Astrochemically Relevant Radicals and Radical-Molecule Complexes: A New Insight from Matrix Isolation

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.

Radiation-induced transformations of matrix-isolated ethanol molecules at cryogenic temperatures: an FTIR study

Ethanol (C2H5OH) is one of the most common alcohol molecules observed in various space media (molecular clouds, star formation regions, and, highly likely, interstellar ices), where it is exposed to light and ionizing radiation, leading to more complex organic molecules and eventually to the biologically important species. To better understand the radiation-induced evolution of ethanol molecules in icy media, we have examined the transformations of isolated C2H5OH and C2D5OH under the action of X-rays and vacuum ultraviolet (VUV) radiation in solid inert matrices (Ne, Ar, Kr, and Xe) at 4.4 K using Fourier transform infrared (FTIR) spectroscopy. The results obtained with X-ray irradiation demonstrate the formation of a variety of radiolysis products corresponding to dehydrogenation (CH3CHOH˙, CH3CHO, CH2CHOH, CH3CO˙, H2CCO-H2, H2CCO, HCCO˙, CCO) and C-C bond rupture (H2CO, HCO˙, CO, CH4, and CH3˙). The absorptions of the CH3CHOH˙ radical related to the CCO stretching (the bands at 1249.1, 1247.0, 1246.2, and 1245.1 cm-1, in Ne, Ar, Kr, and Xe, respectively) were first tentatively characterized on the basis of comparison with available computational data. In addition, the C2H2⋯H2O complex, which corresponds to dehydrogenation, was found followed by C-O bond cleavage. The results were confirmed by experiments with isotopic substitution. It was found that dehydrogenation strongly predominated in a xenon matrix, while skeleton bond rupture is more feasible in neon and argon. The matrix effect was attributed to a significant role of "hot" reaction channels in neon and argon, which are efficiently quenched due to relaxation in more polarizable xenon. The VUV photolysis (185 nm) in Ar and Xe matrices yields a similar set of products, except for CH3CHOH˙ and CH2CHOH, which were not found (the nonobservation of the former species may be explained by its efficient secondary photolysis). The plausible mechanisms of product formation and astrochemical implications of the results are discussed.

An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C₂H₅OH and CH₃CD₂OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH₃˙) and formyl (HCO˙) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of "hot" ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of α-hydroxyethyl radicals (CH₃˙CHOH or CH₃˙CDOH(D) in the cases of C₂H₅OH and CH₃CD₂OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH₃CD₂O˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH₃˙CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.

Reductive Removal and Recovery of As(V) and As(III) from Strongly Acidic Wastewater by a UV/Formic Acid Process

The removal of arsenic (As(V) and As(III)) from strongly acidic wastewater using traditional neutralization or sulfuration precipitation methods produces a large amount of arsenic-containing hazardous wastes, which poses a potential threat to the environment. In this study, an ultraviolet/formic acid (UV/HCOOH) process was proposed to reductively remove and recover arsenic from strongly acidic wastewater in the form of valuable elemental arsenic (As(0)) products to avoid the generation of hazardous wastes. We found that more than 99% of As(V) and As(III) in wastewater was reduced to highly pure solid As(0) (>99.5 wt %) by HCOOH under UV irradiation. As(V) can be efficiently reduced to As(IV) (H₂AsO₃ or H₄AsO₄) by hydrogen radicals (H^•) generated from the photolysis of HCOOH through dehydroxylation or hydrogenation. Then, As(IV) is reduced to As(III) by H^• or through its disproportionation. The reduction of As(V) to H₄AsO₄ by H^• and the disproportionation of H₄AsO₄ are the main reaction processes. Subsequently, As(III) is reduced to As(0) not only by H^• through stepwise dehydroxylation but also through the disproportionation of intermediate arsenic species As(II) and As(I). With additional density functional theory calculations, this study provides a theoretical foundation for the reductive removal of arsenic from acidic wastewater.

The Radiation Chemistry of NH3···CO Complex in Cryogenic Media as Studied by Matrix Isolation

The NH₃···CO complex can be considered an important building block for cold synthetic astrochemistry leading to the formation of complex organic molecules, including key prebiotic species. In this work, we have studied the radiation-induced transformations of this complex in Ar, Kr, and Xe matrices using FTIR spectroscopy. On the basis of comparison with the quantum chemical calculations at the CCSD(T)/L2a_3 level of theory, it was found that the initial complex had the configuration with hydrogen bonding through the carbon atom of CO. Irradiation of the matrix isolated complex with X-rays at 6 K leads to the formation of a number of synthetic products, namely, HNCO (in all matrices), formamide NH₂CHO, NH₂CO, and HNCO-H₂ (in argon and krypton). The matrix effect on the product distribution was explained by the involvement of different excited states of the complex in their formation. It was suggested that formamide results from the singlet excited states while other species mainly originate from triplet excited states. The latter states are efficiently populated through ion-electron recombination (in all matrices) and through intersystem crossing (particularly, in xenon). High yield of the recombination triplet states is a feature of the processes induced by high-energy radiation (in contrast to direct photolysis). NCO, CN, and NO were found as minor secondary products at high adsorbed doses. The astrochemical implications of the obtained results are discussed.

IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

In this article, IR-induced and tunneling-driven reactions observed in cryogenic matrices are described in a historical perspective, the entangling of the two types of processes being highlighted. The story of this still ongoing fascinating scientific endeavor is presented here following closely our own involvement in the field for more than 30 years, and thus focuses mostly on our work. It is, because of this reason, also an incomplete story. Nevertheless, it considers a large range of examples, from very selective IR-induced conformational isomerizations to IR-induced bond-breaking/bond-forming reactions and successful observations of rare heavy atom tunneling processes. As a whole, this article provides a rather general overview of the major progress achieved in the field.

Radiation-induced transformations of acetaldehyde molecules at cryogenic temperatures: a matrix isolation study

Acetaldehyde is one of the key small organic molecules involved in astrochemical and atmospheric processes occurring under the action of ionizing and UV radiation. While the UV photochemistry of acetaldehyde is well studied, little is known about the mechanism of processes induced by high-energy radiation. This paper reports the first systematic study on the chemical transformations of CH₃CHO molecules resulting from X-ray irradiation under the conditions of matrix isolation in different solid noble gases (Ne, Ar, Kr, and Xe) at 5 K. CO, CH₄, H₂CCO, H₂CCO-H₂, C₂H₂⋯H₂O, CH₂CHOH, CH₃CO˙, CH₃˙, HCCO˙, and CCO were identified as the main radiolysis products. The dominant pathway of acetaldehyde degradation involves C-C bond cleavage leading to the formation of carbon monoxide and methane. The second important channel is dehydrogenation resulting in the formation of ketene, a potentially highly reactive species. It was found that the matrix significantly affected both the decomposition efficiency and distribution of the reaction channels. Based on these observations, it was suggested that the formation of the methyl radical as well as vinyl alcohol and the C₂H₂⋯H₂O complex presumably included a significant contribution of ionic pathways. The decomposition of acetyl radicals under photolysis with visible light leading to the CH₃˙-CO radical-molecule pair was observed in all matrices, while the recovery of CH₃CO˙ in the dark at 5 K was found only in Xe. This finding represents a prominent example of matrix-dependent chemical dynamics (probably, involving tunnelling), which deserves further theoretical studies. Probable mechanisms of acetaldehyde radiolysis and their implications for astrochemistry, atmospheric chemistry and low-temperature chemistry are discussed.

Reactions of radiation-induced electrons with carbon dioxide in inert cryogenic films: matrix tuning of the excess electron interactions in solids

A single-electron reduction of carbon dioxide is supposed to be an important basic step in various processes, ranging from interstellar chemistry to photocatalytic transformations. In this work, we report an FTIR spectroscopic study on the reactions of carbon dioxide (¹²CO₂ and ¹³CO₂) with the radiation-induced excess electrons in deposited cryogenic matrices with different physical characteristics (Ne, N₂, Ar, Xe) occurring at 6 K. The reaction was monitored by the observation of carbon dioxide radical anions. It was found that attachment of excess electrons to CO₂ occurred in neon and nitrogen matrices, but not in argon and xenon. In the case of nitrogen, the formation of matrix-related cationic species (N₄⁺˙ and NNCO⁺˙) was also observed. Since the CO₂ molecules have a negative intrinsic electron affinity, it was suggested that the electron attachment to CO₂ is controlled by the energy of excess electrons in the solid matrix, which is determined by the value of the corresponding conduction band bottom energy (V₀). The implications of the obtained results are discussed.

Radiation-Induced Transformations of C6H6 Molecules in Solid Noble-Gas Matrices: Is Benzene Intrinsically Resistant in Condensed Media?

