Tunneling Lifetime of the <i>ttc</i>/VIp Conformer of Glycine in Low-Temperature Matrices

Conformer <i><b>ttc</b></i>/<b>VIp</b> of glycine and glycine-N,N,O-<i>d</i>₃ has been prepared in low-temperature Ar, Kr, Xe, and N₂ matrices by near-infrared (NIR) laser irradiation of the first OH stretching overtone of conformer <i><b>ttt</b></i>/<b>Ip</b>. Glycine (and glycine-N,N,O-<i>d</i>₃) <i><b>ttc</b></i>/<b>VIp</b> was found to convert back to <i><b>ttt</b></i>/<b>Ip</b> in the dark by hydrogen-atom tunneling. The observed half-lives of <i><b>ttc</b></i>/<b>VIp</b> in Ar, Kr, and Xe matrices at 12 K were 4.4 ± 1 s (50.0 ± 1 h), 4.0 ± 1 s (48.0 ± 1 h), and 2.8 ± 1 s (99.3 ± 2 h), respectively. In correspondence with the observation for the <i>cis</i>-to-<i>trans</i> conversion of formic and acetic acid, the tunneling half-life of glycine <i><b>ttc</b></i>/<b>VIp</b> in a N₂ matrix is more than 3 orders of magnitude longer (6.69 × 10³ and 1.38 × 10⁴ s for two different sites) than in noble gas matrices due to complex formation with the host molecules. The present results are important to understand the lack of experimental observation of some computationally predicted conformers of glycine and other amino acids