Xenon Matrices Research Articles

4-hydroxyquinolin-2(1H)-one isolated in cryogenic argon and xenon matrices: Tautomers and photochemistry

4-Hydroxyquinolin-2(1H)-one (4HQ2O) was synthesized, isolated in cryogenic matrices (argon and xenon), and studied by infrared spectroscopy. Quantum chemical calculations carried out at the DFT(B3LYP)/6–311++G(3df,3pd) level of theory were used to determine the conformational and tautomeric properties of the molecule. Two tautomeric forms were identified in the as-deposited matrices with the help of the theoretical data. To investigate the photochemistry of the compound, in situ broadband ultraviolet (λ > 283 nm) irradiation of the as-deposited argon matrix was performed. This irradiation led to the generation of an additional tautomer, together with the products of fragmentation of the heterocyclic ring of the molecule, specifically isocyanic acid and carbon monoxide. Photoproducts such as 1,3-dihydro-2H-indol-2-one and cyclohepta-1,2,4,6-tetraene were also observed in the photolyzed argon matrix. A comprehensive assignment of the infrared spectra of all the species observed experimentally is presented.

Phenylpropiolic acid isolated in cryogenic nitrogen and xenon matrices: NIR and UV-induced study.

Phenylpropiolic acid (C6H5C≡CCOOH, PPA) isolated in nitrogen and xenon cryogenic matrices was studied by infrared spectroscopy. The experimental studies were complemented by a series of quantum chemical calculations carried out at the density functional theory (B3LYP) and MP2 levels of theory (with different basis sets). The calculations predicted the existence of two planar PPA conformers, differing in the arrangement of the carboxylic group. The higher-energy trans-PPA conformer has a negligible population in the gas phase at room temperature and was prepared in situ in the N2 cryomatrix through vibrationally-induced rotamerization of the lower-energy cis-PPA conformer, achieved using selective narrowband infrared excitation of the OH stretching coordinate of the latter species. Broadband UV (λ > 235nm) irradiation of matrix-isolated cis-PPA was also undertaken, leading to the observation of cis-PPA → trans-PPA isomerization. No other UV-induced photoreactions were observed. The in situ generated trans-PPA conformer was found to decay back to cis-PPA in the dark by tunneling, and its lifetimes under different experimental conditions were determined. The assignment of the infrared spectra of both conformers is presented, considerably extending the vibrational information available on this molecule.

Radiation-induced transformation of the C2H2⋯NH3 complex in cryogenic media: Identification of C2H2⋯NH2∙ complex and evidence of cold synthetic routes

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 5K 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 -60.6cm-1 for Ar, Kr, and Xe, respectively. Additionally, in krypton and xenon matrices, the blue-shifted features in the CHasym bend 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 2.51kcal mol-1, respectively). We believe that the C2H2⋯NH2 ∙ complex may be an important intermediate in cold synthetic astrochemistry.

Phosphorescence of C5N− in Rare Gas Solids

Phosphorescence of C5N− was discovered following the ArF-laser (193 nm) photolysis of cyanodiacetylene (HC5N) isolated in cryogenic argon, krypton, and xenon matrices. This visible emission, with an origin around 460 nm, is vibrationally resolved, permitting the measurement of frequencies for eight ground-state fundamental vibrational modes, including the three known from previous IR absorption studies. Phosphorescence lifetime amounts to tens or even hundreds of ms depending on the matrix host; it is five times longer than in the case of HC5N.

Evidence of IR-Induced Chemistry in a Neat Solid: Tautomerization of Thiotropolone by Thermal, Electronic, and Vibrational Excitations.

Thiotropolone isolated in argon and xenon matrices (as monomers) or in a neat solid (as the crystalline or amorphous state) at low temperature was found to exist only in the thione-enol form. Visible light irradiation (λ ≥ 400 nm) leads to thione-enol → thiol-keto tautomerization in matrices and under neat solid conditions at 15 K. The assignment of the IR spectra of the two thiotropolone tautomers (thione-enol and thiol-keto) was carried out with the support of B3LYP/6-311+G(2d,p) computations. The thiol-keto form generated in situ in a neat solid was found to tautomerize back to the thione-enol upon annealing up to 100 K. Gaussian-4 (G4) computations estimate that such a tautomerization process has an energy barrier of ∼25 kJ mol-1, which is consistent with the observations. Moreover, it was found that narrowband IR irradiation of the thiol-keto form in a neat solid, at the frequency of its CH stretching overtones/combination modes, also induces tautomerization to the thione-enol form. Such a result constitutes an important demonstration of vibrationally induced chemistry under neat solid conditions.

