Carbene Spin Multiplicity in Solution Probed Using Time-Resolved EPR Spectroscopy

Authors

  • Jin Wang The Ohio State University
  • Natalia V. Lebedeva The University of North Carolina
  • Matthew S. Platz The Ohio State University
  • Malcolm D. E. Forbes The University of North Carolina

DOI:

https://doi.org/10.29356/jmcs.v53i3.999

Keywords:

Time-resolved EPR Spectroscopy, Free Radicals, Carbene Spin Multiplicity, Laser Flash Photolysis

Abstract

Time-resolved (CW) EPR spectroscopy at X-band has been used to detect free radicals created by H-atom abstraction reactions of photochemically generated carbenes in n-hexadecane solutions at room temperature. Three different carbenes were studied, formed via loss of nitrogen after photo-excitation of a substituted biphenyl diazo precursor. The TREPR signals from two different alkyl radicals and the benzylic-type radical from the carbene are detected for two of the three diazo substrates. The radicals were unambiguously identified by spectral simulation using literature parameters (g-factors and hyperfine coupling constants). Previous transient optical absorption spectroscopy experiments have concluded that these two carbenes have triplet ground states, which is confirmed by the low field emissive, high field absorptive phase of the TREPR signals reported here. The spectra are strongly spin polarized by the Radical Pair Mechanism of CIDEP, and their overall intensities track with the measured quantum yields for carbene production. A third substrate, known to react via a concerted Wolff reaction or excited state rearrangement via a singlet state, showed no TREPR signal, as expected. Reactions run in acetonitrile, where the spin multiplicity is reversed to give a singlet ground state, did not lead to detectable TREPR signals for all three substrates. The observations are consistent with previously published laser flash photolysis results.

 

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Author Biographies

Jin Wang, The Ohio State University

Department of Chemistry

Natalia V. Lebedeva, The University of North Carolina

Department of Chemistry

Matthew S. Platz, The Ohio State University

Department of Chemistry

Malcolm D. E. Forbes, The University of North Carolina

Department of Chemistry

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Published

2019-06-24

Issue

Vol. 53 No. 3 (2009): Special Issue of the J. Mex. Chem. Soc. in Honor of Dr. Ernest L. Eliel (1921-2008)

Section

Regular Articles

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