Equilibrating parent aminomercaptocarbene and CO 2 with 2-amino-2-thioxoacetic acid via heavy-atom quantum tunneling.

The search for methods to bind CO 2 and use it synthetically as a C 1 -building block under mild conditions is an ongoing endeavor of great urgency. The formation of heterocyclic carbene-carbon dioxide adducts occurs rapidly when the carbene is generated in solution in the presence of CO 2 . Here we demonstrate the reversible formation of a complex of the hitherto unreported aminomercaptocarbene (H 2 N-C̈-SH) with CO 2 isolated in solid argon by photolysis of 2-amino-2-thioxoacetic acid. Remarkably, the complex disappears in the dark as deduced by time-dependent matrix infrared measurements, and equilibrates back to the covalently bound starting material. This kinetically excluded process below ca. 8 K is made possible through heavy-atom quantum mechanical tunneling, as also evident from density functional theory and ab initio computations at the CCSD(T)/cc-pVTZ level of theory. Our results provide insight into CO 2 activation using a carbene and emphasize the role of quantum mechanical tunneling in organic processes, even involving heavy atoms.