Structure and IR Spectra of 3(5)-Aminopyrazoles and UV-Induced Tautomerization in Argon Matrix.

The prototropic tautomerism in 3(5)-aminopyrazoles was investigated by matrix isolation infrared (IR) spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. In consonance with the experimental data, the calculations predict tautomer 3-aminopyrazole (3AP) to be more stable than the 5-aminopyrazole (5AP) tautomer (calculated energy difference: 10.7 kJ mol−1; Gibbs free energy difference: 9.8 kJ mol−1). The obtained matrix isolation IR spectra (in both argon and xenon matrices) were interpreted, and the observed bands were assigned to the tautomeric forms with help of vibrational calculations carried out at both harmonic and anharmonic levels. The matrix-isolated compound (in argon matrix) was then subjected to in situ broadband UV irradiation (λ > 235 nm), and the UV-induced transformations were followed by IR spectroscopy. Phototautomerization of the 3AP tautomer into the 5AP form was observed as the strongly prevalent reaction.

Hidden Isomer of Trifluoroacetylacetone Revealed by Matrix Isolation Infrared and Raman Spectroscopy.

Enol forms of trifluoroacetylacetone (TFacac) isolated in molecular and rare gas matrices were studied using infrared (IR) and Raman spectroscopy. Additionally, calculations using DFT B3LYP and M06-2X as well as MP2 methods were performed in order to investigate the possibility of coexistence of more than one stable enol form isomer of TFacac. Calculations predict that both stable enol isomers of TFacac, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one (1) and 5,5,5-trifluoro-4-hydroxy-3-penten-2-one (2), could coexist, especially in matrices where the room temperature population is frozen, 1 being the most stable one. Raman and IR spectra of TFacac isolated in nitrogen (N2) and carbon monoxide (CO) matrices exhibit clear absorption bands, which cannot be attributed to this single isomer. Their relative band positions and intensity profiles match well with the theoretical calculations of 2. This allows us to confirm that in N2 and CO matrices both isomers exist in similar amounts. Careful examination of the spectra of TFacac in argon, xenon, neon, normal, and para-hydrogen (Ar, Xe, Ne, nH2, and pH2 respectively) matrices revealed that both isomers coexist in all the explored matrices, whereas 2 was not considered in the previous spectroscopic works. The amount of the second isomer (2) in the as-deposited samples depends on the host. The analysis of TFacac spectra in the different hosts and under various experimental conditions allows the vibrational characterization of both chelated isomers. The comparison with theoretical predictions is also investigated.

Carbene-insertion noble gas compounds: FKrCF and FXeCF

FKrCF and FXeCF were synthetized in solid krypton and xenon matrices at cryogenic temperatures. These molecules representing a new class of noble-gas compounds are formed as a result of reaction between mobile F atoms and trapped CF fragment in solid Kr and Xe, which occur after X-ray radiolysis of CH2F2 in the corresponding matrices. The assignment was based on comparison with previously published computational data (Chopra et al., Chem. Phys., 2017, 494, 20) and the results of additional quantum chemical calculations. New molecules were found to be sensitive to near-IR, visible and UV light.

UV-induced conformational isomerization and photochemistry of 3-chloro-4-methoxybenzaldehyde in cryogenic inert matrices.

The interest in investigating the structure and reactivity of halogen- and methoxy-substituted benzaldehydes has been motivated by the practical relevance of these substances in various fields, from the fragrance industry to agrochemicals and drug manufacturing. In this study, 3-chloro-4-methoxybenzaldehyde (3CMBA) was investigated by matrix isolation infrared spectroscopy and quantum chemistry calculations. Molecules of the compound were isolated in cryogenic argon and xenon matrices and the conformational composition of the as-deposited materials characterized by infrared spectroscopy. Two nearly degenerated conformers, differing by the orientation of the aldehyde group relatively to the chlorine ring substituent, were successfully trapped into the matrices from the room temperature vapor of the compound, the cis conformer being slightly higher in energy than the trans form. Both conformers belong to the Cs point group and have the methoxy group pointing to the opposite direction of the chlorine atom. The as-deposited matrices of 3CMBA were then irradiated by tunable monochromatic UV-light. Irradiation within the 302-299 nm range induced conversion of the cis into the trans conformer, while irradiation in the 298-296 nm range induced the inverse photoisomerization. Irradiation at higher energy (250 nm) was found to lead to decarbonylation of both conformers. Very interestingly, the observed UV-induced processes were found to take place in an essentially selective way as a function of the irradiation wavelength. In particular, the conformational isomerization reactions of 3CMBA occur with only a residual number of molecules undergoing simultaneous decarbonylation.

