Identification and Reactivity of <i>s</i>-<i>cis</i>,<i>s</i>-<i>cis</i>-Dihydroxycarbene, a New [CH₂O₂] Intermediate

Abstract

We report the first preparation of the <i>s</i>-<i>cis</i>,<i>s</i>-<i>cis</i> conformer of dihydroxycarbene (<b>1</b><i>cc</i>) by means of pyrolysis of oxalic acid, isolation of the lower-energy <i>s</i>-<i>trans</i>,<i>s</i>-<i>trans</i> (<b>1</b><i>tt</i>) and <i>s</i>-<i>cis</i>,<i>s</i>-<i>trans</i> (<b>1</b><i>ct</i>) product conformers at cryogenic temperatures in a N₂ matrix, and subsequent narrow-band near-infrared (NIR) laser excitation to give <b>1</b><i>cc</i>. Carbene <b>1</b><i>cc</i> converts quickly to <b>1</b><i>ct</i> via quantum-mechanical tunneling with an effective half-life of 22 min at 3 K. The potential energy surface features around <b>1</b> 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 <b>1</b><i>cc</i> → <b>1</b><i>ct</i> rotamerization and suggest that direct <b>1</b><i>cc</i> → H₂ + CO₂ 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.