Identification and Reactivity of s - cis , s - cis -Dihydroxycarbene, a New [CH 2 O 2 ] Intermediate.

We report the first preparation of the s - cis , s - cis conformer of dihydroxycarbene ( 1 cc ) by means of pyrolysis of oxalic acid, isolation of the lower-energy s - trans , s - trans ( 1 tt ) and s - cis , s - trans ( 1 ct ) product conformers at cryogenic temperatures in a N ₂ matrix, and subsequent narrow-band near-infrared (NIR) laser excitation to give 1 cc . Carbene 1 cc converts quickly to 1 ct via quantum-mechanical tunneling with an effective half-life of 22 min at 3 K. The potential energy surface features around 1 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 1 cc → 1 ct rotamerization and suggest that direct 1 cc → H ₂ + CO ₂ 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.