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The reactive open-shell species play a very important role in the radiation-induced molecular evolution occurring in the cold areas of space and presumably leading to the formation of biologically relevant molecules. This review presents an insight into the mechanism of such processes coming from matrix isolation studies with a main focus on the experimental and theoretical studies performed in the author's laboratory during the past decade. The radicals and radical cations produced from astrochemically relevant molecules were characterized by Fourier transform infrared (FTIR) and electron paramagnetic resonance (EPR) spectroscopy. Small organic radicals containing C, O, and N atoms are considered in view of their possible role in the formation of complex organic molecules (COMs) in space, and a comparison with earlier results is given. In addition, the radical–molecule complexes generated from isolated intermolecular complexes in matrices are discussed in connection with their model significance as the building blocks for COMs formed under the conditions of extremely restricted molecular mobility at cryogenic temperatures. [ABSTRACT FROM AUTHOR]

Acetylene and ammonia are important constituents of the interstellar medium, and their coupled chemistry induced by high-energy radiation may be responsible for the formation of a variety of prebiotically important organic–nitrogen compounds. In this work, we first comprehensively characterized the vibrational spectrum of the 1:1 C2H2⋯NH3 complex obtained by deposition of the C2H2/NH3/Ng (Ng = Ar, Kr, or Xe) gaseous mixtures at 5 K using Fourier transform infrared spectroscopy and ab initio calculations at the CCSD(T)/L2a_3 level of theory and examined its radiation-induced transformations. The parent complex adopts a C3v symmetric top molecular structure with C2H2 acting as a proton donor. The x-ray-induced transformations of this complex result in the formation of the C2H2⋯NH2∙ complex and various CN-containing species (CH2CNH, CH3NC, CH2NCH, CH2NC∙, CCN∙, and CNC∙). The radical–molecule complex was identified based on comparison of experimental data with the results of the UCCSD(T)/L2a_3 computations. It is characterized by distinct features in the region of acetylene CHasym str mode, red-shifted from the corresponding absorptions of non-complexed acetylene by −72.9, −70.4, and −60.6 cm−1 for Ar, Kr, and Xe, respectively. Additionally, in krypton and xenon matrices, the blue-shifted features in the CHasymbend region of acetylene were observed, which can be also tentatively attributed to the C2H2⋯NH2∙ complex. The extrapolated to the complete basis set limit unrestricted coupled cluster method with single and double, and perturbative triple excitations binding energy of the C2H2⋯NH2∙ complex (including zero-point vibration energy correction) is lower than that of the C2H2⋯NH3 complex (1.90 and 2.51 kcal mol−1, respectively). We believe that the C2H2⋯NH2∙ complex may be an important intermediate in cold synthetic astrochemistry. [ABSTRACT FROM AUTHOR]

Trapping and decay of hydrogen atoms generated by fast electron irradiation of solid xenon doped with small hydrogen-containing molecules (acetylene, water) were studied by EPR using monoisotopic 136Xe matrix (I=0) and highly isotopically enriched 129Xe matrix (I=12). It was found that more than 99% of H atoms observed by EPR are initially trapped in the octahedral interstitial trapping sites, whereas initial population of the substitutional traping sites is very small (less than 1%). The 129Xe hyperfine coupling tensor parameters for major trapping site were determined from direct measurements in a 136Xe matrix doped with small amount of 129Xe: A0 (129Xe)=-92.1 MHz and B(129Xe)=-22 MHz. Final proof for the trapping site structure was obtained from comparison between experiment and simulation for the highly enriched 129Xe matrix. The mean interspin distance of approximately 4 nm was estimated from the EPR signal linewidth in a 136Xe matrix, the hydrogen atom loss upon irradiation being negligible at low doses. Decay of trapped H atoms occurring at 38–45 K leads to population (or creation) of metastable traps of lower symmetry. [ABSTRACT FROM AUTHOR]

IR spectra of xenon hydrides (HXeCCH, HXeCC, and HXeH) obtained from different xenon isotopes (129Xe and 136Xe) exhibit a small but detectable and reproducible isotopic shift in the absorptions assigned to H–Xe stretching (by 0.17–0.38 cm-1). To our knowledge, it is the first direct experimental evidence for the H–Xe bond in HXeY type compounds. The shift magnitude is in good agreement with quantum-chemical calculations. [ABSTRACT FROM AUTHOR]

Conformers of carboxyl radical (HOCO) have been studied by IR spectroscopy in argon and nitrogen matrices. In an argon matrix, only the lower-energy conformer trans-HOCO is observed, whereas both cis and trans conformers are found for deuterated carboxyl radical DOCO. In a nitrogen matrix, both conformers of HOCO and DOCO isotopologues can be prepared, indicating strong stabilization of the higher-energy cis conformer by a nitrogen matrix. Selective vibrational excitation promotes the trans-to-cis and cis-to-trans conversions of DOCO in an argon matrix and HOCO and DOCO in a nitrogen matrix, which is the first conformational photoswitching of an open-shell species. In a nitrogen matrix, the cis-to-trans and trans-to-cis conversions of HOCO is also found upon broadband IR light of the spectrometer, and the ratio of the quantum yields of these processes is about 3.3. The photoswitching peculiarities are in agreement with the available theoretical energy barriers. The higher-energy cis conformer decays to the lower-energy trans conformer via hydrogen-atom tunneling through the torsional barrier, which is also a unique observation for an open-shell species. The tunneling mechanism of the cis-to-trans switching is supported by the low-temperature limit of the reaction rate and by the H/D kinetic isotope effect. Our results suggest a large difference in the H/D kinetic isotope effects in nitrogen and argon matrices (∼5 and >100, respectively). The stabilizing effect on cis-DOCO by a nitrogen matrix (by 2 orders of magnitude versus an argon matrix) is much smaller than that on cis-HOCO (estimated to be >104). [ABSTRACT FROM AUTHOR]

