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

Near-Infrared Laser Induced Conformational Change of Alanine in Low-Temperature Matrixes and the Tunneling Lifetime of Its Conformer VI

The near- and mid-IR spectra of α-alanine isolated in low-temperature Ar, Kr, and N₂ matrixes were measured. Production of the short-lived conformer <b>VI</b> at the expense of the predominant conformer <b>I</b> was observed upon short irradiation with NIR laser light at the first O–H stretching overtone band of conformer <b>I</b>. Conformer <b>VI</b> decays by H-atom tunneling at 12 K with half-lives of 5.7 ± 1 s, 2.8 ± 1 s in Ar (two different sites), 7.0 ± 1 s in Kr, and 2.8 × 10³ ± 1.2 × 10³ s in N₂. Upon prolonged irradiation, conformer <b>I</b> slowly transformed into conformer <b>IIa</b>. On the basis of these irradiation experiments, the unambiguous vibrational assignments of conformers <b>I</b>, <b>IIa</b>, and <b>VI</b> are given. In contrast to similar experiments for glycine, the irradiation experiments did not lead to the formation of conformer <b>IIIb</b>. This is explained by a very low <b>IIIb</b> → <b>I</b> barrier height computed for alanine, which results in a very fast depletion of conformer <b>IIIb</b> even in low-temperature matrixes

Exploring the Conformational Space of Cysteine by Matrix Isolation Spectroscopy Combined with Near-Infrared Laser Induced Conformational Change

Six conformers of α-cysteine were identified by matrix isolation IR spectroscopy combined with NIR laser irradiation. Five of these conformers are identical with the five out of six conformers that have recently been identified by microwave spectroscopy. The sixth conformer observed in the present study is a short-lived conformer, which decays by H-atom tunneling; its half-life in a 12 K N₂ matrix is (1.1 ± 0.5) × 10³ s. This study proves that matrix isolation IR spectroscopy combined with NIR laser irradiation is a suitable method to identify conformers of a complex system for which computations predict several dozens of conformers, and that the reliability of this method for conformational assignment is comparable to that of microwave spectroscopy

Near-Infrared Laser-Induced Structural Changes of Glycine·Water Complexes in an Ar Matrix

The structures of glycine·H₂O complexes have been reinvestigated in low-temperature inert matrices. To go beyond the former matrix-isolation IR studies, NIR laser irradiation was used to change the relative abundances of the different complexes in the matrix. It is shown that the irradiation of the first overtone of the OH stretching mode of glycine as well as of the first overtone of the OH stretching mode of the water molecule in the complex can induce structural changes. Comparison of the experimental IR spectra with the IR spectra computed for different structures resulted in more reliable assignments of spectral patterns and identification of more structures than in former studies

Photochemical Formation of Diazenecarbaldehyde (HNNCHO) and Diazenecarbothialdehyde (HNNCHS) in Low-Temperature Matrices

The photochemical decomposition of 1,2,4-oxadiazole-3,5-diamine and 1,2,4-thiadiazole-3,5-diamine was investigated in low-temperature Ar and Kr matrixes at different wavelengths. The analysis of matrix-isolation infrared (MI-IR) spectra aided by high-level quantum chemical computations showed not only that these photochemical reactions yield [NH₂, C, N, X] (X = O, S) isomers but also that the bands of a novel, formerly unobserved species were observed. The comparison of computed IR spectra of potential products with the observed spectra suggests that these species are the diazenecarbaldehyde (HNNCHO) and diazenecarbothialdehyde (HNNCHS). Neither of the reactive HNNCHO and HNNCHS molecules was observed experimentally before. Both molecules are identified in the matrix as a complex with the other photoproduct, NH₂CN. Comparison of the present experiments with former photochemical experiments on 1,2,5-oxadiazole-3,4-diamine and 1,2,5-thiadiazole-3,4-diamine and the analysis of the rate of formation of the different photoproducts indicate that HNNCHO and HNNCHS are formed in a different reaction path than H₂NNCX and H₂NC­(NX) (X = O, S), and not by photoisomerization from these latter products