Argon Matrix Research Articles

Structural and FTIR spectroscopic studies of matrix-isolated 3-thio-1,2,4-triazole complexes with dinitrogen. First experimental evidence for the UV-induced formation of thiol⋯N2 complexes

Complexes of 3-thio-1,2,4-triazole (ST) with N2 were studied theoretically by the B3LYP-D3 method and experimentally by FTIR spectroscopy combined with the matrix isolation technique and UV laser irradiation. The computational results show that ST interacts specifically with dinitrogen in several different ways. For the 1:1 complexes of the most abundant ST thione tautomer, three stable minima have been located on the potential energy surface. The two most stable structures, containing both the N-H⋯N hydrogen bond and the S⋯N van der Waals contact, were detected experimentally in the argon matrix after deposition. Interestingly, it was found that laser irradiation at λ = 270 nm, apart from generating the thiol tautomer of ST, also leads to the formation of its corresponding complexes, characterized by N-H⋯N hydrogen bonds. What is more, it appeared that the efficiency of hydrogen atom transfer in ST complexed with N2 is lower than in the monomeric form.

Progress towards the FRIB-EDM[formula omitted]-Frontend: A tool to provide radioactive molecules from isotope harvesting for fundamental symmetry studies

The under-construction FRIB-EDM3-instrument was designed to study polar radioactive molecules (such as RaF) in transparent cryogenic solids by laser spectroscopy. The instrument is divided into a frontend- and a backend section. The frontend accepts an aqueous sample from isotope harvesting and provides a mass-separated molecular ion beam in an ultra-high vacuum environment. In the backend, the ions are guided into alkali-metal vapor and the resulting neutrals are co-deposited in a solid argon matrix to perform laser spectroscopy. This work addresses the frontend of the instrument. The efficient ionization of harvested radioisotopes from aqueous samples is achieved with a spray-ionization method. Subsequently, the molecular ion beam is analyzed by mass-to-charge ratio by a quadrupole mass filter. To verify the feasibility of the approach, numerical simulations with the COMSOL and SIMION packages have been conducted. While the former was applied to study transport in ion funnels, the latter was used to investigate ion beam transmission through the lower pressure sections. Following promising simulation results, a first experimental setup is under construction.

Conformational-Dependent Photodissociation of Glycolic Acid in an Argon Matrix

Ultraviolet-induced photodissociation and photo-isomerization of the three most stable conformers (SSC, GAC, and AAT) of glycolic acid are investigated in a low-temperature solid argon matrix using FTIR spectroscopy and employing laser radiation with wavelengths of 212 nm, 226 nm, and 230 nm. The present work broadens the wavelength range of photochemical studies of glycolic acid, thus extending the understanding of the overall photochemistry of the compound. The proposed kinetic model for the photodissociation of glycolic acid proceeds from the lowest energy conformer (SSC). The model suggests that ultraviolet light induces isomerization only between the SSC and GAC conformers and between the SSC and AAT conformers. The relative reaction rate coefficients are reported for all proposed reactions. These results suggest that the direct photodissociation of GAC and AAT conformer does not occur in an argon matrix. The main photodissociation channel via the SSC conformer produces formaldehyde–water complexes. The proposed photodissociation mechanism emphasizes that the conformers’ relative abundancies can significantly affect the photodissociation rate of the molecule. For example, in the case of high relative GAC and AAT concentrations, the ultraviolet photodissociation of glycolic acid requires the proceeding photo-isomerization of GAC and AAT to SSC.

A Metastable Ketenyl Radical-Water Complex from UV Photolysis of the Carboxymethyl Radical.

The unpaired electron impacts the binding between radicals and ordinary closed-shell molecules in noncovalent complexes. Conversely, the complexation partner can enhance, decrease, or even control the reactivity of the interacting radical. Previously, such radical-molecule (and especially radical-water) complexes were studied by controlled assembly of the interacting partners which mostly leads to formation of the thermodynamically most stable species. Here, we show that UV photolysis of the resonance-stabilized carboxymethyl radical isolated in a cryogenic argon matrix at 4 K leads to the intermediary formation of a metastable, noncovalent complex of the ketenyl radical with a water molecule. In this complex, the ketenyl radical binds water at its terminal carbon atom, although a more stable isomer exists in which water interacts with the C-H bond of the radical. Rigorous W1 theory computations confirm that the ketenyl radical is a stronger donor in C-H···O interactions than ketene itself, while it performs comparably well as an acceptor. We propose that complex formation proceeds via an initial excited-state C-O bond breaking reaction in carboxymethyl under release of an OH radical, which is supported by multireference QD-NEVPT2 computations.

