HOCO Radical Research Articles

Cracking the puzzle of CO2 formation on interstellar ices

Context. Carbon dioxide (CO2) is one of the dominant components of interstellar ices. Recent observations show CO2 exists more abundantly in polar (H2O-dominated) ice than in apolar (H2O-poor) ice. Formation of CO2 ice is primarily attributed to the reaction between CO and OH, which has a barrier. Aims. We investigate the title reaction in H2O ice and CO ice to quantify the efficiency of the reaction in polar ice and apolar ice. Methods. Highly accurate quantum chemical calculations were employed to analyze the stationary points of the potential energy surfaces of the title reaction in the gas phase on H2O and CO clusters. Microcanonical transition state theory was used as a diagnostic tool for the efficiency of the reaction under interstellar medium conditions. We simulated the kinetics of ice chemistry, considering different scenarios involving non-thermal processes and energy dissipation. Results. The CO + OH reaction proceeds through the remarkably stable intermediate HOCO radical. On the H2O cluster, the formation of this intermediate is efficient, but the subsequent reaction leading to CO2 formation is not. Conversely, HOCO formation on the CO cluster is inefficient without external energy input. Thus, CO2 ice cannot be formed by the title reaction alone either on an H2O cluster or a CO cluster. Conclusions. In the polar ice, CO2 ice formation is possible via CO + OH → HOCO followed by HOCO + H → CO2 + H2, as demonstrated by abundant experimental literature. In apolar ice, CO2 formation is less efficient because HOCO formation requires external energy. Our finding is consistent with the JWST observations. Further experimental work using low-temperature OH radicals is encouraged.

Computational studies on the possible formation of glycine via open shell gas-phase chemistry in the interstellar medium.

Glycine is the simplest proteinogenic amino acid. It has significant astrobiological implications owing to the ongoing investigation for its detection in the interstellar medium (ISM). Hence, a suitable mechanistic elucidation for its formation in the ISM is of current research interest. In the present work, by employing electronic structure calculations [UCCSD(T) and density functional theory (DFT)], various plausible chemical pathways in the gas phase have been examined for the formation of glycine (whose existence has been indirectly proposed in the ISM) and other simple amino acids (yet to be detected in the ISM) from some simpler molecules present in the ISM. This work suggests that step 1: HO-CO (radical) + CH2NH → NHCH2COOH (radical) and step 2a: NHCH2COOH (radical) + H2 → glycine + H (radical) have very small barriers of 0.14 kcal mol-1 and ∼3 kcal mol-1, respectively (easily surmountable at a temperature of ∼50 K under putative interstellar conditions). Hence this should likely be feasible in interstellar gas-phase chemistry. Therefore, HO-CO (radical), CH2NH, and H2 could be the possible precursors for the formation of glycine (subject to the presence of the HO-CO radical). The energetic information related to the interstellar reactions, and how this work takes the putative interstellar conditions into account are presented. This paper also highlights how a reaction found to be unsuitable for interstellar molecular evolution via surface chemistry could nonetheless occur via gas-phase chemistry. Based on our results, this work also recommends detecting three new open-shell molecules, HO-CO radical, NHCH2COOH radical, and NH2CHCOOH radical, in the ISM.

Hydrocarboxyl Radical as a Product of α-Alanine Ultraviolet Photolysis.

UV photodissociation of α-alanine was studied by parahydrogen matrix isolation infrared spectroscopy. The temporal behavior of Fourier transform infrared spectra revealed that UV irradiation at 213 nm yielded the HOCO radical as a direct photoproduct from the S2 excited state. The concentration of HOCO quickly approached a steady state due to secondary photodissociation of HOCO to produce CO2 + H or CO + OH. On the other hand, no photoproducts were detected by S1 excitation at 266 nm. Irradiation of fully deuterated α-alanine at 213 nm yielded ∼2 times more cis-DOCO radicals than the lower energy isomer trans-DOCO, indicating that the conformation of the hydroxyl group is fairly well-preserved upon photodissociation of α-alanine. The present study suggests that HOCO may be a good tracer species in the search for amino acids in interstellar space.

