H-Tunneling Rotamerization in Glycine Imine

Quantum mechanical tunneling governs the chemical reactivity at cryogenic temperatures. Here we present the near-infrared (NIR) light-induced generation of a higher energy conformer of glycine imine and its H-tunneling CO bond rotamerization in solid argon (Ar), para-hydrogen (p-H₂), and dinitrogen (N₂) at cryogenic temperatures. The tunneling half-life for the CO bond rotamerization highly depends on the host matrix and is approximately 5 h in Ar and 18 h in both p-H₂ and N₂. Surprisingly, experiments in p-H₂ revealed a much longer half-life than in Ar, possibly due to the formation of a dinitrogen complex. Deuteration of the carboxylic acid group completely inhibits the CO bond D-tunneling rotamerization. We performed Wentzel - Kramer - Brillouin (WKB) and canonical variational transition state theory (CVT) calculations, incorporating multidimensional small curvature tunneling corrections (SCT), using the B3LYP/cc-pVTZ level of theory. The gas-phase tunneling half-lives are 11 h according to the one-dimensional WKB model and 1 h according to the multidimensional CVT/SCT model, both aligning well with our experimental results.