Photochemical Investigations of Ions in the Gas Phase Utilizing Ion Cyclotron Resonance Spectroscopy

Citation

Freiser, Ben Sherman (1977) Photochemical Investigations of Ions in the Gas Phase Utilizing Ion Cyclotron Resonance Spectroscopy. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HMVE-Q508. https://resolver.caltech.edu/CaltechETD:etd-02242009-110155

Abstract

The photochemistry and photophysical processes of a broad range of ions have been studied in the gas phase using ion cyclotron resonance techniques. A discussion summarizing much of this work, and two papers on related topics are presented. In paper I, "Acid-Base Properties of Molecules in Excited Electronic States Utilizing Ion Cyclotron Resonance Spectroscopy", a general method is described for obtaining excited state acid-base properties of molecules and ions in the gas Phase which utilizes ion cyclotron resonance spectroscopy for studying photochemical processes involving ions. These processes, including photodissociation and photodetachment, yield in favorable instances, electronic excitation energies of ions. A comparison of the excitation energies of a base B with the corresponding acid-base complex AB yields the excited state basicity of B. Similarly, a comparison of the excitation energies of a chromophoric acid A with the complex AB yields information about the excited state acidity of A. Studies of the first type are described using the reference acids H[superscript +] and Li[superscript +] with the bases C6H5X (X = H, CN, NH2, CHO, COCH3, NO2, OCH3, O[superscript -], and S[superscript -) pyridine, and ferrocene. In several instances photodissociation spectra of solvated acid-base complexes of the type BLib[with a + above the L] have been obtained and analyzed to determine the effects of further solvation on the excitation spectra of these complexes. A comparison of the gas phase excitation spectra of a number of ions to their solution absorption is made. Studies of the second type (excited state acidities) are described using the reference base H[superscript -] with the acids C6H5CO[superscript +] and C6H5CHOH[superscript +]. Calculated changes in acidity and basicity are used to infer changes in electron distributions and dipole moments for excited states, and yield insight into the types of transitions involved. In particular, these studies are used to assess the controversial role played by intra-molecular charge transfer in the lowest two singlet π→π* transitions of monosubstituted benzenes. These results are compared with findings from related experiments and calculations when available. Paper II, entitled "Electron Impact Dissociation of Cyanobenzene Radical Cations by Ion Cyclotron Resonance Spectroscopy", describes using trapped ion cyclotron resonance spectroscopy for the first time to study the electron impact dissociation of ions. Fragmentation of C6H5CN[superscript +] to produce C6H4[superscript +] and HCN is observed to occur at low electron energies (3-9 eV). The extent of dissociation is observed to be linear in emission current, rising from a threshold at 3.0 ± 0.5 eV to a maximum cross section estimated to be 6 Å[superscript 2] at 7.5 ± 0.5 eV. The implications of these results are discussed.

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