AmericanChemicalSociety.com

Despite the fact that aryl(trifluoromethyl)carbenes are among the most widely utilized reactive intermediates in photoaffinity labeling of biological systems, surprisingly few studies have probed their chemistry, reactivity, and electronic structures. We have found via DFT calculations, and confirmed by matrix isolation studies, that trifluoromethyl groups actually stabilize the singlet states of carbenes compared to H.  We attribute this unusual singlet stabilization to inductive withdrawal from the trifluoromethyl group, which leads to rehybridization at the adjacent carbene center.  Aryl(trifluoromethyl)carbenes thus have smaller S-T energy gaps than phenylcarbenes, and we have found that singlet or triplet multiplicities can be finely tuned through appropriate substitution. We have been able to generate a variety of substituted phenyl- (trifluoromethyl)carbenes in inert matrices at cryogenic temperatures, and have probed their spectroscopy and reactivity.  In particular, electron donating aryl substituents favor singlet state carbenes.  In addition, we have found that the electronic absorptions of the trifluoromethyldiazirine precursors are unusually dependent on their conformations.  This unanticipated geometry dependence can lead to uncharacteristic UV absorptions, or lack thereof.  These results may have particular relevance to the wide variety of trifluoromethyldiazirines that have been produced for photoaffinity labeling applications.