Reactions of Photoionization-Induced CO-H2O Cluster: Direct Ab Initio Molecular Dynamics Study.

The hydrocarboxyl radical (HOCO) is an important species in combustion and astrochemistry because it is easily converted to CO2 after hydrogen reduction. In this study, the formation mechanism of the HOCO radical in a CO–H2O system was investigated by direct ab initio molecular dynamics calculations. Two reactions were examined for HOCO formation. First, the reaction dynamics of the CO–H2O cluster cation, following the ionization of the neutral parent cluster CO(H2O)n (n = 1–4), were investigated. Second, the bimolecular collision reaction between CO and (H2O)n+ was studied. In the ionization of the CO(H2O)n clusters (n = 3 and 4), proton transfer, expressed as CO(H2O)n+ → CO–(OH)H3O+(H2O)n–2, occurred within the (H2O)n+ cluster cation, and the HOCO radical was yielded as a product upon addition of CO and OH. This reaction proceeds under zero-point energy. Also, this radical was effectively formed from the collision reaction of CO with water cluster cation (H2O)n+, expressed as CO + OH(H3O+)(H2O)n–2 → HOCO–H3O+ + (H2O)n–2. If the intermolecular vibrational stretching mode is excited in the CO(H2O)n cluster (vibrational stretching between CO and the water cluster), the HOCO radical was detected after ionization when n = 2. The reaction mechanism was discussed based on the theoretical results.

The Formation of C3O3H6 Structural Isomers in the Gas Phase through Barrierless Pathways: Formation and Spectroscopic Characterization of Methoxy Acetic Acid

Abstract Gas phase formation processes feasible at low temperatures are determined theoretically for 38 isomers obeying the C3O3H6 empirical formula, one of them, the simplest ketose dihydroxyacetone, has been observed in gas phase sources. A preliminary search for isomeric forms first targets ethoxy formic acid (C2H5–O–COOH) as the most stable isomer followed by lactic acid. Profiles corresponding to the minimum energy pathways reveal that the favored conformers of 14 of these isomers can be formed in the gas phase through 29 barrierless processes involving the OH*, CH3O*, HCO*, CH3*, CH2OH, HCOO*, and OHCO* radicals, all of them observed in the interstellar medium. Kinetic rates are provided at 200, 298, and 500 K, confirming the suitability of 16 processes at low temperatures. Faster processes involve the OH hydroxyl radical whereas, to a lesser degree, the processes involving the HOCO radical and the methoxy methyl radical CH3O*, are quite significant. Spectroscopic parameters (rovibrational and torsional) are obtained for methoxy acetic acid (CH3–O–CH2COOH) for which two low-lying isoenergetic conformers can be produced from the CH3OCH2* radical predicted to be a precursor of abundant observed molecules. Profiles and spectroscopic properties make methoxy acetic acid a good candidate to be detected in the gas phase of extraterrestrial sources.

Effect of water on dynamics of HOCO radical

The activated HOCO* is one of the most important intermediates in combustion and atmospheric processes and could become thermally stabilized through the collision with the third species. In this work, the effect of a single water molecule on an energized HOCO* radical for the collision of HOCO + H 2 O has been investigated by using direct dynamics methodology. The properties of the stationary points on the potential energy surface have been characterized at M06-2X/aug-cc-pVDZ level of theory. Interesting dynamical features and detailed atomic level mechanisms are presented. It is found that the water molecule is not involved in the chemical reaction, but it can substantially decrease the activation barrier when compared with the water-free system. Energy distributions of products indicate that H 2 O can transfer the vibrational energy of the energized HOCO to its own rotational energy and thus serves as an efficient third body.

VUV photochemistry of the H2OCO complex in noble-gas matrices: formation of the OHCO complex and the HOCO radical.