[Display omitted] • CF 2 + cation was obtained and comprehensively characterized in solid Ar. • Absorptions of C-F stretching and F-C-F bending modes of CF 2 + were first observed. • IR spectral features of CF 2 + are in good agreement with ab initio calculations. The vibrational spectrum of CF 2 + radical cation was first explicitly characterized by FTIR spectroscopy in an argon matrix. The radical cation was produced from X-ray radiolysis of difluoromethane isolated in solid argon at 6 K in the presence of electron scavengers. The assignment was made on the basis of ab initio calculations at the CCSD(T)/L3a_3 level of theory. The absorptions at 1567.6 cm−1, 1379.4, and 613.8 cm−1 were assigned to antisymmetric and symmetric stretching, and bending modes of CF 2 +, respectively. [ABSTRACT FROM AUTHOR]

A hydrogen-bonded CHF⋯HF complex was characterized by FTIR matrix isolation spectroscopy and ab initio calculations. Three possible structures of this complex were found at the coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)/L3a_3] level of theory. The comparison between the experiment and theory reveals that the most stable structure with the binding energy of 6.48 kcal/mol is formed upon x-ray irradiation of isolated CH2F2 molecules in noble gas matrices (Ne, Ar, Xe). This species appears to be the first known intermolecular complex of monofluorocarbene, and its identification was unambiguously proved by IR absorptions corresponding to HF deformation (libration), CF stretching, H–C–F bending, and CH and HF stretching modes. It is worth noting that the corresponding spectral features in an argon matrix were previously tentatively ascribed to CH2F2+· and HF⋯CHF−· [L. Andrews and F. T. Prochaska, J. Chem. Phys. 70, 4714 (1979)], but the calculations performed in the present study definitely support the re-assignment. The observed CHF⋯HF complex can be converted to the parent CH2F2 under the action of light with λ < 525 nm. The plausible mechanism of this conversion using the conical intersection concept is discussed. [ABSTRACT FROM AUTHOR]

The α-hydroxyethyl radical (CH3·CHOH, 2A) is a key intermediate in ethanol biochemistry, combustion, atmospheric chemistry, radiation chemistry, and astrochemistry. Experimental data on the vibrational spectrum of this radical are crucially important for reliable detection and understanding of the chemical dynamics of this species. This study represents the first detailed experimental report on the infrared absorption bands of the α-hydroxyethyl radical complemented by ab initio computations. The radical was generated in solid para-H2 and Xe matrices via the reactions of hydrogen atoms with matrix-isolated ethanol molecules and radiolysis of isolated ethanol molecules with x rays. The absorption bands with maxima at 3654.6, 3052.1, 1425.7, 1247.9, 1195.6 (1177.4), and 1048.4 cm−1, observed in para-H2 matrices appearing upon the H· atom reaction, were attributed to the OHstr, α-CHstr, CCstr, COstr + CCObend, COstr, and CCstr + CCObend vibrational modes of the CH3·CHOH radical, respectively. The absorption bands with the positions slightly red-shifted from those observed in para-H2 were detected in both the irradiated and post-irradiation annealed Xe matrices containing C2H5OH. The results of the experiments with the isotopically substituted ethanol molecules (CH3CD2OH and CD3CD2OH) and the quantum-chemical computations at the UCCSD(T)/L2a_3 level support the assignment. The photolysis with ultraviolet light (240–300 nm) results in the decay of the α-hydroxyethyl radical, yielding acetaldehyde and its isomer, vinyl alcohol. A comparison of the experimental and theoretical results suggests that the radical adopts the thermodynamically more stable anti-conformation in both matrices. [ABSTRACT FROM AUTHOR]

The radiation-induced transformations of small molecules at low temperatures play an important role in the interstellar, planetary and atmospheric chemistry. This work presents a review of our recent model studies on the radiation chemistry of relevant molecules in solid noble gas (Ng) matrices, including some preliminary new results. Among the triatomic molecules, water and carbon dioxide were studied in detail. The radiation-induced degradation of isolated H 2 O yields hydrogen atoms and OH radicals, while oxygen atoms are produced at higher doses. Isolated CO 2 molecules are decomposed to yield CO and trapped oxygen atoms. Upon annealing the trapped O and H atoms are mobilized selectively at different temperatures and react with other trapped species. The formation of HCO and HOCO radicals was observed in the mixed H 2 O/CO 2 /Ng systems. Other studies were concerned with the radiation-induced degradation of simple organic molecules (methanol, formic acid) and chlorofluorocarbons (CFCl 3 , CF 2 Cl 2 ). Preliminary results for methanol revealed deep dehydrogenation yielding HCO and CO, whereas CO 2 , CO and HOCO were detected as primary products for formic acid. In the case of chlorofluorocarbons, significance of ionic channels was demonstrated. The implications of the results for modeling the processes in astrochemical ices and atmosphere are discussed. [ABSTRACT FROM AUTHOR]

The radiation-induced transformationsoccurring upon X-ray irradiationof solid CO2/H2O/Ng systems (Ng = Ar, Kr, Xe)at 8–10 K and subsequent annealing up to 45 K were studiedby Fourier transform infrared spectroscopy. The infrared (IR) spectraof deposited matrices revealed the presence of isolated monomers,dimers, and intermolecular H2O···CO2complexes. Irradiation resulted in effective decompositionof matrix-isolated carbon dioxide and water yielding CO moleculesand OH radicals, respectively. Annealing of the irradiated samplesled to formation of O3, HO2, and a number ofxenon hydrides of HXeY type (in the case of xenon matrices). The formationof these species was used for monitoring of the postirradiation thermallyinduced chemical reactions involving O and H atoms generated by radiolysis.It was shown that the radiolysis of CO2in noble-gas matricesproduced high yields of stabilized oxygen atoms. In all cases, thetemperatures at which O atoms become mobile and react are lower thanthose of H atoms. Dynamics and reactivity of oxygen atoms was foundto be independent of the precursor nature. In addition, the formationof HOCO radicals was observed in all the noble-gas matrices at remarkablylow temperatures. The IR spectra of HOCO and DOCO were first characterizedin krypton and xenon matrices. It was concluded that the formationof HOCO was mainly due to the radiation-induced evolution of the weaklybound H2O···CO2complexes. Thisresult indicates the significance of weak intermolecular interactionsin the radiation-induced chemical processes in inert low-temperaturemedia. [ABSTRACT FROM AUTHOR]

