Gábor Bazsó

Tautomers of cytosine and their excited electronic states: a matrix isolation spectroscopic and quantum chemical study

We have measured the IR and UV spectra of cytosine in a low-temperature argon matrix. An attempt was made to determine the tautomeric ratios existing in the matrix, making use of the matrix-isolation IR spectrum and computed IR intensities of the tautomers in a least squares fitting procedure. The mole fractions are about 0.22 for oxo(-amino) form, 0.26 and 0.44 for the two rotamers, respectively, of the hydroxy(-amino) form and 0.08 for the (oxo-)imino tautomer. These ratios were then used to simulate the matrix-isolation UV spectrum as a composite of the individual spectra, the latter calculated ab initio at high levels of electron correlation theory. The agreement between simulated and experimental UV spectra seems satisfactory. This indicates that, in contrast to the solid state and solution spectra described up to now by the oxo(-amino) form alone, the reproduction of the matrix-isolation UV spectrum needs at least the hydroxy(-amino) and oxo(-amino) forms, and probably also the (oxo-)imino form.

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

Conformer ttc/VIp of glycine and glycine-N,N,O-d(3) has been prepared in low-temperature Ar, Kr, Xe, and N(2) matrices by near-infrared (NIR) laser irradiation of the first OH stretching overtone of conformer ttt/Ip. Glycine (and glycine-N,N,O-d(3)) ttc/VIp was found to convert back to ttt/Ip in the dark by hydrogen-atom tunneling. The observed half-lives of ttc/VIp 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 cis-to-trans conversion of formic and acetic acid, the tunneling half-life of glycine ttc/VIp in a N(2) matrix is more than 3 orders of magnitude longer (6.69 × 10(3) and 1.38 × 10(4) 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 VI at the expense of the predominant conformer I was observed upon short irradiation with NIR laser light at the first O-H stretching overtone band of conformer I. Conformer VI 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(3) ± 1.2 × 10(3) s in N2. Upon prolonged irradiation, conformer I slowly transformed into conformer IIa. On the basis of these irradiation experiments, the unambiguous vibrational assignments of conformers I, IIa, and VI are given. In contrast to similar experiments for glycine, the irradiation experiments did not lead to the formation of conformer IIIb. This is explained by a very low IIIb → I barrier height computed for alanine, which results in a very fast depletion of conformer IIIb even in low-temperature matrixes.

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 trans (Z) and the cis (E) conformers but also the assignment of the spectral bands to different matrix sites. The tunneling decay of the higher-energy cis conformer prepared from both trans 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 cis → trans 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.

First Isolation and Spectroscopic Observation of Thiofulminic acid (HCNS)

For the first time: Thiofulminic acid (HCNS), the parent member of the nitrile sulfide family of reactive intermediates and potential interstellar species, was produced and characterized by IR spectroscopy for the first time. HCNS was generated in cryogenic matrices by 254 nm UV irradiation of 1,2,5-thiadiazole (see figure).

Identification of Serine Conformers by Matrix-Isolation IR Spectroscopy Aided by Near-Infrared Laser-Induced Conformational Change, 2D Correlation Analysis, and Quantum Mechanical Anharmonic Computations

The conformers of α-serine were investigated by matrix-isolation IR spectroscopy combined with NIR laser irradiation. This method, aided by 2D correlation analysis, enabled unambiguously grouping the spectral lines to individual conformers. On the basis of comparison of at least nine experimentally observed vibrational transitions of each conformer with empirically scaled (SQM) and anharmonic (GVPT2) computed IR spectra, six conformers were identified. In addition, the presence of at least one more conformer in Ar matrix was proved, and a short-lived conformer with a half-life of (3.7 ± 0.5) × 10(3) s in N2 matrix was generated by NIR irradiation. The analysis of the NIR laser-induced conversions revealed that the excitation of the stretching overtone of both the side chain and the carboxylic OH groups can effectively promote conformational changes, but remarkably different paths were observed for the two kinds of excitations.

Structure, Stability, and Generation of CH3CNS

The unstable acetonitrile N-sulfide molecule CH3CNS has been photolytically generated in inert solid argon matrix from 3,4-dimethyl-1,2,5-thiadiazole by 254-nm UV irradiation, and studied by ultraviolet spectroscopy and mid-infrared spectroscopy. The molecule is stable in the matrix to 254-nm UV irradiation, but decomposes to CH3CN and a sulfur atom when broad-band UV irradiation is used. Chemiluminescence due to S2 formation from triplet sulfur atoms was detected on warming the matrix to ∼20–25 K. The ground-state structure and potential uni- and bimolecular reactions of CH3CNS are investigated using B3LYP, CCSD(T), and MR-AQCC quantum-chemical methods. CH3CNS is demonstrated to be stable under isolated conditions at room temperature, i.e. in the dilute gas phase or in an inert solid matrix, but unstable owing to bimolecular reactions, i.e. in the condensed phase.

A matrix isolation and computational study of the [C, N, F, S] isomers

The potential energy surface (PES) of the [C, N, F, S] system was investigated by quantum chemical and experimental methods. Seven minima were located on the ground state PES by density functional and ab initio electronic structure calculations. Four of these isomers, FSCN, FSNC, FCNS and FNCS, have an acyclic structure, while the other three, FC(NS), FS(CN) and FN(SC), form a three-membered fluorine-substituted ring. Out of these seven theoretically predicted isomers, FCNS and FC(NS) were successfully prepared in low-temperature Ar and Kr matrices by photochemical methods. The identification of these species was based on experimental considerations as well as on comparison of their IR spectra to computed anharmonic vibrational frequencies and infrared intensities. The present paper describes not only the first generation of both FCNS and FC(NS) species, but also reports the first time that a substituted CNS ring has been experimentally identified.

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) × 10(3) 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.