Browsing by Author "Tarczay, Gyorgy"

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Hydrogen-Atom-Assisted Tautomerization on Solid Surfaces-The Case Study of Thioacetamide
(American Chemical Society, 2024) Góbi, Sándor; Reva, Igor; Ragupathy, Gopi; FAUSTO, RUI; Tarczay, Gyorgy
Amorphous thioacetamide (TA) ice was prepared by deposition on a low-temperature substrate in an ultrahigh-vacuum simulation chamber, exposing the sample to a beam of hydrogen atoms. The structural changes were monitored by reflection-absorption FT-IR spectroscopy. The spectral data unambiguously evidence the formation of the higher-energy thiol tautomeric forms of TA upon interaction with H atoms, resulting from the more stable thione tautomer, which is otherwise exclusively present in unprocessed ice. The regeneration of the thione species from the thiol tautomeric forms occurs upon heating the sample above 100 K. Quantum-chemical computations were conducted to confirm the conclusions drawn from the experimental results. According to the theoretical findings, the thiol molecules, formed on the surface of amorphous ice upon H-atom-assisted thione -> thiol tautomerization, cannot survive in the crystalline material. A mechanism for the process at the molecular level is also proposed. This work provides the first example of H-atom-assisted tautomerization occurring on the surface of amorphous ice without the action of UV light as the source of energy.
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Near-infrared in situ generation of the higher-energy trans conformer of tribromoacetic acid: Observation of a large-scale matrix-site changing mediated by conformational conversion
(Amer Inst Physics, 1305 Walt Whitman Rd, Ste 300, Melville, Ny 11747-4501 USA, 2018-01-28) Apostolo, Rui F. G.; Bazso, Gabor; Tarczay, Gyorgy; Fausto, Rui; ILDIZ, GÜLCE ÖĞRÜÇ; 107326
The first observation of the higher-energy conformer of tribromoacetic acid (trans-TBAA) is reported. The conformer was produced in cryogenic matrices (Ar, Kr, and N-2) by in situ selective narrowband near-infrared excitation of the lower-energy cis-TBAA conformer and characterized both structurally and vibrationally. The novel trans-TBAA conformer is shown to spontaneously decay to the most stable cis-TBAA form in all studied matrix media, by tunneling, and the measured decay rates in the different matrices were compared with those of the trans conformers of other carboxylic acids in similar experimental conditions. In the N-2 matrix, where trans-TBAA establishes a specific stabilizing intermolecular interaction with the host N-2 molecules via its OH group and is about 11 times more stable than in rare gas matrices, the effect of changing the irradiation wavenumber within the 2 nu OH absorption profile was investigated in detail. An interesting phenomenon of matrix-site changing mediated by conformational conversion was observed in the N-2 matrix: vibrational excitation of cis-TBAA in the 2 nu OH wavenumber range predominantly converts the molecules located in a specific "matrix site" into trans-TBAA; then, relaxation (by tunneling) of the produced higher-energy conformer back to the cis form populates almost exclusively another "matrix site." The experimental studies received support from quantum chemistry calculations, which allowed a detailed characterization of the relevant regions of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers in the various matrices. Published by AIP Publishing.