Vacuum ultraviolet (VUV, 130-170 nm) photochemistry of the H2OCO complex is studied by matrix-isolation infrared spectroscopy. The H2OCO complexes in Ne, Ar, Kr, and Xe matrices are generated by ultraviolet (UV, 193 and 250 nm) photolysis of formic acid (HCOOH). VUV photolysis of the H2OCO complexes is found to lead to the formation of the OHCO radical-molecule complexes and trans-HOCO radicals. It is shown that the matrix material, local matrix morphology, and possibly the H2OCO complex geometry strongly affect the VUV photolysis pathways. The intrinsic reactivity of the matrix-isolated OHCO complex resulting in the formation of trans-HOCO is directly demonstrated for the first time. This reaction occurs in Ar, Kr, and Xe matrices upon annealing above 25 K and may proceed over the barrier. The case of a Ne matrix is very special because the formation of trans-HOCO from the OHCO complex is observed even at the lowest experimental temperature (4.5 K), which is in sharp contrast to the other matrices. It follows that quantum tunneling is probably involved in this process in the Ne matrix at such a low temperature. Infrared light also promotes this reaction in the Ne matrix at 4.5 K, which is not the case in the other matrices. The last findings show the effect of the environment on the tunneling and infrared-induced rates of this fundamental chemical reaction.

Matrix-isolation studies on the radiation-induced chemistry in H₂O/CO₂ systems: reactions of oxygen atoms and formation of HOCO radical.

The radiation-induced transformations occurring upon X-ray irradiation of solid CO2/H2O/Ng systems (Ng = Ar, Kr, Xe) at 8-10 K and subsequent annealing up to 45 K were studied by Fourier transform infrared spectroscopy. The infrared (IR) spectra of deposited matrices revealed the presence of isolated monomers, dimers, and intermolecular H2O···CO2 complexes. Irradiation resulted in effective decomposition of matrix-isolated carbon dioxide and water yielding CO molecules and OH radicals, respectively. Annealing of the irradiated samples led to formation of O3, HO2, and a number of xenon hydrides of HXeY type (in the case of xenon matrices). The formation of these species was used for monitoring of the postirradiation thermally induced chemical reactions involving O and H atoms generated by radiolysis. It was shown that the radiolysis of CO2 in noble-gas matrices produced high yields of stabilized oxygen atoms. In all cases, the temperatures at which O atoms become mobile and react are lower than those of H atoms. Dynamics and reactivity of oxygen atoms was found to be independent of the precursor nature. In addition, the formation of HOCO radicals was observed in all the noble-gas matrices at remarkably low temperatures. The IR spectra of HOCO and DOCO were first characterized in krypton and xenon matrices. It was concluded that the formation of HOCO was mainly due to the radiation-induced evolution of the weakly bound H2O···CO2 complexes. This result indicates the significance of weak intermolecular interactions in the radiation-induced chemical processes in inert low-temperature media.

Correction to “Sub-Doppler Spectroscopy of the trans-HOCO Radical in the OH Stretching Mode”

Correction to “Sub-Doppler Spectroscopy of the <i>trans</i>-HOCO Radical in the OH Stretching Mode”

CO Capture and Conversion to HOCO Radical by Ionized Water Clusters

The CO molecule can interact with the hydroxyl radical ((•)OH) via either a weak noncovalent interaction or strong covalent bonding. Upon the ionization of neutral water clusters, the resulting water cluster cations produce protonated water clusters and hydroxyl radicals. In this regard, we investigate the interactions of a CO molecule with water dimer and trimer cations using density functional theory (DFT), Möller-Plesset second-order perturbation theory (MP2), and coupled cluster theory with single, double, and perturbative triple excitations [CCSD(T)]. It is found that the reaction products of CO by the water dimer and trimer cations form a HOĊO radical solvated by a protonated water monomer and dimer, respectively. These radicals are useful intermediates to make oxalic acids, formic acids, metal ligands, and so on, which is important in green chemistry.

Spectroscopy and dynamics of the HOCO radical: insights into the OH + CO → H + CO2 reaction.

