Identification and Reactivity of -,--Dihydroxycarbene, a New [CHO] Intermediate.

We report the first preparation of the -,- conformer of dihydroxycarbene () by means of pyrolysis of oxalic acid, isolation of the lower-energy -,- () and -,- () product conformers at cryogenic temperatures in a N matrix, and subsequent narrow-band near-infrared (NIR) laser excitation to give . Carbene converts quickly to via quantum-mechanical tunneling with an effective half-life of 22 min at 3 K. The potential energy surface features around were pinpointed by convergent focal point analysis targeting the AE-CCSDT(Q)/CBS level of electronic structure theory. Computations of the tunneling kinetics confirm the time scale of the → rotamerization and suggest that direct → H + CO decomposition may also be a minor pathway. The intriguing latter possibility cannot be confirmed spectroscopically, but hints of it may be present in the measured kinetic profiles.