Phototransformations of 2-(1,2,4-Triazol-3-yl)benzoic Acid in Low Temperature Matrices.

Phototransformations of 2-(1,2,4-triazol-3-yl)benzoic acid induced by tunable UV laser were studied in low temperature matrices by FTIR spectroscopy. Out of 36 possible isomers of this molecule the most stable one, comprising the intramolecular N-H···O hydrogen bond, was detected experimentally in argon and xenon matrices after deposition. Upon irradiation with λ = 325 nm, two new conformers of the precursor were formed. The corresponding reverse photorotamerizations were also detected at 250 nm partly reproducing the initial molecules. In addition to the conformational changes, an interesting photoisomerization took place in the studied matrices leading to a proton transfer from N atom of the triazole ring to the carbonyl oxygen of the carboxylic group. As a result of this reaction, a new product with two OH groups was identified for the first time. The experimental studies were supported by DFT calculations in both ground and excited electronic states.

The role of dispersion and anharmonic corrections in conformational analysis of flexible molecules: the allyl group rotamerization of matrix isolated safrole.

Conformational changes of the monomeric safrole (5-(2-propenyl)-1,3-benzodioxole) isolated in low temperature xenon matrices were induced thermally or using narrow-band UV radiation. The rotation of the allyl group taking place in the studied matrices was followed by FTIR spectroscopy. Safrole represents a challenging example of a flexible molecule highlighting the importance of dispersion interactions and anharmonic effects in the structural, spectroscopic and energetic analysis. Structures of the safrole conformers, their energetics and infrared spectra have been calculated using various computational methods ranging from density functional theory (DFT) to coupled cluster (CC). The best theoretical results were obtained by integrating CCSD(T) energies including complete basis set extrapolation and core-valence corrections with B2PLYP-D3 equilibrium structures and hybrid B2PLYP-D3/B3LYP-D3 anharmonic computations for IR spectra and thermodynamics.

UV laser-induced photolysis of matrix isolated o-guaiacol

A matrix isolation study of structure and infrared spectra of o-guaiacol (o-methoxyphenol, GU) has been performed showing that the most stable conformer of the compound is present in argon and xenon matrices. Photochemical reactions of o-guaiacol were induced using UV-laser radiation and followed by the FTIR spectroscopy. Four photolysis pathways have been considered based on the positions of new bands appearing during irradiation of the matrices. It was concluded that the cleavage of the OH bond is an initial step of the reactions and a number of photoproducts have been identified originating from the subsequent ring opening and decarbonylation processes as well as the OCH3 bond cleavage. Among the photoproducts are cyclopetadienone and long-chain ketenes with conjugated/cumulated CO and CC bonds.

Medium-induced change of the optical response of metal clusters in rare-gas matrices

Interaction with the surrounding medium modifies the optical response of embedded metal clusters. For clusters from about ten to a few hundreds of silver atoms, embedded in rare-gas matrices, we study the environment effect within the matrix random phase approximation with exact exchange (RPAE) quantum approach, which has proved successful for free silver clusters. The polarizable surrounding medium screens the residual two-body RPAE interaction, adds a polarization term to the one-body potential, and shifts the vacuum energy of the active delocalized valence electrons. Within this model, we calculate the dipole oscillator strength distribution for Ag clusters embedded in helium droplets, neon, argon, krypton, and xenon matrices. The main contribution to the dipole surface plasmon red shift originates from the rare-gas polarization screening of the two-body interaction. The large size limit of the dipole surface plasmon agrees well with the classical prediction.

Photoisomerization of azobenzenes isolated in cryogenic matrices.