The radiation resistance of aromatic compounds is one of the key concepts of basic and applied radiation chemistry in condensed phases. Usually, it is attributed to the intrinsic radiation stability of the benzene ring. In this work, we have demonstrated for the first time that the isolated benzene molecules undergo rather efficient radiation-induced degradation in rigid inert media at cryogenic temperatures (comparable to that of aliphatic hydrocarbons), and their stability is essentially determined by the intermolecular relaxation correlating with matrix polarizability. The principal primary products of benzene radiolysis in matrices are phenyl radicals and fulvene. The matrix environment strongly affects the proportion of these species because of external heavy atom effect on the intersystem crossing, which may trigger further reaction pathways. The obtained results may have important implications for the prediction of radiation stability of complex organic systems and polymers. Furthermore, they may contribute to a better understanding of the radiation-induced evolution of aromatic species in cold interstellar media.

Formation and interconversion of CCN and CNC radicals resulting from the radiation-induced decomposition of acetonitrile in solid noble gas matrices

Acetonitrile and the species resulting from its dehydrogenation play an important role in the radiation-induced evolution of organic matter in the space environment. In this work, we report on FTIR spectroscopic studies of the degradation of isolated CH3CN and CD3CN molecules induced by prolonged X-ray irradiation in solid noble gas matrices at 5 K. The principal products observed at high conversion degree of the parent acetonitrile molecules (70-90%) are CCN and CNC radicals, which result from prompt or two-step dehydrogenation of the corresponding precursors, H2CN and CH2NC radicals, respectively. CHCN and CHNC were also found as products of dehydrogenation at high absorbed doses, whereas the fragmentation products (CH3, CN, HCN, and HNC) were detected only in minor amounts over the whole dose range studied. CCN and CNC are produced in nearly equal amounts at high absorbed doses. Selective isomerization of CCN to CNC was observed under the illumination with visible light (460-470 nm), while subsequent action of the UV light (254 nm) induced reverse transformation leading to a photostationary state with the relative population of CNC/CCN being ca. 0.7. The astrochemical implications of the obtained results are discussed in connection with the recent discovery of CCN in extraterrestrial objects.

Characterization of the HCNCO complex and its radiation-induced transformation to HNCCO in cold media: an experimental and theoretical investigation

The HCNCO complex and its X-ray induced transformation to HNCCO in solid noble gas (Ng) matrices (Ng = Ne, Ar, Kr, Xe) was first characterized by matrix isolation FTIR spectroscopy at 5 K. The HCNCO complex was obtained by deposition of HCN/CO/Ng gaseous mixtures. The assignment was based on extensive quantum chemical calculations at the CCSD(T) level of theory. The calculations predicted two computationally stable structures for HCNCO and three stable structures for HNCCO. However, only the most energetically favorable linear structures corresponding to the co-ordination between the H atom of HCN (HNC) and the C atom of CO have been found experimentally. The HCNCO complex demonstrates a considerable red shift of the H-C stretching vibrations (-24 to -38 cm^-1, depending on the matrix) and a blue shift of the HCN bending vibrations (+29 to +32 cm^-1) with respect to that of the HCN monomer, while the C[double bond, length as m-dash]O stretching mode is blue-shifted by 15 to 20 cm^-1 as compared to the CO monomer. The HNCCO complex reveals a strong red shift of the H-N bending (-77 to -118 cm^-1) and a strong blue shift of the HNC bending mode (ca. +100 cm^-1) as compared to the HNC monomer, whereas the C[double bond, length as m-dash]O stretching is blue-shifted by 24 to 29 cm^-1 with respect to that of the CO monomer. The interaction energies were determined to be 1.01 and 1.87 kcal mol^-1 for HCNCO and HNCCO, respectively. It was found that the formation of complexes with CO had a remarkable effect on the radiation-induced transformations of HCN. While the dissociation of HCN to H and CN is suppressed in complexes, the isomerization of HCN to HNC is strongly catalyzed by the complexation with CO. The astrochemical implications of the results are discussed.

Experimental determination of the absolute infrared absorption intensities of formyl radical HCO

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±6.6kmmol^-1 for the ν₁, ν₂, and ν₃ modes, respectively) have been compared to the calculated IR intensities obtained by DFT and UCCSD(T) computations.

VUV photochemistry of the H2O⋯CO complex in noble-gas matrices: formation of the OH⋯CO complex and the HOCO radical

VUV photolysis of the H₂O⋯CO complexes leads to the formation of the OH⋯CO radical–molecule complexes and trans-HOCO radicals.

Mechanisms of Radiation-Induced Degradation of CFCl3 and CF2Cl2 in Noble-Gas Matrixes: An Evidence for "Hot" Ionic Channels in the Solid Phase

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.

Radiation-induced transformations of methanol molecules in low-temperature solids: a matrix isolation study

Radiation-induced transformations of methanol in inert solids at 6 K reveal remarkable matrix effects, and mechanisms and astrochemical implications are discussed.