title: Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions.
abstract: We examine the photoelectron spectra and laboratory-frame angular distributions in the photodetachment of furanide (C4H3O(-)), thiophenide (C4H3S(-)), and thiazolide (C3H2NS(-)) and compare the results to the previously reported studies of pyridinide (C5H4N(-)) and oxazolide (C3H2NO(-)). Using the mixed s-p model for the angular distributions, the results are interpreted in terms of the effective fractional p character of the highest-occupied molecular orbitals of these heterocyclic anions, revealing trends related to the aromaticity. We conclude that aromatic stabilization across a series of systems may be tracked using the photoelectron angular distributions. In addition, we report an improved (higher-precision) electron affinity (EA) for the thiophenyl radical, EA((•)C4H3S) = 2.089(8) eV. The EA of thiazolyl falls within the 2.5(1) eV range, but it is not clear if this determination corresponds to the 2- or 5-cyclic species or the 2-ring-open isomer. These results are analyzed in conjunction with the properties of other heterocyclic radicals (pyridinyl, furanyl, and oxazolyl) and interpreted in terms of the C-H bond dissociation energies (BDEs) of the corresponding closed-shell molecules. The BDEs of all five-membered-ring heterocyclics studied fall within the 116-120 kcal/mol range, contrasting the lower BDE = 110.4(2.0) kcal/mol of the more aromatic six-membered-ring pyridine. The observed aromaticity trends are consistent with the findings derived from the anion photoelectron angular distributions.
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Lori M
authors.initials: LM
authors.lastname: Culberson
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Blackstone
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Adam A
authors.initials: AA
authors.lastname: Wallace
authors.affiliation: Department of Chemistry and Biochemistry, University of Arizona , Tucson, Arizona 85721, United States.
authors.forename: Andrei
authors.initials: A
authors.lastname: Sanov