Difluoromethane is considered among the environment friendly alternatives to the ozone depleting chlorofluorocarbons. Due to its chemical inertness and lack of UV absorption above 200 nm, this compound can easily come to the upper layers forming complexes with widely abundant atmospheric components, such as water. The radiation-induced degradation of this compound and its complexes may be significant for reliable prediction of its long-term evolution in the environment as well as for development of new ways for its removal. In this work we have studied the vibrational spectroscopic properties and mechanisms of the radiation-induced decay of the CH 2 F 2 ⋯H 2 O under the action of X-rays using matrix isolation FTIR spectroscopy and ab initio calculations. The IR spectrum of the complex in an argon matrix was characterized for the first time and assigned to a hydrogen-bonded structure with a binding energy of 11.1 kJ/mol (2.65 kcal/mol) (CCSD(T)/CBS level of theory). Complexation with water leads to a certain suppression of the efficiency of the radiation-induced decomposition of difluoromethane. The obtained results provide evidence for the radiation-induced formation of previously unreported CF 2 ⋯H 2 O complex (in addition to other oxygen containing molecules, such as COF 2 and CO). As demonstrated by calculations, the new difluorocarbene complex reveals a hydrogen bond and it is characterized by a binding energy of 5.73 kJ/mol (1.37 kcal/mol) (CCSD(T)/CBS level of theory). [Display omitted] • The CH 2 F 2 ⋯H 2 O complex was examined by FTIR spectroscopy and ab initio calculations. • The effect of complexation on the CH 2 F 2 radiation-induced transformations was studied. • The CF 2 ⋯H 2 O complex was first identified on the basis of experiment and calculations. • The ecological aspects of the CH 2 F 2 ⋯H 2 O complex radiation chemistry are discussed. [ABSTRACT FROM AUTHOR]

Difluoromethane is considered among the environment friendly alternatives to the ozone depleting chlorofluorocarbons. Due to its chemical inertness and lack of UV absorption above 200 nm, this compound can easily come to the upper layers forming complexes with widely abundant atmospheric components, such as water. The radiation-induced degradation of this compound and its complexes may be significant for reliable prediction of its long-term evolution in the environment as well as for development of new ways for its removal. In this work we have studied the vibrational spectroscopic properties and mechanisms of the radiation-induced decay of the CH 2 F 2 ⋯H 2 O under the action of X-rays using matrix isolation FTIR spectroscopy and ab initio calculations. The IR spectrum of the complex in an argon matrix was characterized for the first time and assigned to a hydrogen-bonded structure with a binding energy of 11.1 kJ/mol (2.65 kcal/mol) (CCSD(T)/CBS level of theory). Complexation with water leads to a certain suppression of the efficiency of the radiation-induced decomposition of difluoromethane. The obtained results provide evidence for the radiation-induced formation of previously unreported CF 2 ⋯H 2 O complex (in addition to other oxygen containing molecules, such as COF 2 and CO). As demonstrated by calculations, the new difluorocarbene complex reveals a hydrogen bond and it is characterized by a binding energy of 5.73 kJ/mol (1.37 kcal/mol) (CCSD(T)/CBS level of theory). [Display omitted] • The CH 2 F 2 ⋯H 2 O complex was examined by FTIR spectroscopy and ab initio calculations. • The effect of complexation on the CH 2 F 2 radiation-induced transformations was studied. • The CF 2 ⋯H 2 O complex was first identified on the basis of experiment and calculations. • The ecological aspects of the CH 2 F 2 ⋯H 2 O complex radiation chemistry are discussed. [ABSTRACT FROM AUTHOR]

The radiation-induced assembling of the naphthalene molecule was first directly observed under X-ray irradiation of the isolated styrene-acetylene complex in a solid krypton matrix at 6 K using FTIR spectroscopy. The observation of naphthalene radical cation in the presence of electron scavenger suggests a cationic pathway of this reaction. This finding makes it possible to propose a new mechanism for the formation of polycyclic aromatic hydrocarbons in astrochemical ices. • Formation of naphthalene from a styrene-acetylene complex was observed at 6 K. • Naphthalene radical cation was detected in the presence of an electron scavenger. • Neutral and ionized naphthalene were characterized by FTIR spectroscopy. • The astrochemical implications of the results are discussed. [ABSTRACT FROM AUTHOR]

Bridged bifunctional molecules of general structure X(CH2) nY are useful models for investigations into the effects of intra- and intermolecular interactions in primary radiation-induced processes, with a number of implications for radiation chemistry, radiobiology, and molecular electronics. This short Review presents an outline of recent studies on the structure and reactivity of aliphatic bifunctional radical ions in low-temperature matrices using EPR spectroscopy and quantum-chemical calculations. Both EPR data and DFT calculations for radical cations show that the delocalization of charge and spin density occurs if XY or X and Y have close electronic properties. If the difference in the ionization potentials between X and Y is large, localized radical cations are observed with ground-state properties close to those of the corresponding monofunctional species. Nevertheless, the remote second functional group may affect the photochemistry of such radical cations at a specific bridge length, probably due to intramolecular interactions in a certain conformation. The stabilization of bridged bifunctional radical anions containing two carbonyl groups was found to be very sensitive to the local environment, which may be described in terms of a microsolvation model at the MP2 level of theory. Two independent reactions pathways with excess electrons, yielding different types of localized radical anions, were demonstrated for asymmetrical bifunctional molecules. The obtained results and their implications are discussed in terms of fine-tuning effects in radiation-induced chemical processes in condensed phases. [ABSTRACT FROM AUTHOR]