After more than forty years of scrutiny, crucial new details regarding the elementary reaction OH + CO → H + CO2 are still emerging from experimental and theoretical studies of the HOCO radical intermediate. In this perspective, previous studies of this elementary reaction and the structure and energetics of the HOCO radical will be briefly reviewed. Particular attention will be paid to the experimental techniques used in our laboratory to prepare excited HOCO radicals by both photodetachment and dissociative photodetachment of HOCO(-). These experiments directly probe the dynamics occurring on the ground and excited states of the HOCO radical, and are sensitive to both direct and tunneling-induced dissociation. Photoelectron-photofragment coincidence experiments on HOCO(-) in particular have been used to study tunneling from the HOCO well to form H + CO2 products. In addition, new experimental insights into the OH + CO entrance channel for the reaction will be presented. These studies have provided a number of constraints on the potential energy surface for this system from an energetic and dynamical perspective, and have helped spur a renewed effort to characterize the global potential energy surface and reaction dynamics of this fundamental chemical reaction. Outstanding questions and new directions for future work on the HOCO radical will be discussed.

Reactions of HOCO radicals through hydrogen-atom hopping utilizing clathrate hydrates as an observational matrix

The carboxyl (HOCO) radical, which is an important species in atmospheric chemistry and combustion, is an intermediate in the reaction: CO + OH → CO2 + H and serves as a hydrogen donor to the reaction partners. The cis-HOCO radical, one of the ground-state HOCO radicals, is supposed to be decomposed into CO2 and the hydrogen atom by a tunneling effect. In order to prove the hypothesis, we performed electron spin resonance (ESR) measurements to investigate the decay mechanisms of the ground-state HOCO and DOCO radicals in gamma-ray-irradiated CO2 hydrates, which may hold the radicals stably. The ground-state HOCO and DOCO radicals decayed according to a second-order decay model and transformed into formic acid and CO2. The ratio of the decay rate constants of HOCO and DOCO radicals shows a good agreement with that in the kinetic isotope effect for the hydrogen and deuterium abstraction reactions. These results indicate that they react with another HOCO radical in the adjacent hydrate cage without the tunneling effect. This implies that the ground-state HOCO radicals are not decomposed by the tunneling effect but are decayed through reactions with some atoms, molecules, and/or radicals even in the gas phase. In addition, the hydrogen-atom hopping through the temporary hydrogen bonds between the HOCO radical and CO2 results in a seeming diffusion of the HOCO radicals in the CO2 hydrate; this would be an important concept for the studies of the radical diffusions and the supply of hydrogen atoms in gas, liquid, and solid phases.

Rovibrational Energies of the Hydrocarboxyl Radical from a RCCSD(T) Study

A RCCSD(T)/cc-pVQZ potential energy surface is constructed for the HOCO radical in the ground electronic state and used to compute rotation-vibration levels of HOCO and DOCO. Two numerical strategies are employed to study in detail the wave function properties. The importance of stretch-bend coupling, such as ν4/ν5 and ν3/ν4, for the internal dynamics is demonstrated. The rotational constants computed for the vibrational ground state of trans and cis conformers are in good agreement with experimental values.

Dipole Surface and Infrared Intensities for the cis- and trans-HOCO and DOCO Radicals

The vibrational spectra for the HOCO radical in both of its conformers and deuterated isotopologues are shown here for the first time. Building on previous work with coupled cluster quartic force fields (QFFs) in the computation of the fundamental vibrational frequencies for both cis- and trans-HOCO, coupled cluster dipole surfaces are now provided for both HOCO conformers and their corresponding deuterated isotopologues. These surfaces and subsequent vibrational configuration interaction (VCI) computations produce the intensities of transitions into vibrational states including the fundamentals, overtones, and first few combination bands of less than 4000 cm(-1), slightly beyond the O-H stretch. Simulated spectra with an artificial full width at half-maximum broadening of 10 cm(-1) are also provided in order to aid in the characterization of HOCO's vibrational frequencies and to assist detection in various laboratory or astronomical observations.

