Direct Spectroscopic Observation of Intramolecular Hydrogen Shifts in Carbenes

Article abstract of DOI:10.1021/ja00237a008

Triplet o-tolylmethylene (13a) decays thermally to singlet o-xylylene (14a) in an argon matrix at temperatures as low as 4.6 K.The thermal, intramolecular <1,4>-hydrogen shift has been observed directly by IR and UV spectroscopy.The carbene disappearance kinetics follow the standard (time)^1/2 dependence, due to multiple reaction sites in the matrix.The small temperature dependence and non-Arrhenius behavior of the rate implicate a tunneling mechanism.In contrast, triplet 1-phenylethylidene (17a) is thermally stable in argon or xenon matrices at 10 K.Warming 17a to 65 K in a xenon matrix produces styrene (18a).The intramolecular <1,2>-hydrogen shift has been observed directly by IR spectroscopy.The carbene disappearance kinetics show apparent first-order behavior.We estimate an upper limit of ca. 4.7 kcal/mol for the singlet-triplet energy gap in 17a.