Difluoromethane (CH 2 F 2) is considered as an ecologically friendly alternative to conventionally used chlorofluorocarbons for various applications. Being photochemically inert in the broad UV range, this molecule can be decomposed only through the action of hard vacuum UV or ionizing radiation, while the corresponding mechanisms are poorly studied. In this work we report the results of model investigations of the X-ray induced decomposition of isolated CH 2 F 2 in solid noble gas matrices at 6 K. The radiation-chemical yields of difluoromethane were found to decrease from 1.3 to 0.1 molecule/100 eV with decreasing the ionization energy of matrix (from Ne to Xe), which indicates significance of the "hot" ionic channels. Difluorocarbene (CF 2) was detected as one of the principal radiolysis products in all the matrices, whereas the CHF⋯HF complex was detected in Ne, Ar, and Kr. Annealing of the irradiated samples results in thermally induced reactions of trapped H and F atoms yielding a number of various species, including the noble gas compounds. The radiolysis mechanism, nature of matrix effects and possible implications for atmospheric chemistry are discussed. • Radiolysis of CH 2 F 2 in solid noble gas matrices was investigated by FTIR spectroscopy. • The mechanism of CH 2 F 2 decomposition involving ionic and neutral pathways was proposed. • Thermal reactions of the radiolysis products with H and F atoms are considered. • Possible implications for atmospheric chemistry are outlined. [ABSTRACT FROM AUTHOR]

In this work, we report the first direct evidence for formation of the key simplest molecule with the peptide bond (acetamide) under radiation-induced transformations occurring in the CH3CN···H2O complex isolated in an argon matrix at 6 K. The 1:1 CH3CN···H2O intermolecular complex was obtained by deposition of CH3CN/H2O/Ar mixtures and characterized by FTIR spectroscopy. The irradiation of the icy matrix with X-rays results first in formation of acetimidic acid, which is transformed to acetamide at higher absorbed doses. In addition, the products of acetonitrile oxidation (hydroxyacetonitrile and acetonitrile N-oxide) have been detected, which probably originate from the reactions of 'hot' radiation-induced oxygen atoms. The formation of acetamide was confirmed by deuterium isotopic substitution in acetonitrile. We believe that the observed transformation may be of basic importance for understanding the radiation-induced synthesis of prebiotic molecules in occurring in cold space media. [ABSTRACT FROM AUTHOR]

Abstract: Structure and properties of the radical cations of “bridged” amidoesters (functional groups separated by 0–4 methylene groups and the corresponding dimethyl-substituted amides) were investigated by EPR, optical absorption spectroscopy and DFT calculations. The EPR spectra and thermal reactions of these species are similar. Meanwhile, for amidoesters with 2–4 methylene groups, the methylene “bridge” is involved in photochemistry, which can be explained by specific conformation of amidoester radical cations determined by interaction between the functional groups. [Copyright &y& Elsevier]

Abstract: Chemical reactions induced by fast electron irradiation of benzene in solid xenon were studied by EPR and FTIR spectroscopy. Irradiation at 16K leads to H atoms and phenyl radicals. Annealing at 45K results in formation of HXeH and cyclohexadienyl radical ( C6H7). A strong IR absorption at 618/620cm−1 and several weaker bands were assigned to cyclohexadienyl radical from comparison with EPR data and DFT calculations. A computationally stable molecule HXeC6H5 was not found experimentally, which means that it is either not formed or not stabilized under the experimental conditions used. [Copyright &y& Elsevier]

Isolated radical cations of substituted oligomers and polymers of phenylene vinylenes (PV) produced by irradiation in glassy toluene solutions were characterized by EPR. It was shown that the linewidth of the EPR signal from radical cations in oligomers decreased with increasing the number of repeat PV units, at least, up to n=8. This result implies extensive delocalization of hole on the isolated PV chain. In the case of polymer (MEH-PPV), the EPR signal linewidth is substantially smaller than the extrapolated value for oligomer radical cations, which was tentatively explained by the effect of substitution pattern. [Copyright &y& Elsevier]

The discovery of carbon-rich species, including polycyclic aromatic hydrocarbons (PAH) and fullerenes, in the space environment is among the most intriguing findings in molecular astrophysics and astrochemistry of the recent decades. The mechanism of their formation remains unclear and the one of the key issues is related to making "first ring", i.e., formation of benzene from simple small molecules abundant in the outer space. Different routes involving C2, C3 and C4 hydrocarbons are considered in literature. Here we report the first direct evidence for a single-step radiation-induced formation of benzene from pre-existing isolated acetylene trimer occurring in frozen inert environment at very low temperature (6 K). The finding is based on the correlation between trimer decay and build-up of benzene as monitored by FTIR spectroscopy. This concerted pathway may be applicable to acetylene aggregates trapped in the bulky ices or in the grain pores and it emphasizes the importance of concept of molecular organization in the chemical history of the Universe. • The radiation-induced transformation of acetylene to benzene in solid noble gas matrices at 6 K was detected by FTIR • Consumption of acetylene trimers correlates with build-up of benzene • The obtained data provide direct evidence in favor of a single-step concreted mechanism • The astrochemical implications of the results are discussed [ABSTRACT FROM AUTHOR]

