Publication TypeJournal Article
Year of Publication2010
AuthorsGerbig, D, Reisenauer, HP, Wu, C-H, Ley, D, Allen, WD, Schreiner, PR
JournalJournal of the American Chemical Society
Date PublishedJun 2
ISBN Number0002-7863
Accession NumberISI:000278190600020
Keywordscarbon, correlated molecular calculations, coupled-cluster method, density functionals, energy gradients, gaussian-basis sets, ground-state, rearrangements, tunneling rate, wave-functions

Phenylhydroxycarbene (Ph-C-OH, 1), the parent of all arylhydroxycarbenes, was generated by high-vacuum flash pyrolysis of phenylglyoxylic acid at 600 degrees C and spectroscopically characterized (IR, UV-vis) via immediate matrix isolation in solid Ar at 11 K. The identity of 1 was unequivocally confirmed by the precise agreement between the observed IR bands and (unscaled) anharmonic vibrational frequencies computed from a CCSD(T)/cc-pVDZ quartic force field. The UV-vis spectrum of 1 displays a broad band with maximum absorption at 500 +/- 25 nm (2.5 +/- 0.1 eV) that extends to similar to 640 nm (1.9 eV), in full accord with combined CCSD(T)/cc-pVQZ and EOM-CCSD/cc-pVTZ computations that yield a gas-phase vertical (adiabatic) excitation energy of 2.7 (1.9) eV. Unlike singlet phenylchlorocarbene, 1 does not undergo photochemical ring expansion. Instead, 1 exhibits quantum-mechanical hydrogen tunneling to benzaldehyde underneath a formidable barrier of 28.8 kcal mol(-1), even at cryogenic temperatures. The remarkable hydrogen tunneling mechanism is supported by the temperature insensitivity of the observed half-life (2.5 h) and substantiated by a comparable theoretical half-life (3.3 h) determined from high-level barrier penetration integrals computed along the intrinsic reaction path. As expected, deuteration turns off the tunneling mechanism, so d-1 is stable under otherwise identical conditions.

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