|Title||Ultraviolet Photoelectron Spectroscopy of Organic Anions|
|Year of Publication||2006|
|Academic Department||Department of Chemistry and Biochemistry|
|Number of Pages||202|
|University||University of Colorado|
Ultraviolet photoelectron spectroscopy has been used to study several negative organic ions and their corresponding neutrals: cyclopentadienide, pyrrolide, imidazolide, pyrazolide, vinyl diazomethyl anion, and 2-oxepinoxide. The photoelectron spectra obtained for these systems gives structural, vibrational, and electronic information about the corresponding neutral radicals.
Several of the ions listed above are related: cyclopentadienide, pyrrolide, imidazolide and pyrazolide. Each is a 5-membered ring system, with 0, 1, or 2 nitrogen atoms in the ring. The cyclopentadienyl radical is a well known radical exhibiting Jahn-Teller distortion, and these effects are evident in the photoelectron spectrum of cyclopentadienide. Replacement of one C—H group in cyclopentadienide by an N atom gives the pyrrolide ion. Jahn-Teller effects are no longer present in the radical counterpart, pyrrolyl; however, electronic interactions between the ground and first excited states are evident in the photoelectron spectrum. Imidazolide and pyrazolide are isomers of one another, and each has two N atoms in the ring. In imidazolide the N atoms are separated, while in pyrazolide they are adjacent. The corresponding radicals (imidazolyl and pyrazolyl) also show interesting electronic interactions between their ground and first excited states. In imidazolyl the interactions are weak, and the effects are not evident in the photoelectron spectrum. In pyrazolyl they are significantly stronger, and unexpected vibrational bands appear in the photoelectron spectrum as a result.
Also observed in the spectra of imidazolide and pyrazolide were photoelectron signals from other isomers. These isomers are nearly structurally identical to the imidazolide and pyrazolide ions, with the exception that a hydrogen atom is displaced from a carbon site to a nitrogen site. The photoelectron spectra of these isomers appear very different. Electronic interactions in the analogous radicals are not present, and the photoelectron spectra appear very Franck-Condon-like. Gas-phase acidities for these ions were also obtained through bracketing experiments. Combination of the electron affinities obtained from the photoelectron spectra with these gas-phase acidities allows for determination of a C—H bond dissociation enthalpy of the parent molecules imidazole and pyrazole.
Although not a ring system, the vinyldiazomethyl anion is an isomer of imidazolide and pyrazolide. It can be thought of as a ring opened pyrazolide ion (by breaking a C—N bond). Its photoelectron spectrum is rather simple, appearing very Franck-Condon-like. Interestingly, it appears that the spectrum originates mostly from the E-vinyldiazomethyl anion only; there is little indication of the presence of the Z-isomer. Franck-Condon simulations show each isomer would have a different spectral signature.
The 2-oxepinoxide anion is a seven-membered ring containing oxygen. The corresponding neutral, the 2-oxepinoxy radical, is a proposed intermediate in the combustion of benzene. The phenyl radical is expected to be formed first in a benzene flame, and many pathways for reaction of phenyl radical with O2 have been studied theoretically. The phenyl-peroxy radical is thought to isomerize into the 2-oxepinoxy radical as a first step to the formation of CO, CO2, and other combustion products. Production of the 2-oxepinoxide ion and subsequent photodetachment to form the 2-oxepinoxy radical is one of the first experimental steps to be taken to investigate these combustion pathways.