2,2'-Dihydroxyazobenzene (DAB), 2,2'-azotoluene (AT) and azobenzene (AB) were isolated in argon and xenon matrices and their molecular structures and photochemical transformations were characterized by infrared spectroscopy and theoretical calculations. All these compounds can adopt the E and Z isomeric forms around the central CNNC moiety, which can be enriched by several conformational and tautomeric modifications for DAB and AT. A number of DAB and AT isomeric forms were identified for the first time. For DAB, the E azo-enol isomer with two intramolecular six-membered quasi-rings formed via OHN hydrogen bonds was found after deposition. Irradiation with UV light generated a different E azo-enol form with two intramolecular H-bonded five-membered quasi-rings. Phototransformation was shown to be reversible and the forms could be interconverted by irradiation at different wavelengths. The isomerization between these two forms constitutes a direct experimental observation of an E → E isomerization in azobenzene-type molecules. Further irradiation generated a form(s) bearing both OH and NH groups. For AT, two E isomers with the CH3 groups forming five-membered and five/six-membered quasi-rings with the azo group were observed in the as-deposited matrices. Irradiation of AT with UV light generated a Z form that can be converted back to the E form at different irradiation wavelengths. E-AB was deposited in a xenon matrix and both E → Z and Z → E phototransformations were observed (contrary to what was previously reported in an argon matrix where only the Z → E conversion occurred). AB photoisomerization becomes more pronounced at elevated temperatures, thus indicating that the matrix effects responsible for hindering the AB photoisomerization are essentially due to steric restrictions. The different photoisomerization channels observed for these compounds are discussed in terms of a small-amplitude pedal motion.

Infrared spectra and UV-induced photochemistry of methyl aziridine-2-carboxylate isolated in argon and xenon matrices

Methyl aziridine-2-carboxylate (MA2C) has been isolated in low temperature argon and xenon matrices and its structure and photochemistry were studied by FTIR spectroscopy. The reactant as well as the main photoproducts were characterized by comparison of their experimental IR spectra with spectra calculated at the DFT(B3LYP)/6-311++G(d,p) level. The theoretical calculations predicted the existence of two low energy MA2C conformers, differing by the orientation of the OCCN dihedral angle. Both conformers were identified in the studied matrices. Both narrowband tunable and broadband UV irradiations of matrix-isolated MA2C yielded isomerization photoproducts resulting from cleavage of the CC and weakest CN bonds of the aziridine ring. Irradiation with UV laser-light at λ=235nm resulted in the formation of the E isomer of methyl 2-(methylimino)-acetate (MMIA) and the Z isomer of methyl 3-iminopropanoate (M3IP). Subsequent irradiation at 290nm led to observation of new bands resulting from E→Z isomerization of MMIA, while bands due to M3IP remained unchanged. The photoproduced Z isomer of MMIA could be subsequently consumed upon higher-wavelength irradiation (λ=330nm). The initially produced MMIA conformer was found to obey the nonequilibrium of excited rotamers (NEER) principle. No photoproducts resulting from the cleavage of the strongest CN bond of the MA2C aziridine ring were observed, nor that of methyl 3-aminoacrylate (M3AA), which could in principle be obtained also by cleavage of the weakest CN bond of the MA2C aziridine ring, but would imply a different H-atom migration simultaneous with the ring opening process. These results indicate that both the differential electronic characteristics of the CN bonds of substituted aziridine rings and the type of required H-atom migration are major factors in determining the specific photochemistries of substituted aziridines. Photofragmentation reactions of MA2C were also observed, through identification of various related products, e.g., acetonitrile, methanol, methane, CO and CO2.

Spectroscopy of Ba andBa+deposits in solid xenon for barium tagging in nEXO

Progress on a method of barium tagging for the nEXO double beta decay experiment is reported. Absorption and emission spectra for deposits of barium atoms and ions in solid xenon matrices are presented. Excitation spectra for prominent emission lines, temperature dependence and bleaching of the fluorescence reveal the existence of different matrix sites. A regular series of sharp lines observed in Ba$^+$ deposits is identified with some type of barium hydride molecule. Lower limits for the fluorescence quantum efficiency of the principal Ba emission transition are reported. Under current conditions, an image of $\le10^4$ Ba atoms can be obtained. Prospects for imaging single Ba atoms in solid xenon are discussed.