Calculation of the local environment of a barium monofluoride molecule in an argon matrix: a step towards using matrix-isolated BaF for determining the electron electric dipole moment

The local environment of a barium monofluoride (BaF) molecule embedded in an argon matrix is calculated. A substitution of a BaF molecule for four Ar atoms is found to be strongly favoured compared to substitutions for other numbers of Ar atoms. The equilibrium positions of the BaF molecule and its nearby Ar neighbours are found by minimising the total energy. The potential barrier that prevents the migration of the BaF molecule within the solid and the barrier that prevents its rotation are calculated. At the cryogenic temperatures used by the EDM collaboration, these barriers are sufficiently large to fix the position and orientation of the molecule. Knowledge of the local environment of matrix-isolated BaF molecules is essential for the EDM collaboration, which is using them in a precision measurement of the electron electric dipole moment.

IR Spectrometric Studies of CCL4 and Ar Gas Mixtures

IR spectrometric studies were performed to examine the cryo-deposition processes and properties of thin cryo-vacuum condensate films containing a mixture of carbon tetrachloride and argon, which were obtained by physical cryo-vacuum deposition at various concentrations. The measurements were carried out in a temperature range of 11–100 K, and the pressure of the gas phase was 10–8 Torr-10–4 Torr. The film thickness in all experiments performed was d = 2 μm. The vibrational spectra of CCl4 in an argon matrix were measured in a frequency range of 400–4200 cm−1 at mixture concentrations (CCl4 + Ar) of 10–90%, 5–95%, and 1–99% and after the argon matrix underwent sublimation. Thermal desorption curves for the yield of the matrix gas (Ar) were obtained in the temperature range of 40–90 K for the (CCl4 and Ar) system. It was found that the CCl4 molecules in the Ar matrix substance underwent cryocapture. The results obtained suggest that carbon tetrachloride can be used as a matrix in low-temperature studies on self-organization processes in thin films.

Dual Photochemistry of Benzimidazole.

Monomers of benzimidazole trapped in an argon matrix at 15 K were characterized by vibrational spectroscopy and identified as 1H-tautomers exclusively. The photochemistry of matrix-isolated 1H-benzimidazole was induced by excitations with a frequency-tunable narrowband UV light and followed spectroscopically. Hitherto unobserved photoproducts were identified as 4H- and 6H-tautomers. Simultaneously, a family of photoproducts bearing the isocyano moiety was identified. Thereby, the photochemistry of benzimidazole was hypothesized to follow two reaction pathways: the fixed-ring and the ring-opening isomerizations. The former reaction channel results in the cleavage of the NH bond and formation of a benzimidazolyl radical and an H-atom. The latter reaction channel involves the cleavage of the five-membered ring and concomitant shift of the H-atom from the CH bond of the imidazole moiety to the neighboring NH group, leading to 2-isocyanoaniline and subsequently to the isocyanoanilinyl radical. The mechanistic analysis of the observed photochemistry suggests that detached H-atoms, in both cases, recombine with the benzimidazolyl or isocyanoanilinyl radicals, predominantly at the positions with the largest spin density (revealed using the natural bond analysis computations). The photochemistry of benzimidazole therefore occupies an intermediate position between the earlier studied prototype cases of indole and benzoxazole, which exhibit exclusively the fixed-ring and the ring-opening photochemistries, respectively.

A sulfur monoxide complex of platinum fluoride with a positively charged ligand.

A sulfur monoxide complex of platinum fluoride in the form of PtF2(η1-SO) was generated via the isomerization of a molecular complex Pt(SOF2) in cryogenic matrixes under UV-vis irradiation. The infrared absorptions observed at 1205.4, 619.8 and 594.9 cm-1 are assigned to the S-O, antisymmetric and symmetric F-Pt-F stretching vibrations of the PtF2(η1-SO) complex, which possesses nonplanar Cs symmetry with a singlet ground state according to density functional theory calculations. The experimental vibrational frequency and computed distance (1.449 Å) of the SO ligand indicate that the SO ligand features a positively charged character, which is further confirmed by natural bond orbital analysis and Mayer bond order. Such character is completely different from that for early transition metal-SO complexes and dioxygen complexes of platinum. Formation of the PtF2(η1-SO) complex was found to occur via the consecutive transfer of the two fluorine atoms from SOF2 to Pt in the sulfur bound Pt(SOF2) complex, which involves a series of intermediates on the basis of the mechanism study at the B3LYP level. Although the whole process is hindered by the large energy barrier encountered during the transfer of the first fluorine atom, UV-vis irradiation can provide sufficient energy to surmount this barrier and facilitates the formation of the nonplanar PtF2(η1-SO) complex stabilized in matrix.

