H-Tunneling Rotamerization in Glycine Imine

Quantum mechanical tunneling governs 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, 18 h in p-H2 and N2. Surprisingly, experiments in p-H2 revealed a much longer half-life than in Ar, indicating the formation of a dinitrogen complex after photolysis of the azide precursor. Deuteration of the carboxylic acid group completely inhibits the CO bond D-tunneling rotamerization. We conducted Wentzel−Kramers−Brillouin (WKB) and canonical variational transition state theory in combination with multidimensional small curvature tunneling corrections (CVT/SCT) tunneling calculations at the B3LYP/cc-pVTZ level of theory. The gas-phase tunneling half-life is 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.