We report on the preparation and vibrational characterization of the C2H3...CO2 complex, the first example of a stable intermolecular complex involving vinyl radicals. This complex was prepared in Ar and Kr matrices using UV photolysis of propiolic acid (HC3OOH) and subsequent thermal mobilization of H atoms. This preparation procedure provides vinyl radicals formed exclusively as a complex with CO2, without the presence of either CO2 or C2H3 monomers. The absorption bands corresponding to the v5(C2H3), v7(C2H3), v8(C2H3), v2(CO2), and v3(CO2) modes of the C2H3...CO2 complex were detected experimentally. The calculations at the UCCSD(T)/L2a level of theory predict two structures of the C2H3...CO2 complex with Cs and C1 symmetries and interaction energies of -1.92 and -5.19 kJ mol-1. The harmonic vibrational frequencies of these structures were calculated at the same level of theory. The structural assignment of the experimental species is not straightforward because of rather small complexation-induced shifts and matrix-site splitting of the bands (for both complex and monomers). We conclude that the C1 structure is the most probable candidate for the experimental C2H3...CO2 complex based on the significant splitting of the bending vibration of CO2 and on the energetic and structural considerations. [ABSTRACT FROM AUTHOR]

The radiation-induced transformations of isolated dimethyl disulphide molecules in a solid argon matrix occurring under the action of X-rays at 4.5 K were studied by FTIR spectroscopy. It was shown that decomposition was quite efficient (the corresponding radiation-chemical yield of Me 2 S 2 degradation was estimated as 4.5 molecules per 100 eV). The radiolysis proceeds via two primary channels yielding CH 3 SH plus H 2 CS and CH 4 plus H 2 CSS, respectively. These two channels of dimethyl disulphide decay during radiolysis lead to various secondary products appearing during prolonged radiolysis. Spectroscopic characteristics of the observed products and possible implications of the results are discussed. • Efficient radiolysis of isolated dimethyl disulphide molecules in solid argon was showed. • Two primary channels of radiolysis of dimethyl disulphide molecules were discovered. • With prolonged radiolysis of dimethyl disuphide molecules, a variety set of products is discovered. [ABSTRACT FROM AUTHOR]

The photo-induced transformations of HXeSH and HXeH under the action of IR and visible light have been studied using FTIR spectroscopy. The xenon hydrides were produced by the X-ray induced decomposition of H2S and its isotopomers in a solid xenon matrix at 7.5 K followed by thermal annealing at the temperatures up to 45 K. Selective IR-induced photodissociation of HXeSH at 3500-2500 cm-1 was attributed to vibrational excitation of the 3νH-Xe mode. The IR-photodecomposed HXeSH molecules can be almost quantitative recovered below 22 K with very small effective activation energy (∼20 meV) indicating local character of this process. Analysis of the photoactivity of xenon hydrides in the visible region revealed previously unknown absorptions for HXeSH (in the region of 400-700 nm) and HXeH (above 700 nm). The decomposition of HXeH occurs due to both direct photolysis and reactions of 'hot' H atoms produced from the photodissociation of HXeSH. The efficiency of thermal recovery for both xenon hydrides after photolysis with visible light was found to be dependent on the excitation wavelength, which was explained by the effect of photon energy on spatial distribution of the dissociation fragments. [ABSTRACT FROM AUTHOR]

The structural and spectroscopic features of the CHF3...H2O complex have been investigated using high-level ab initio calculations and IR matrix isolation spectroscopy. In contrast to previous findings, the computations at the CCSD(T)/L3a_3 and MP2/L3a_3 levels of theory revealed only one structure of the complex stabilized by two C-H...O and O-H...F hydrogen bonds. The interaction energy extrapolated to a complete basis set with ZPVE and BSSE corrections is 2.73 kcal/mol at the CCSD(T) level, taking into account the zero-point vibrational energy (ZPVE) and basis set superposition error (BSSE) corrections. According to Bader's analysis, complex exhibits a cyclic structure, which confirms the existence of ring critical point as well as bond critical points in the system. The calculated complexation-induced shifts of the fluoroform fundamentals are in good agreement with the matrix isolation results. In addition to previously reported IR absorptions of the complex, the feature corresponding to the C-F bending mode of CHF3...H2O was first observed, in good correlation with the computational predictions. [ABSTRACT FROM AUTHOR]

The radiation-induced transformations of acetylene, ethylene and ethane molecules under X-ray radiolysis in cryogenic media (noble gas matrices and neat hydrocarbon ices) at 5 K were characterized by FTIR spectroscopy. The radiation-chemical yields of decomposition of isolated hydrocarbon molecules were estimated in different solid noble gas matrices (argon, krypton and xenon). It was found that the radiation resistance of hydrocarbons at molecular level increases in the order ethane < ethylene < acetylene. For ethylene and ethane, the decomposition yields in argon are remarkably higher than those in krypton and xenon, which was explained by possible involvement of “hot” ionic fragmentation in argon and matrix effect on the excess energy dissipation. In all the cases, irradiation of matrix isolated molecules leads to progressive dehydrogenation, which should finally lead to the C2 carbon species. In the case of ethane, an evidence for prompt deep dehydrogenation yielding acetylene and vinyl radical was found even at low doses, particularly in argon. In contrast to the radiolysis of isolated molecule, irradiation of acetylene ice leads to predominating radiation-induced condensation. Possible implications of the results for basic radiation chemistry and astrochemistry are highlighted. [ABSTRACT FROM AUTHOR]

The reversible photoinduced transformations of 2,3-dimethyloxirane radical cations in freonic matrices at 77K are related to the conversion between the cis- and trans-isomers of an open form and a cyclic form. [ABSTRACT FROM AUTHOR]

The X-ray irradiation of 2,2-dimethyloxirane in freonic matrices at 77K results in the trapping of a cyclic radical cation form, presumably, complexed with a matrix molecule, which transforms into a C-centered distonic radical cation under the action of light. [ABSTRACT FROM AUTHOR]

The well-resolved EPR spectra of small radicals were obtained in a monoisotopic 136Xe matrix, which opens new prospects for the studies of structure, motional and chemical dynamics of such species over a wide temperature range. [Copyright &y& Elsevier]

The radical cation (RC) of methoxyacetone exhibits a delocalization of spin density between two oxygen atoms with considerable spin population on methylene hydrogen atoms, which results in the selective deprotonation of this RC. [Copyright &y& Elsevier]

Cites an experimental evidence for the formation of a hydrocarbon with an inserted rare-gas atom. Discovery of rare-gas hydrides in low-temperature matrixes; Characterization of a number of species by infrared spectroscopy; Dissociation of a precursor in solid xenon by ultraviolet light.