Computational study of hydrogen-bonded complexes of HOCO with acids: HOCO⋯HCOOH, HOCO⋯H2SO4, and HOCO⋯H2CO3

Quantum chemistry calculations at the density functional theory (DFT) (B3LYP), MP2, QCISD, QCISD(T), and CCSD(T) levels in conjunction with 6-311++G(2d,2p) and 6-311++G(2df,2p) basis sets have been performed to explore the binding energies of open-shell hydrogen bonded complexes formed between the HOCO radical (both cis-HOCO and trans-HOCO) and trans-HCOOH (formic acid), H(2)SO(4) (sulfuric acid), and cis-cis-H(2)CO(3) (carbonic acid). Calculations at the CCSD(T)∕6-311++G(2df,2p) level predict that these open-shell complexes have relatively large binding energies ranging between 9.4 to 13.5 kcal∕mol and that cis-HOCO (cH) binds more strongly compared to trans-HOCO in these complexes. The zero-point-energy-corrected binding strengths of the cH⋯Acid complexes are comparable to that of the formic acid homodimer complex (∼13-14 kcal∕mol). Infrared fundamental frequencies and intensities of the complexes are computed within the harmonic approximation. Infrared spectroscopy is suggested as a potential useful tool for detection of these HOCO⋯Acid complexes in the laboratory as well as in various planetary atmospheres since complex formation is found to induce large frequency shifts and intensity enhancement of the H-bonded OH stretching fundamental relative to that of the corresponding parent monomers. Finally, the ability of an acid molecule such as formic acid to catalyze the inter-conversion between the cis- and trans-HOCO isomers in the gas phase is also discussed.

Vibrational frequencies and spectroscopic constants from quartic force fields for cis-HOCO: The radical and the anion

The use of accurate quartic force fields together with vibrational configuration interaction recently predicted gas phase fundamental vibrational frequencies of the trans-HOCO radical to within 4 cm(-1) of experimental results for the two highest frequency modes. Utilizing the same approach, we are providing a full list of fundamental vibrational frequencies and spectroscopic constants for the cis-HOCO system in both radical and anionic forms. Our predicted geometrical parameters of the cis-HOCO radical match experiment and previous computation to better than 1% deviation, and previous theoretical work agrees equally well for the anion. Correspondence between vibrational perturbation theory and variational vibrational configuration interaction for prediction of the frequencies of each mode is strong, better than 5 cm(-1), except for the torsional motion, similar to what has been previously identified in the trans-HOCO radical. Among other considerations, our results are immediately applicable to dissociative photodetachment experiments which initially draw on the cis-HOCO anion since it is the most stable conformer of the anion and is used to gain insight into the portion of the OH + CO potential surface where the HOCO radical is believed to form, and we are also providing highly accurate electron binding energies relevant to these experiments.

Electron Affinities, Well Depths, and Vibrational Spectroscopy of cis- and trans-HOCO

We report vibrationally resolved photoelectron spectra of internally cold HOCO(-) and DOCO(-) anions at wavelengths near and well above the detachment threshold. These spectra are dominated by a strong Franck-Condon progression of three low-energy modes of the cis isomer, the first gas-phase measurement of these vibrations. Using highly resolved, near-threshold spectra we are able to reassign the electron affinities (EAs) of cis- and trans-HOCO to 1.51 ± 0.01 and 1.37 ± 0.01 eV, respectively. Using these EAs, well depths with respect to OH + CO are determined to be 1.07 ± 0.02 eV for trans-HOCO and 0.99 ± 0.02 eV for cis-HOCO. High-level ab initio calculations show excellent agreement with all experimental results. These values will be of direct use in thermochemical calculations and will help to aid in the identification of the HOCO radical in complex reactions.

The trans-HOCO radical: Quartic force fields, vibrational frequencies, and spectroscopic constants

In the search for a full mechanism creating CO(2) from OH + CO, it has been suggested that creation of the hydroxyformyl or HOCO radical may be a necessary step. This reaction and its transient intermediate may also be responsible for the regeneration of CO(2) in such high quantities in the atmosphere of Mars. Past spectroscopic observations of this radical have been limited and a full gas phase set of the fundamental vibrational frequencies of the HOCO radical has not been reported. Using established, highly accurate quantum chemical coupled cluster techniques and quartic force fields, we are able to compute all six fundamental vibrational frequencies and other spectroscopic constants for trans-HOCO in the gas phase. These methods have yielded rotational constants that are within 0.01 cm(-1) for A(0) and 10(-4) cm(-1) for B(0) and C(0) compared with experiment as well as fundamental vibrational frequencies within 4 cm(-1) of the known gas phase experimental ν(1) and ν(2) modes. Such results lead us to conclude that our prediction of the other four fundamental modes of trans-HOCO are also quite reliable for comparison to future experimental observation, though the discrepancy for the torsional mode may be larger since it is fairly anharmonic. With the upcoming European Space Agency/NASA ExoMars Trace Gas Orbiter, these data may help to establish whether HOCO is present in the Martian sky and what role it may play in the retention of a CO(2)-rich atmosphere. Furthermore, these data may also help to clear up questions built around the fundamental chemical process of how exactly the OH + CO reaction progresses.