The enol of propionic acid.

We demonstrate the gas-phase synthesis of prop-1-ene-1,1-diol, the hitherto unreported higher energy tautomer of propionic acid. The enol was trapped in an argon matrix and characterized by IR and UV/Vis spectroscopy in combination with density functional theory computations. Upon photolysis, the enol rearranges to propionic acid and methylketene.

Infrared Spectra and Phototransformations of meta-Fluorophenol Isolated in Argon and Nitrogen Matrices

Monomers of meta-fluorophenol (mFP) were trapped from the gas phase into cryogenic argon and nitrogen matrices. The estimated relative energies of the two conformers are very close, and in the gas phase they have nearly equal populations. Due to the similarity of their structures (they only differ in the orientation of the OH group), the two conformers have also similar predicted vibrational signatures, which makes the vibrational characterization of the individual rotamers challenging. In the present work, it has been established that in an argon matrix only the most stable trans conformer of mFP exists (the OH group pointing away from the fluorine atom). On the other hand, the IR spectrum of mFP in a nitrogen matrix testifies to the simultaneous presence in this matrix of both the trans conformer and of the higher-energy cis conformer (the OH group pointing toward the fluorine atom), which is stabilized by interaction with the matrix gas host. We found that the exposition of the cryogenic N2 matrix to the Globar source of the infrared spectrometer affects the conformational populations. By collecting experimental spectra, either in the full mid-infrared range or only in the range below 2200 cm-1, we were able to reliably distinguish two sets of experimental bands originating from individual conformers. A comparison of the two sets of experimental bands with computed infrared spectra of the conformers allowed, for the first time, the unequivocal vibrational identification of each of them. The joint implementation of computational vibrational spectroscopy and matrix-isolation infrared spectroscopy proved to be a very accurate method of structural analysis. Some mechanistic insights into conformational isomerism (the quantum tunneling of hydrogen atom and vibrationally-induced conformational transformations) have been addressed. Finally, we also subjected matrix-isolated mFP to irradiations with UV light, and the phototransformations observed in these experiments are also described.

Direct evidence for formation of acetimidic acid and acetamide under irradiation of isolated acetonitrile–water complexes at cryogenic temperatures

ABSTRACTIn 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.

Infrared spectrum of CF2+ cation in a solid argon matrix

The vibrational spectrum of CF2+ 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 CF2+, respectively.

Structure, conformational landscape and UV-induced selective rotamerization of 2-aminoacetanilide

The molecular structure, conformational space and UV-induced rotamerization and decarbonylation of 2-aminoacetanilide (2AA) were investigated by matrix isolation infrared spectroscopy and quantum chemical electronic structure calculations. The compound was isolated in a cryogenic (15 K) argon matrix, and four conformers were spectroscopically identified: two trans-amide (OCNH dihedral ∼ 180°) conformers (trans-I and trans-II), and two higher-energy cis-amide (OCNH dihedral ∼ 0°) conformers (cis-I and cis-II). These four conformers were found to be present in the as-deposited matrix of 2AA in a population ratio matching well that predicted by the calculations for the gas-phase equilibrium at the temperature of the vapor of the compound before deposition (60 °C = 333.15 K). The calculations predicted also the existence of an additional higher-energy trans-amide conformer (trans-III) which was not observed experimentally. In situ broadband UV irradiation (λ > 235 nm) of the matrix-isolated compound was found to induce selective conversion of conformer trans-I into conformer trans-II, in a few minutes (55 % after 2 min. of irradiation; 70 % after 10 min.), while the populations of the cis-amide conformers did not change. Prolonged UV irradiation was found to result in decomposition of the compound, leading to generation of carbon monoxide and 2-amine-N-methylaniline. The infrared spectra of the experimentally relevant conformers of 2AA were interpreted and assigned with help of normal coordinate analysis, and the different behavior of the conformers of 2AA upon UV-irradiation of the argon matrix was explained with help of time-dependent DFT (TD-DFT) calculations.