We report on the complex of a noble-gas hydride HXeOH with carbon monoxide. This species is prepared via the annealing-induced H + Xe + OH···CO reaction in a xenon matrix, the OH···CO complexes being produced by VUV photolysis of the H2O···CO complexes. The H-Xe stretching mode of the HXeOH···CO complex absorbs at 1590.3 cm-1 and it is blue-shifted by 12.7 cm-1 from the H-Xe stretching band of HXeOH monomer. The observed blue shift indicates the stabilization of the H-Xe bond upon complexation, which is characteristic of complexes of noble-gas hydrides. The HXeOH···CO species is the first complex of a noble-gas hydride with carbon monoxide and the second observed complex of HXeOH. On the basis of the MP2/aug-cc-pVTZ-PP calculations, the experimental complex is assigned to the structure, where the carbon atom of CO interacts with the oxygen atom of HXeOH. [ABSTRACT FROM AUTHOR]

Formyl radical HCO is an important reactive intermediate in combustion, atmospheric and extraterrestrial chemistry. Like in the case of other transients, the lack of knowledge of the absolute IR intensities limits the quantitative spectroscopic studies on this species. We report the first experimental determination of the absorption intensities for the fundamental vibrational bands of HCO. The measurements have been performed using matrix-isolation FTIR spectroscopy. Determination of the values was based on the repeated photodissociation and thermal recovery of the HCO radical using the known value of the absorption coefficient of CO. The experimentally determined values (93.2 ± 6.0, 67.2 ± 4.5, and 109.2 ± 6.6 km mol − 1 for the ν 1 , ν 2 , and ν 3 modes, respectively) have been compared to the calculated IR intensities obtained by DFT and UCCSD(T) computations. [ABSTRACT FROM AUTHOR]

The transformations of isolated CH 3 CN molecules in various solid noble-gas matrices (Ne, Ar, Kr, and Xe) under the action of X-ray irradiation at 5 K were investigated by FTIR spectroscopy. The main products are CH 3 NC, CH 2 CNH and CH 2 NCH molecular isomers as well as CH 2 CN and CH 2 NC radicals. The matrix has a strong effect on the distribution of reaction channels. In particular, the highest relative yield of keteneimine (CH 2 CNH) was found in Ne matrix, whereas the formation of CH 3 NC predominates in xenon. It was explained by differences in the matrix ionization energy (IE) resulting in different distributions of hot ionic reactions. The reactions of neutral excited states are mainly involved in Xe matrix with low IE, while the isomerization of the primary acetonitrile positive ions may be quite effective in Ne and Ar. Annealing of the irradiated samples results in mobilization of trapped hydrogen atoms followed by their reactions with radicals to yield parent molecule and its isomers. The scheme of the radiation-induced processes and its implications for the acetonitrile chemistry in cosmic ices are discussed. [ABSTRACT FROM AUTHOR]

The X-ray induced transformations in the CHF 3 /Ng systems (Ng=Ne, Ar, Kr or Xe) at 6 K were studied by FTIR spectroscopy. The radiation-induced decomposition of CHF 3 was found to be rather inefficient in solid xenon with low ionization energy, which suggests primary significance of the positive hole transfer from matrix to the fluoroform molecule. CF 3 • , : C F 2 , CHF 2 • and CF 4 were identified as the products of low-temperature radiolysis in all the noble gas matrixes. In addition, the anionic complex HF ⋯ CF 2 − was detected in Ne and Ar matrixes. The radiolysis also resulted in formation of noble gas compounds (HArF in argon, HKrF in krypton, and XeF 2 in xenon). While XeF 2 and HArF were essentially formed directly after irradiation (presumably due to reactions of ’hot’ fluorine atoms), HKrF mainly resulted from annealing of irradiated samples below 20 K due to thermally induced mobility of trapped fluorine atoms. In both krypton and xenon matrixes, the thermally induced reactions of F atoms occur at lower temperatures than those of H atoms, while the opposite situation is observed in argon. The mechanisms of the radiation-induced processes and their implications are discussed. [ABSTRACT FROM AUTHOR]

Review: 36 refs. [ABSTRACT FROM AUTHOR]

Review: 93 refs. [ABSTRACT FROM AUTHOR]

Effect of syn/anti- stereoisomerism of the macrocycle on radiation-chemical transformations of dicyclohexano-18-crown-6 complexes with BaCl 2 was studied by EPR spectroscopy. It was found that the stereoisomerism affected the composition of radicals produced under radiolysis at 77 K. In addition, it was demonstrated that the yield of post-radiation macrocycle cleavage after annealing the irradiated complexes at 333 K was also different for the two studied stereoisomers. The radiation-induced destruction of both cis-syn-cis- and cis-anti-cis -complex was predominately localized at the crown ether moiety, and the Cl 2 - radical anions were not detected in irradiated samples. The last finding was ascribed to a high reactivity of chlorine atoms towards the crown ether. Generally, it may be concluded that the macrocyclic complex with anti- configuration had a lower radiation stability. Thus, it is very important to characterize the stereoisomeric composition of dicyclohexano-18-crown-6 and its complexes in the radiation-chemical tests for practical purposes. [ABSTRACT FROM AUTHOR]

