|Title||Negative Ion Photoelectron Spectroscopy of Alkyl Peroxides, Alkoxides, and Group VIII Transition Metal Oxides|
|Year of Publication||2001|
I employed negative ion photoelectron spectroscopy to investigate the structure and energetics of three groups of anions and their corresponding neutrals: alkyl peroxides ROO−, (R = H, D, CH3, CD3, and CH3CH2); alkoxides RO− (R=CH3, CD3, CH3CH2, CD3CD2, (CH3)2CH, and (CH3)3C); and Group VIII transition metal oxides XO− and OXO− (X = Ni, Pd, Pt). The peroxides and the alkoxides are of great interest to those who study atmospheric or combustion chemistry, while the metal oxides play an important role in catalysis reactions. However, each of these groups of molecules displays interesting behavior that is itself a motivation for their investigation.
The spectra of HOO− and DOO− are relatively straightforward to analyze and understand and provide a good basis from which to compare the larger alkyl peroxides. The ROO− spectra exhibit the normal Franck-Condon behavior leading to clear assignments of the expected vibrational progressions in both the ground and first excited state of the neutral species. Although the molecules increase in size from HOO to CH3CH2OO, many of the spectral characteristics such as electron affinity (EA) and prominence of the O-O stretch vibration do not appreciably change. The EA of HOO is revised, which becomes important as part of a newly revised thermochemistry of HOO and HOOH.
The RO− species exhibit an additional layer of complexity. Both the CH3O− and (CH3)3CO− molecules possess relatively high C3v symmetry about the CO axis as well as a doubly degenerate ground electronic state of the neutral RO molecule. Both of these elements are expected to produce a Jahn-Teller effect, where in order to break the molecular symmetry and electronic state degeneracy, the Born- Oppenheimer approximation breaks down and nuclear and electronic wavefunctions become coupled. The extent to which Jahn-Teller effects affect the RO molecule photoelectron spectra is discussed.
Although the transition metal monoxides are diatomics and thus perhaps presumed to be uncomplicated molecules, they are the most difficult to understand in this thesis and the most difficult to obtain in the laboratory. The d orbitals of the metals are closely spaced together, leading to congested spectra and mixing of the properties of orbitals that complicates analysis and ab initio calculations. Furthermore, the high nuclear charge of the atoms involved leads to non-negligible spin-orbit and other relativistic effects. Perhaps for these reasons there is relatively little information in the literature on these molecules. However, despite the complexities involved, comparison of all three metal molecules has allowed for a consistent interpretation of the spectra. Assignments are made including electron affinities, spin-orbit excited states of both anion and neutral XO molecules, other excited electronic states of anion and neutral, and vibrational frequencies and bond length changes. Analysis of the OXO molecules yields electron affinities, vibrational frequencies, and anion to neutral geometry changes.