15 research outputs found
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><sub>3</sub> has been
prepared in low-temperature Ar, Kr, Xe, and N<sub>2</sub> 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><sub>3</sub>) <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 N<sub>2</sub> matrix is more than 3 orders of magnitude longer (6.69 ×
10<sup>3</sup> and 1.38 × 10<sup>4</sup> 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
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 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
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 [NH<sub>2</sub>, 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, NH<sub>2</sub>CN.
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 H<sub>2</sub>NNCX and H<sub>2</sub>NC(NX) (X = O, S), and
not by photoisomerization from these latter products
Experimental Evidence of Long-Range Intramolecular Vibrational Energy Redistribution through Eight Covalent Bonds: NIR Irradiation Induced Conformational Transformation of <i>E</i>‑Glutaconic Acid
Long-range
intramolecular vibrational energy redistribution (IVR)
driven conformational changes were investigated in a matrix-isolated
open-chain, asymmetrical dicarboxylic acid, <i>E</i>-glutaconic
acid. Although the analysis was challenging due to the presence of
multiple backbone conformers and short lifetimes of the prepared higher
energy <i>cis</i> conformers, it was shown that the selective
excitation of the O–H stretching overtone of one of the carboxylic
groups can induce the conformational change (<i>trans</i> to <i>cis</i>) of the other carboxylic group, located
at the other end of the <i>E</i>-glutaconic acid molecule.
This is a direct proof that the IVR process can act through eight
covalent bonds in a flexible molecule before the excess energy completely
dissipates into the matrix. The lifetime of the prepared higher energy
conformers (averaged over the different backbones) was measured to
be 12 s
Photolysis of Dimethylcarbamoyl Azide in an Argon Matrix: Spectroscopic Identification of Dimethylamino Isocyanate and 1,1‑Dimethyldiazene
The
UV photolysis of dimethylcarbamoyl azide has been investigated
in an argon matrix at cryogenic temperatures. The products of the
photolysis were identified by infrared spectroscopy supported by quantum-chemical
calculations. Sequential formation of dimethylamino isocyanate (Me<sub>2</sub>N–NCO), 1,1-dimethyldiazene (Me<sub>2</sub>NN),
and ethane was established. Therefore, the major decomposition channel
is identified as Me<sub>2</sub>NC(O)N<sub>3</sub> → Me<sub>2</sub>N–NCO → Me<sub>2</sub>NN → Me–Me,
via consecutive N<sub>2</sub>, CO, and N<sub>2</sub> eliminations.
Ground-state geometries, vibrational frequencies, IR intensities,
and UV excitation energies of the transient dimethylamino isocyanate
and 1,1-dimethyldiazene have been computed using the B3LYP and SAC-CI
methods and the aug-cc-pVTZ basis set
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 N<sub>2</sub> 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 × 10<sup>3</sup> ± 1.2 × 10<sup>3</sup> s in N<sub>2</sub>. 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 N<sub>2</sub> matrix is (1.1 ± 0.5) × 10<sup>3</sup> 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·H<sub>2</sub>O 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
Melting of Hydrogen Bonds in Uracil Derivatives Probed by Infrared Spectroscopy and ab Initio Molecular Dynamics
The thermal response of hydrogen bonds is a crucial aspect
in the
self-assembly of molecular nanostructures. To gain a detailed understanding
of their response, we investigated infrared spectra of monomers and
hydrogen-bonded dimers of two uracil-derivative molecules, supported
by density functional theory calculations. Matrix isolation spectra
of monomers, temperature dependence in the solid state, and ab initio
molecular dynamics calculations give a comprehensive picture about
the dimer structure and dynamics of such systems as well as a proper
assignment of hydrogen-bond affected bands. The evolution of the hydrogen
bond melting is followed by calculating the CO···H–N
distance distributions at different temperatures. The result of this
calculation yields excellent agreement with the H-bond melting temperature
observed by experiment