The X-ray-induced transformations of simple chlorofluorocarbons (CFCl3 and CF2Cl2) in solid noble-gas matrixes (Ne, Ar, Kr, and Xe) at 7 K were studied in order to elucidate basic mechanisms of the radiation-chemical degradation with possible implications for stratospheric and extraterrestrial ice chemistry. The decomposition of parent molecules and formation of products were monitored by FTIR spectroscopy, and the identification was supported by ab initio calculations at the CCSD(T) level. It was shown that the ionic reaction channels were predominating in most cases (except for CF2Cl2/Xe system). The primary radical cations (CFCl3+• and CF2Cl2+•) are either stabilized in matrixes or undergo fragmentation to yield the corresponding secondary cations (CFCl2+, CCl3+, CF2Cl+) and halogen atoms. The probability of fragmentation through different channels demonstrates a remarkable matrix dependence, which was explained by the effect of excess energy resulting from the exothermic positive hole transfer from matrix atoms to freon molecules. A qualitative correlation between "hot" ionic fragmentation at low temperatures and gas-phase ion energetics was found. However, dissociative electron attachment leads to formation of neutral radicals (CFCl2• or CF2Cl•) and chloride anions. One more possible way of dissociative electron attachment in the case of CF2Cl2 is formation of CF2•• and Cl2-•. A general scheme of the radiation-induced processes is proposed. [ABSTRACT FROM AUTHOR]

Gold nanoparticles (AuNPs) stabilized with poly(1-vinyl-1,2,4-triazole) (PVT) have been synthesized via a one-pot manner in irradiated solutions of 1-vinyl-1,2,4-triazole (VT) and Au(III) ions. The transmission electron microscopy examinations have shown that the sizes of nanoparticles formed range from 1 to 11 nm and are affected by the ratio of VT to gold ions. To study the kinetics peculiarities of the VT polymerization and assembling of AuNPs, UV-Vis spectroscopy was used. The analysis of the data obtained reveals that an inhibition period, influenced by Au(III) concentration, is followed by the polymerization of a monomer. Importantly, the absorbed doses, corresponding to the onset of rapid polymerization, correlate with the doses at which the accelerated formation of AuNPs begins. The kinetics aspects, which could lead to such an effect, are discussed. [ABSTRACT FROM AUTHOR]

Gold nanoparticles (GNPs) are promising radiosensitizers for cancer radiotherapy. Moreover, they can be used in the same way for radiation processing and sterilization. Such application of GNPs is of practical interest since it may significantly reduce the dose load and expand the application of radiation treatment. In the present study, the high radiosensitization effect of GNPs in relation to viral particles is demonstrated for the first time. The preparations of tobacco mosaic virus (TMV) are used as an experimental model, insofar as this virus has the same properties as animal and human ones but is safe for humans. Irradiation with 45 kVp X‐ray to the doses of 4 and 7 kGy leads to a decrease in the infectious activity of TMV virions up to 1.92‐ and 2.70‐fold, respectively. At the same time, irradiation in the presence of 0.4 mg mL−1 of 12 nm spherical GNPs increases the efficiency of virus inactivation up to 15‐ and 22‐fold. The GNPs enhance both the damage to capsid protein due to the enhanced generation of reactive oxygen species and genome RNA due to the emission of secondary radiation. These results show the great prospects of the application of high‐Z nanoparticles in radiation treatment. [ABSTRACT FROM AUTHOR]

The interpolyelectrolyte complexes (IPECs) are widely used for various applications, ranging from agriculture and medicine to advanced technological processes. In many cases, they are subjected to the action of ionizing radiation or hard UV light, either intentionally (e.g., for fabrication of nanocomposites) or in the course of routine treatment (sterilization or weathering), or unexpected accidents. In this communication, we report the first preliminary results on the effect of ionizing radiation on the poly(acrylic acid) (PAA) – polyethyleneimine (PEI) complexes and their components as revealed by EPR spectroscopic studies. The obtained results clearly reveal that the yield of primary radicals trapped after irradiation is comparable in all the studied systems and the main processes are decarboxylation in the PAA moiety and proton loss in the PEI moiety. There are no evidences for main-chain scission, which might lead to the degradation of material properties (at least, at moderate doses), instead of this, the radiation-induced crosslinking looks plausible. Generally, it provides a positive forecast for the radiation resistance of such complexes. • Effect of ionizing radiation on interpolyelectrolyte complexes (IPECs) was studied by EPR. • The radicals produced in the polyacrylic acid – polyethyleneimine IPECs were identified. • No evidence for the radiation-induced destruction of the IPECs was found. • A plausible mechanism of the radiation-induced processes was proposed. [ABSTRACT FROM AUTHOR]

In this work we report the results of systematic studies on the radiation-induced transformations in HCN/Ng systems (Ng=Ne, Ar, Kr or Xe) at 7 K using a combination of FTIR and EPR spectroscopy. It was shown that HCN underwent efficient decomposition producing H atoms, CN radicals and HNC isomer. The thermally induced reactions of H atoms in different matrices result in the formation of two isomeric radicals, H 2 CN and trans-HCNH, the former being predominated. The temperature dependent dynamics of CN and H 2 CN radicals in a krypton matrix was observed by EPR spectroscopy in solid krypton. The vibrational frequencies, IR intensities and magnetic resonance parameters of H 2 CN and trans-HCNH radicals calculated at the CCSD(T) level are in reasonable agreement with the experimental results. It was found that HCNH radical could be effectively bleached with visible light. The comparison of experimental and computational data made it possible to assign a new vibrational band at 918 cm −1 in an Ar matrix (and the corresponding bands in Kr and Xe) to trans-HCNH radical. In addition, HKrCN was found in the case of krypton, whereas HXeCN and HXeNC were produced in solid xenon. The reaction mechanisms and contribution of different channels are discussed. [ABSTRACT FROM AUTHOR]

