Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions

Lori M. Culberson, Christopher C. Blackstone, Adam A. Wallace, Andrei M Sanov

Research output: Contribution to journalArticle

8 Citations (Scopus)

Abstract

We examine the photoelectron spectra and laboratory-frame angular distributions in the photodetachment of furanide (C<inf>4</inf>H<inf>3</inf>O<sup>-</sup>), thiophenide (C<inf>4</inf>H<inf>3</inf>S<sup>-</sup>), and thiazolide (C<inf>3</inf>H<inf>2</inf>NS<sup>-</sup>) and compare the results to the previously reported studies of pyridinide (C<inf>5</inf>H<inf>4</inf>N<sup>-</sup>) and oxazolide (C<inf>3</inf>H<inf>2</inf>NO<sup>-</sup>). 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(<sup>•</sup>C<inf>4</inf>H<inf>3</inf>S) = 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.

Original languageEnglish (US)
Pages (from-to)9770-9777
Number of pages8
JournalJournal of Physical Chemistry A
Volume119
Issue number38
DOIs
StatePublished - Sep 24 2015

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Angular distribution
Photoelectrons
Electron affinity
Anions
photoelectrons
angular distribution
Stabilization
stabilization
electron affinity
anions
trends
Imaging techniques
dissociation
rings
photodetachment
Molecular orbitals
Isomers
energy
pyridines
molecular orbitals

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry

Cite this

Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions. / Culberson, Lori M.; Blackstone, Christopher C.; Wallace, Adam A.; Sanov, Andrei M.

In: Journal of Physical Chemistry A, Vol. 119, No. 38, 24.09.2015, p. 9770-9777.

Research output: Contribution to journalArticle

Culberson, Lori M. ; Blackstone, Christopher C. ; Wallace, Adam A. ; Sanov, Andrei M. / Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions. In: Journal of Physical Chemistry A. 2015 ; Vol. 119, No. 38. pp. 9770-9777.
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AB - 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.

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