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 Radiation Induced Conformational Change and Hydrogen Atom Tunneling of 2-Chloropropionic Acid in Low-Temperature Ar Matrix

Former assignments of the matrix-isolation infrared (MI-IR) spectrum of 2-chloropropionic acid are revised with the help of near-infrared (NIR) laser irradiation induced change in conformer ratios. This method allows not only the unambiguous assignment of each band in the MI-IR spectrum to the two <i>trans</i> (<i>Z</i>) and the <i>cis</i> (<i>E</i>) conformers but also the assignment of the spectral bands to different matrix sites. The tunneling decay of the higher-energy <i>cis</i> conformer prepared from both <i>trans</i> conformers in different sites is also investigated. It is shown that the tunneling decay time is very sensitive to the matrix site, especially if the in situ prepared high-energy conformer has a strained geometry in the matrix cage. The analysis shows that the kinetics of some <i>cis</i> → <i>trans</i> back conversion processes cannot be fitted by a single exponential decay. The possible reasons of this observation are examined and discussed. The present and former results clearly show that, in addition to tunneling processes, the decay rates strongly depend on solid-state effects. Therefore, simple theoretical predictions of decay rates, which do not take into account the solid-state effects, can only be compared to experimental observations only if experimentally proven that these effects do not significantly affect the experimentally measured tunneling rates

Matrix Isolation and Computational Study of the [H, C, N, Se] Isomers

Nine minima on the ground-state singlet and ten minima on the lowest-energy triplet potential energy surfaces of the [H, C, N, Se] system were located at the B3LYP/aug-cc-pVTZ level of theory. The singlet isomers were further investigated by the higher-level CCSD­(T) method. Besides their structure and relative energies, isomerization barriers and the dissociation energies of the most important fragmentation channels were determined. Anharmonic vibrational wavenumbers, infrared intensities, relative Raman intensities, and UV excitation energies were also computed to assist the detection of these species. Two of the singlet isomers were generated and investigated by IR and UV spectroscopic methods. First, HNCSe and its deuterated isotopomer, DNCSe, were prepared by the reaction of HBr/DBr with AgNCSe and deposited in an 8 K Ar matrix. Photolysis of (H/D)­NCSe at 254 nm led to the formation of the novel (H/D)­SeNC isomer, which decomposed upon broad-band UV irradiation

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