The structure and conformation of radicals produced by X-ray irradiation of three polymorphic forms of d -mannitol were investigated using EPR spectroscopy. In all the cases, primary species were identified as radicals resulting from hydrogen abstraction from position 3 or 4 of the mannitol molecule. It was found that molecular packing in crystals of different polymorphic modifications had noticeable effect on the conformation of radicals observed after irradiation at room temperature and the dehydration of the primary radicals occurring at 400 K. The radicals trapped in stable modifications ( β - and δ -forms) were found to be very stable at room temperature. Relatively high radical yields and remarkable stability of radicals suggest that d -mannitol can be used as an EPR dosimeter or irradiation marker. [ABSTRACT FROM AUTHOR]

Structure and spectroscopic features of the CF2⋯HFcomplexes were studied by ab initio calculations at the CCSD(T) level and matrix isolation FTIR spectroscopy. The calculations predict three stable structures. The most energetically favorable structure corresponds to hydrogen bonding of HF to the lone pair of the C atom (the interaction energy of 3.58 kcal/mol), whereas two less stable structures are the H⋯F bonded complexes (the interaction energies of 0.30 and 0.24 kcal/mol). The former species was unambiguously characterized by the absorptions in the FTIR spectra observed after X-ray irradiation of fluoroform in a xenon matrix at 5 K. The corresponding features appear at 3471 (H–F stretching), 1270 (C–F symmetric stretching, shoulder), 1175 (antisymmetric C–F stretching), and 630 (libration) cm-1, in agreement with the computational predictions. To our knowledge, it is the first hydrogen-bonded complex of dihalocarbene. Possible weaker manifestations of the H⋯F bonded complexes were also found in the C–F stretching region; however, their assignment is tentative. The H⋯C bonded complex is protected from reaction yielding a fluoroform molecule by a remarkably high energy barrier (23.85 kcal/mol), so it may be involved in various chemical reactions. [ABSTRACT FROM AUTHOR]

The processes occurring in the X-irradiated C 2 H 2 /Kr and HCl/Kr systems in the temperature range of 7–30 K were studied using a combination of FTIR and EPR spectroscopy. In both cases, irradiation results in effective decomposition of isolated molecules (C 2 H 2 or HCl) and production of trapped H atoms. The thermal decay of trapped atoms in solid krypton was attributed to “local” reactions (below 21 K) and long-range mobility activated in the temperature range of 23–27 K. Two krypton hydrides, HKrCCH and HKrCl, were synthesized from the radiation-induced hydrogen atoms. In the case of C 2 H 2 /Kr system, competitive reaction channels of H atoms at various absorbed doses were investigated in details, and HKrCCH was found to be one of the main reaction products. The X-ray radiolysis in krypton matrices was concluded to be a promising method to obtain krypton hydrides. [ABSTRACT FROM AUTHOR]

The metal and oxide nanoparticles (NPs) containing various elements with high atomic number (Z) are considered as prospective sensitizers for the production of radicals upon X-ray irradiation of aqueous solutions, which can be used for different applications in medical treatment and material modification. The common basis for the sensitizing effect in oxygen-free systems is concerned with increase of the absorbed dose due to the presence of high- Z atoms (known as physical enhancement, relatively insensitive to the NP size). In this work we report the first experimental evidence for extra enhancement of the radical yield in a model oxygen-free aqueous organic system irradiated with X-rays (45 kVp) in the presence of W(VI) compounds, both in homogeneous solutions of Na 2 WO 4 and in the form of ultra-small NPs of WO 3. Meanwhile, in the case of larger WO 3 NPs the radical yield is consistent with physical enhancement mechanism. In the case of systems containing HfO 2 NPs only physical enhancement was observed, independent of the NP size. Unusually large enhancement of radical production and size effect for W(VI) in homogeneous and NP forms was tentatively attributed to the radiation-chemical processes involving intermediate formation of metastable W(V) due to reactions of hydrated electron. • Effect of X-ray irradiation on model aqueous organic systems was studied by spin trapping/EPR. • Increase of radical yield was found in the presence of Na 2 WO 4 and WO 3 nanoparticles. • The size effect on radical yield was observed for WO 3 nanoparticles. • The effect was explained by combination of "physical" and "chemical" enhancement. • The radiation-chemical mechanism of the effect and its implications are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Structure of radical anions of “bridged” amidoesters MeOOC(CH2) n CONMe2 (n=0–3) generated by X-ray irradiation was investigated by EPR, optical absorption spectroscopy and quantum-chemical calculations. Amidoester-0 (n=0) has positive calculated electron affinity and the corresponding delocalized radical anion is stabilized independent of matrix polarity. Meanwhile, for amidoesters with n=1–3 groups possessing apparently negative intrinsic electron affinity the role of matrix in stabilization of the corresponding radical anions becomes crucial. Two types of the localized radical anions were observed in the latter case. [Copyright &y& Elsevier]

Abstract: The primary radical cation of cyclopropylacetylene was first characterized by EPR spectroscopy in low-temperature freon matrices. The assignment was confirmed by specific deuteration and quantum-chemical calculations at PBE0 and CCSD(T) levels. Photolysis with visible light led to irreversible transformation of the initial species to a ring-open structure. Detailed computational analysis of energy and magnetic resonance parameters of possible reaction products justified formation of pent-3-en-1-yne radical cation (presumably, a (Z)-isomer). This conclusion was also supported by the effect of specific deuteration. [Copyright &y& Elsevier]

Abstract: Nanocomposite polymer materials containing metal or metal oxide particles attract growing interest due to their specific unique combination of physical and electric behavior. Stoichometric triple interpolyelectrolyte-metal complexes (TIMC) are insoluble in water and in aqueous organic media and may include high content of metal ions; concentration of ions is easy to vary in such polymeric systems. Reduction of metal ions is a common method for obtaining nanoparticles. Interpolyelectrolyte complexes reveal high permeability for polar low-molecular substances and salts. Such swelling behavior is important for the reduction of metal ions included in these solids. The properties of triple interpolyelectrolyte-metal complexes and preparation of nanocomposites from these materials using various methods of metal ion reduction are discussed in this work. [Copyright &y& Elsevier]