HOCO Radical Chemistry

Free radicals are important species in atmospheric chemistry, combustion, plasma environments, interstellar clouds, and biochemistry. Therefore, researchers would like to understand the formation mechanism, structure, stability, reactivity, spectroscopy, and dynamics of these chemical species. However, due to the presence of one or more unpaired electrons, radicals are often very reactive and have short lifetimes, which makes it difficult to conduct experiments. The HOCO radical appears in the atmosphere as well as in combustion environments and plays an important role in the conversion of CO to CO(2). Through the interplay between theoretical and experimental investigations, researchers have only recently understood the chemical role of the HOCO radical. In this Account, we systematically describe the current state of knowledge of the HOCO radical based on recent theoretical and experimental studies. This radical's two stable conformers, trans- and cis-HOCO, have been identified by high-level ab initio calculations and experimental spectroscopy. trans-HOCO is more stable by approximately 1.8 kcal/mol. The heat of formation of HOCO (298 K) was determined to be -43.0 ± 0.5 kcal/mol, giving a potential well depth of 30.1 ± 0.5 kcal/mol relative to the asymptote of the reactants OH + CO. The HOCO radical is very reactive. In most reactions between the HOCO radical and atoms, the HOCO radical acts as a hydrogen donor to reaction partners. Generally, the hydrogen is transferred through the formation of an association intermediate, which then proceeds through a molecular elimination step to produce the reaction products. The reaction rates of HOCO with some small radicals fall in the range of 10(-11)-10(-10) cm(3) molecule(-1) s(-1). These results clearly illustrate important features in the reactivity of the HOCO radical with other molecules.

Catalyzed isomerization and decarbonylation of ionized formic acid and dihydroxycarbene

Catalyzed conversion of HCOOH + into HOCOH + was studied in the cell of a FT-ICR in the presence of different molecules. The reaction of HCOOH + with SO 2, whose proton affinity (PA) lies between that of the HOCO radical at the carbon and at the oxygen sites, yields the HOCOH + carbene isomer as proved by its characteristic reaction with cyclopropane. When the PA of the catalyst lies above the highest PA of both sites of the HOCO radical, formation of HOCOH + cannot be observed since its final state lies above that corresponding to protonation of the catalyst. However, reactions of DCO 2H + and of HCO 2D +, which protonate several catalysts in an identical ratio which is very near of 1/1 at the beginning of the reaction, indicates that both ions, DCO 2H + and of HCO 2D +, convert into ion DO-C-OH + within a complex prior to protonation. The reactions of HCOOH + and HOCOH + with water were also more particularly studied by using theoretical calculations. Both reactions lead to protonated water and to the ionized water dimer which has been shown to possess the [H 2OH +⋯OH ] structure. The first step of the process is the conversion of the [HO(O)C ⋯H +⋯OH 2] complex into [HOCO ⋯H +⋯OH 2]. This latter complex undergoes two main pathways: on the one hand, it leads to protonation of water; on the other hand, it isomerizes to the [O COH⋯H +⋯OH 2] intermediate which dissociates to form [H 2OH +⋯OH ] with CO loss. Formation of H 3O + and [H 2OH +⋯OH ] being rapid, the [HOCO ⋯H +⋯OH 2] complex does not dissociate to yield the ionized carbene product which was not detected. Since the catalyzed isomerization of the 1,2-H transfer, converting HCOOH + into HOCOH +, is only observed within the corresponding complexes, this is a typical case of hidden isomerization. Finally, the differences in the unimolecular fragmentations of ionized formic acid and of its water solvated ion were explained.