Conformational preferences of 3,4-dihydroxyphenylacetic acid (DOPAC)

The conformational space of 3,4-dihydroxyphenylacetic acid (DOPAC), an important dopamine metabolite, has been investigated by quantum chemical methods (B3LYP and MP2, with the 6-311++G(d,p) basis set) and matrix-isolation infrared spectroscopy. Detailed analysis of the calculated potential energy surfaces of the molecule led to identification of thirteen unique conformers, all of them showing the acetic acid side chain out of the aromatic ring plane by 60–95°. According to the calculated Gibbs energies, the five lowest energy conformers make up 99.7% of the conformational mixture at 298.15K, exhibiting individual populations falling between 16% and 24%. The main conformational trends of this molecule were interpreted on the grounds of a thorough analysis of the structural parameters and by the application of the Natural Bond Orbital theory. The role of the intramolecular interactions on the relative stability and structure of the conformers was also investigated. The infrared spectrum of DOPAC was registered after isolation of its monomers in argon and xenon matrices. Only one of DOPAC forms populated in the gas phase could be trapped in both matrix gases. This result is in agreement with the predicted low energy barriers for conformational isomerization and is also supported by annealing experiments. The spectra of matrix-isolated model compounds, phenylacetic acid and catechol, were studied under the same experimental conditions. These data were used as references and assisted in the interpretation of the results obtained for DOPAC.

Near-infrared and ultraviolet induced isomerization of crotonic acid in N2 and Xe cryomatrices: first observation of two high-energy trans C-O conformers and mechanistic insights.

E-crotonic acid was isolated in cryogenic solid N2 and xenon matrices, and subjected to Laser ultraviolet (UV) and near-infrared (NIR) irradiations. In the deposited matrices, the two low-energy cis C-O E-cc and E-ct conformers, which are the only forms significantly populated in the gas phase, were observed. UV irradiation (λ= 250 nm) of the compound in N2 matrix allows for experimental detection, not just of the two low-energy cis C-O isomers of Z-crotonic acid previously observed in the experiments carried out in argon matrix [Z-cc and Z-ct; R. Fausto, A. Kulbida, and O. Schrems, J. Chem. Soc., Faraday Trans. 91, 3755-3770 (1995)] but also of the never observed before high-energy forms of both E- and Z-crotonic acids bearing the carboxylic acid group in the trans arrangement (E-tc and Z-tc conformers). In turn, NIR irradiation experiments in the N2 matrix allow to produce the high-energy E-tc trans C-O conformer in a selective way, from the initially deposited E-cc form. The vibrational signatures of all the 6 rotameric structures of the crotonic acids experimentally observed, including those of the new trans C-O forms, were determined and the individual spectra fully assigned, also with support of theoretically obtained data. On the other hand, as found before for the compound isolated in argon matrix, the experiments performed in xenon matrix failed to experimental detection of the trans C-O forms. This demonstrates that in noble gas matrices these forms are not stable long enough to allow for their observation by steady state spectroscopy techniques. In these matrices, the trans C-O forms convert spontaneously into their cis C-O counterparts, by tunnelling. Some mechanistic details of the studied processes were extracted and discussed.

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.

Matrix isolation infrared spectroscopic and theoretical study of 1,1,1-trifluoro-2-chloroethane (HCFC-133a)

The molecular structure and infrared spectrum of the atmospheric pollutant 1,1,1-trifluoro-2-chloroethane (HCFC-133a; CF3CH2Cl) in the ground electronic state were characterized experimentally and theoretically. Excited state calculations (at the CASSCF, MR-CISD, and MR-CISD+Q levels) have also been performed in the range up to ~9.8 eV. The theoretical calculations show the existence of one (staggered) conformer, which has been identified spectroscopically for the monomeric compound isolated in cryogenic (~10 K) argon and xenon matrices. The observed infrared spectra of the matrix-isolated HCFC-133a were interpreted with the aid of MP2/aug-cc-pVTZ calculations and normal coordinate analysis, which allowed a detailed assignment of the observed spectra to be carried out, including identification of bands due to different isotopologues ((35)Cl and (37)Cl containing molecules). The calculated energies of the several excited states along with the values of oscillator strengths and previous results obtained for CFCs and HCFCs suggest that the previously reported photolyses of the title compound at 147 and 123.6 nm [T. Ichimura, A. W. Kirk, and E. Tschuikow-Roux, J. Phys. Chem. 81, 1153 (1977)] are likely to be initiated in the n-4s and n-4p Rydberg states, respectively.