Spectroscopy of Alkali Atoms in Solid Matrices of Rare Gases: Experimental Results and Theoretical Analysis

We present an experimental and theoretical investigation of the spectroscopy of dilute alkali atoms in a solid matrix of inert gases at cryogenic temperatures, specifically Rubidium atoms in a solid Argon or Neon matrix, and related aspects of the interaction energies between the alkali atoms and the atoms of the solid matrix. The system considered is relevant for matrix isolation spectroscopy, and it is at the basis of a recently proposed detector of cosmological axions, exploiting magnetic-type transitions between Zeeman sublevels of alkali atoms in a magnetic field, tuned to the axion mass, assumed in the meV range. Axions are one of the supposed constituents of the dark matter (DM) of the Universe. This kind of spectroscopy could be also relevant for the experimental search of new physics beyond the Standard Model, in particular the search of violations of time-reversal or parity-charge-conjugation (CP) symmetry. In order to efficiently resolve the axion-induced transition in alkali-doped solid matrices, it is necessary to reduce as much as possible the spectral linewidth of the electronic transitions involved. The theoretical investigation presented in this paper aims to estimate the order of magnitude of the inhomogeneous contribution to the linewidth due to the alkali–matrix interactions (Coulomb/exchange and dispersion), and to compare the theoretical results with our experimental measurements of spectra of dilute Rubidium atoms in Argon and Neon solid matrix. The comparison of the expected or measured spectral linewidths will be important for selecting the most appropriate combination of alkali atoms and matrix inert elements to be used in the proposed axion detection scheme. It is finally suggested that dilute Lithium atoms diffused in a cold parahydrogen solid matrix could be, overall, a good system upon which the proposed detector could be based.

Matrix Isolation Study of Fumaric and Maleic Acids in Solid Nitrogen.

Fumaric and maleic acids ((E)- and (Z)-HOOC-CH═CH-COOH, FA and MA) were studied experimentally by infrared spectroscopy in nitrogen matrixes and theoretically by quantum chemical calculations. The calculations, carried out at the DFT(B3LYP) and MP2 levels of theory, predicted the existence of at least 5 conformers of maleic acid and 10 conformers of fumaric acid. After the deposition of the matrixes, two conformers of maleic acid (I and II) and three conformers (I-III) of fumaric acid were observed and characterized vibrationally. Selective narrowband near-infrared (NIR) excitation of the first OH stretching overtones of the different conformers of maleic and fumaric acids initially present in the matrixes allowed the generation of higher-energy forms, never before observed experimentally. In the case of maleic acid, conformers I (a cis-trans form, where cis and trans designate the conformation of the carboxylic groups of the molecule) and II (cis-cis) were found to generate the novel conformers VI (trans-trans) and VII (cis-trans), respectively. The conversion of conformer II into the most stable conformer I was also observed. For fumaric acid, the cis-cis conformers I-III were found to give rise to the new cis-trans conformers IV-VII, respectively. The tunneling decay of the new conformers produced upon NIR excitation of the lowest-energy conformers initially trapped in the matrixes was observed, and their lifetimes in solid N2 were determined. The increased stability of all of the observed high-energy conformers of the studied acids in the N2 matrix, compared to the argon matrix, where they could not be observed experimentally, demonstrates the stabilizing effect of the interaction between the OH groups of the acids with the matrix N2 molecules, in line with previous observations for other carboxylic acids. In addition, the photochemistry of matrix-isolated maleic and fumaric acids upon broad-band UV irradiation (λ > 235 nm) was also investigated. UV-induced isomerization of both acids around the C═C double bond was observed, together with their decarboxylation to acrylic acid.

Weakly Bound Complex Formation between HCN and CH3Cl: A Matrix-Isolation and Computational Study.

The matrix-isolated infrared spectrum of a hydrogen cyanide-methyl chloride complex was investigated in a solid argon matrix. HCN and CH3Cl were co-condensed onto a substrate held at 10 K with an excess of argon gas, and the infrared spectrum was measured using Fourier-transform infrared spectroscopy. Quantum chemical geometry optimization, harmonic frequency, and natural bonding orbital calculations indicate stabilized hydrogen- and halogen-bonded structures. The two resulting weakly bound complexes are both composed of one CH3Cl molecule bound to a (HCN)3 subunit, where the three HCN molecules are bound head-to-tail in a ring formation. Our study suggests that─in the presence of CH3Cl─the formation of (HCN)3 is promoted through complexation. Since HCN aggregates are an important precursor to prebiotic monomers (amino acids and nucleobases) and other life-bearing polymers, this study has astrophysical implications toward the search for life in space.

