year: 1984

Degree: Dissertation (Ph.D.)

Advisor: Janda, Kenneth C.

Committee Members: Beauchamp, Jesse L.; Roberts, John D.; Sparks, Randal K.; Janda, Kenneth C.

Option: Chemistry

DOI: 10.7907/r2r1-k971

Abstract

The first bands of the photoelectron spectra of simple alkyl and alkenyl free radicals, produced by pyrolysis of organic nitrites, are presented. Thermochemical and structural information about the corresponding carbocations in the gas phase are obtained from interpreting the spectra. In particular, adiabatic ionization potentials are combined with radical heats of formation to obtain heats of formation of the cations. The photoelectron band shapes are discussed in terms of structural changes occurring upon ionization. The focus of this work is· the study of isomeric radicals. Their photoelectron spectra allow the characterization of a series of carbocations with well-defined structures, namely those which closely resemble the radicals, even when the cations can rearrange with little or no activation energy to more stable forms. Radical thermal rearrangement and decomposition pathways are also studied.

Chapter I provides a brief introduction to the field of photoelectron spectroscopy of transient species, including a compilation of photoelectron data of transient species, and an introduction to the specific work presented in this thesis. Chapter II presents the photoelectron spectra of the C4H7 radicals 1-methylallyl, 2-methylallyl, allylcarbinyl and cyclobutyl radical, which provide information about the corresponding C4H+7 carbocations in the gas phase. These cations are of longstanding interest in the cyclopropylcarbinyl, cyclobutyl and allylcarbinyl interconversions observed in solvolysis reactions. The photoelectron spectra of 1-propyl, 1-butyl, isobutyl, neopentyl and 2-butyl radicals are presented in Chapter III. The corresponding primary cations are of particular interest because they are often postulated as intermediates or transition states in gas-phase and solution isomerization processes.

Chapter IV describes the application of photoelectron spectroscopy to the study of low-pressure gas-surface reactions. Organic radicals, as well as stable products, have been detected directly after exiting the catalyst bed. Allylic hydrogen abstraction by heated bismuth oxide catalyst forms gas-phase allyl and 2-methylallyl radicals from propylene and isobutylene in the presence of oxygen. Mechanistic information is obtained by varying the catalyst temperature and reactant partial pressures.

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