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Gas-Phase Negative Ion Chemistry: Photoelectron Spectroscopy, Reactivity, and Thermochemical Studies

TitleGas-Phase Negative Ion Chemistry: Photoelectron Spectroscopy, Reactivity, and Thermochemical Studies
Publication TypeThesis
Year of Publication2008
AuthorsVillano, SM

This thesis describes the reactive and spectroscopic characterization of several anions. An overview of gas-phase ion-molecule reactivity and photoelectron spectroscopy is given in Chapters I-IV.

In Chapter V the competition between nucleophilic substitution and base-induced elimination for the reactions of BrO and ClO with a series of alkyl chlorides is investigated. These results are in stark contrast to prior theoretical work, and they indicate that the elimination channel becomes the dominant pathway as the neutral reagent becomes more sterically hindered. Additionally, the highly debated α-nucleophilicity of these two anions and of HO2 is examined; no enhanced reactivity is displayed suggesting that the α-effect is not due to an intrinsic property of the anion.

The reactivity of simple halogen substituted carbene anions is investigated with a series of oxygen, sulfur, and halogen containing neutral reagents in Chapter VI. These carbanions display diverse reactivity that includes substitution, elimination, proton transfer, and electron transfer reactions. In addition to these classic transformations, isotopic labeling studies indicate that these carbanions can undergo an insertion-elimination mechanism. Such reactivity has been observed in neutral carbenes; however, this is the first report of such gas-phase anion reactivity.

Spectroscopic studies of SF6 and c-C4F8- are reported in Chapter VII. The photoelectron spectra of both anions are consistent with a large geometry change
between anionic and neutral species. The photoelectron spectrum of c-C4F8 is composed of combination bands of two symmetric ring-breathing modes. The photoelectron spectrum of SF6 is dominated by an extended progression in the S−F stretching mode as well as a strong overtone/combination progression.

In Chapter VIII we report the photoelectron spectra of the N-methyl-5-pyrazolide and N-methyl-5-imidazolide anions, which provide electron affinities for the corresponding radicals. The gas-phase acidities of N-methylpyrazole and N-methyimidazole are also measured, and these measurements are used to determine the C-H bond strengths.

The thermochemistry of peroxyformic acid and peroxyacetic acid is investigated in Chapter IX. The electron affinities of the peroxyformyl and peroxyacetyl radicals are determined from the photoelectron spectra of the corresponding anion. The gas-phase acidities of both acids are determined, and these measurements are used to determine the O-H bond strengths.