Conformational composition of propanol in gaseous state and in matrix isolation

Conformational analysis of the experimentally recorded IR absorption spectra of propanol in gaseous state and in a low-temperature argon matrix at 20 K and at 35 K was carried out for different spectral ranges. It showed that the conformational composition of the samples in gas and in matrix isolation is different. In gaseous propanol Gt conformers predominate, while in matrix isolation the most energetically favorable form is Tg conformer, which prevails in percentage at both considered temperatures of an Ar matrix.

Water Clusters in Interaction with Corannulene in a Rare Gas Matrix: Structures, Stability and IR Spectra

The interaction of polycyclic aromatic hydrocarbons (PAHs) with water is of paramount importance in atmospheric and astrophysical contexts. We report here a combined theoretical and experimental study of corannulene-water interactions in low temperature matrices and of the matrix’s influence on the photoreactivity of corannulene with water. The theoretical study was performed using a mixed density functional based tight binding/force field approach to describe the corannulene-water clusters trapped in an argon matrix, together with Born-Oppenheimer molecular dynamics to determine finite-temperature IR spectra. The results are discussed in the light of experimental matrix isolation FTIR spectroscopic data. We show that in the solid phase, π isomers of (C20H10)(H2O)n, with n = 2 or 3, are energetically favored. These π complexes are characterized by small shifts in corannulene vibrational modes and large shifts in water bands. These π structures, particularly stable in the case of the water trimer where the water cluster is trapped “inside” the corannulene bowl, may account for the difference in photoreactivity of non-planar–compared to planar–PAHs with water. Indeed, planar PAHs such as pyrene and coronene embedded in H2O:Ar matrices form σ isomers and react with water to form alcohols and quinones under low energy UV irradiation, whereas no photoreactivity was observed for corannulene under the same experimental conditions.

Propanol Clustering in Argon Matrix: 2D FTIR Correlation Spectroscopy

The dynamic FTIR spectra of propanol in the argon matrix have been measured using a thermal perturbation by heating the sample from T = 11 K to 30 K stepping by 1 K. The 2D Fourier-transform infrared (FTIR) absorption correlation analysis has been carried out, the main attention being concentrated to the region of propanol O–H stretching vibrations at 3000–3700 cm–1. The peaks of monomers, two conformers of open dimer, and the cyclic structures from a dimer to a pentamer have been resolved and studied in more details. Analyzing the dependences of the integral band intensities of various aggregates on the temperature, it has been deduced that monomers and dimers act in the initial clustering stage as the main "building units" whose diffusion sustains the formation of the higher H-bond structures in the matrix. The full width at half height (FWHH) for each band has been processed as afunction of the aggregation number (n). It is found that the FHWW is a perfectly linear function of n for all cyclic aggregates n ≥ 2. The resonance broadening has been proposed to be the most reliable mechanism for the formation of diffuse O–H stretching bands in the matrix isolated clusters.

UV-induced -OCH3 rotamerization in a matrix-isolated methoxy-substituted ortho-hydroxyaryl Schiff base.

A new methoxy-substituted ortho-hydroxyaryl Schiff base, 4-(3-methoxy-2-hydroxybenzylidene-amino) phenol was synthesized from 4-aminophenol and 2-hydroxy-3-methoxybenzaldehyde in methanol solution and characterized by 1H-NMR, 13C-NMR and infrared spectroscopies and elemental analysis. The compound was isolated in a cryogenic (10K) argon matrix, and the analysis of the infrared spectrum of the matrix-isolated compound revealed that it corresponds to the E-enol-imine isomeric form, with 3 different conformers being present in the matrix. These conformers share as common structural features the conformation of the free hydroxyl group (trans relatively to the para-substituent of the ring) and the presence of an OH…N intramolecular H-bond involving the methoxy-substituted phenol ring and the azomethine bridge, while they differ in the orientation of the methoxy-substituent group. The structures and relative energies of the conformers of the molecule, and relevant barriers for their interconversion were obtained through quantum chemical calculations, which were also used to calculate the infrared spectra of the different forms. Calculations were also carried out for the higher-energy Z-enol-imine and keto-amine forms of the compound. Upon UV (230nm) irradiation, -OCH3 rotamerization was observed, leading to conversion of the lowest energy conformer, where the methoxy group is aligned with the plane of the ring, into the other two conformers initially present in the matrix, in which the OCH3 group is out-of-the-plane of the ring. As for other phenolic compounds previously studied, spontaneous quantum mechanical tunneling conversion of the cis-OH conformers present in the gas-phase into the three observed conformers was found to take place during matrix deposition.