Xenon Matrices Research Articles

Matrix Isolation FTIR Spectroscopic and Theoretical Study of 3,3-Dichloro-1,1,1-Trifluoropropane (HCFC-243)

The molecular structure and infrared spectrum of the atmospheric pollutant 3,3-dichloro-1,1,1-trifluoropropane (HCFC-243) were characterized experimentally and theoretically. The theoretical calculations show the existence of two conformers, with the gauche (G) and trans (T) orientation around the HCCC dihedral angle. Conformer G was calculated to be more stable than form T by more than 10 kJ mol (-1). In consonance with the large predicted relative energy of conformer T, only the G form was identified spectroscopically in cryogenic argon (10 K) and xenon (20 K) matrices prepared from room-temperature equilibrium vapor of the compound. The observed infrared spectra of the matrix-isolated HCFC-243 were interpreted with the aid of high-level density functional theory calculations and normal coordinate analysis. For experimental identification of the weakest IR absorption bands, the spectrum of HCFC-243 in the neat solid state at 145 K was obtained. This spectrum also confirmed the sole presence of the G conformer in the sample. Natural bond orbital and atomic charge analyses were carried out for the two conformers to shed light on the most important intramolecular interactions in the two conformers, in particular those responsible for their relative stability.

Low temperature IR spectroscopy and photochemistry of matrix-isolated α-pyridil

α-Pyridil [(C 6H 4NO) 2] has been isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6–311++G(d,p) calculations. Calculations predicted the existence of three different conformers exhibiting skewed conformations around the intercarbonyl bond and the two C 5H 4N C( O) fragments nearly planar. The two higher energy forms, TCG and CCSk were estimated theoretically to be, respectively, 21.0 and 35.1 kJ mol −1 higher in energy than the most stable form, TTG . In consonance with the relatively high energies predicted by the calculations for the two less stable conformers of α-pyridil, only the most stable conformer was found spectroscopically to be present in the studied matrices. Infrared spectra obtained for the neat low temperature amorphous and crystalline states reveals that the TTG conformer is also the sole conformer present in these phases. UV irradiation ( λ > 235 nm) of matrix-isolated α-pyridil led to its isomerization into unusual molecular species bearing Hückel-type pyridine (aza-benzvalene) rings.

Hydrogen atoms in solid xenon: Trapping site structure, distribution, and stability as revealed by EPR studies in monoisotopic and isotopically enriched xenon matrices

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

Reactions of β-Dioxo Compounds with Hydrogen Peroxide at Low Temperatures

The paper presents the results obtained for the malonic aldehyde—hydrogen peroxide and β-oxobutanal—hydrogen peroxide systems. The systems were studied by IR spectroscopy in xenon matrices. The potential energy surfaces and vibrational spectra of the molecules that could occur in these systems were calculated quantum-chemically. As distinct from similar systems with monoaldehydes and α-dioxo compounds, the first stage of the reactions that occurred in the systems under consideration involved the decomposition of hydrogen peroxide with the formation of atomic oxygen, whose secondary reactions determined the set of reaction products.

Infrared spectra of (HCOOH)2 and (DCOOH)2 in rare gas matrices: A comparative study with gas phase spectra

Infrared absorption spectra of (HCOOH)(2) and (DCOOH)(2) in solid argon, krypton, and xenon matrices have been measured and each fundamental band has been assigned. Spectra in Ar and Kr matrices showed notable splitting in contrast to those in Xe, which suggests a difference in structure of the trapping sites. A comparison with the reported jet-cooled spectra has shown that vibrational structures of the spectra of (HCOOH)(2) and (DCOOH)(2) in the O-H stretching region are preserved in the matrices. On the other hand, the C-O stretching band of (HCOOH)(2) shows a drastic change upon matrix isolation, wherein the Fermi-triad feature observed in gas phase [F. Ito, Chem. Phys. Lett. 447, 202 (2007)] could not be identified. No substantial change of the vibrational structure has been found for matrix-isolated (DCOOH)(2). The differences of the vibrational structures in the matrix-isolation spectra and in the jet-cooled spectra have been qualitatively accounted for using the idea of anharmonic couplings among "matrix-shifted harmonic states."

Matrix isolation spectroscopy and molecular dynamics simulations for 2,7,12,17-tetra-tert-butylporphycene in argon and xenon

Electronic absorption spectra of 2,7,12,17-tetra-tert-butylporphycene (TTPC) have been recorded in low-temperature argon and xenon matrices for various deposition conditions. In the region of the S(0)-S(1) electronic transition, the spectra of TTPC in argon reveal a rich site structure, characterized by a series of more than 30 absorption peaks. Studies of the temperature dependence of the electronic spectra in solid argon demonstrated remarkable spectral changes, resulting in the broadening of all bands with increasing temperature. These temperature-induced spectral changes are, to a large degree, reversible, so lowering of temperature is accompanied by the recovery of the original fine-line spectrum. The absorption spectra in xenon reveal broad bands, on which a rich pattern of lines becomes superimposed at low temperatures. Trapping site distribution and the structure of the microenvironment around the TTPC chromophore, embedded in argon and xenon hosts, have been analyzed using molecular dynamics (MD) simulations. The MD results show that the trapping of TTPC in rare-gas solids is influenced by favorable embedding of the bulky tert-butyl groups inside the matrix cage. The crucial role of the tert-butyl groups for the thermodynamics and kinetics of matrix deposition is demonstrated by comparing the results with those obtained for the parent, unsubstituted porphycene.

Reaction of acetaldehyde with hydrogen peroxide at low temperatures: IR spectra in xenon matrices and calculations of intermediates and transition states

The products of the reaction between acetaldehyde and hydrogen peroxide were studied by measuring their IR spectra. Density functional calculations were performed for intermediates and stable reaction products and for saddle points of elementary processes possible in this system. It was shown that two products could be formed at the first stage, the hydroperoxide molecule and CH3CO radical.

A matrix isolation and theoretical study of SiF 4 dimers spectra

The absorption spectra of (SiF 4) 2 dimers were studied in argon, xenon and nitrogen matrices. The dimer ν X, Y , ν Z bands due to resonance dipole–dipole interaction between two SiF 4 subunits were identified. The ab initio calculations of the structure, energies and vibrational frequencies for the different conformations of (SiF 4) 2 dimer were performed using the second-order Møller–Plesset perturbation theory. The model with the point dipole moments located on each of Si–F bonds has been developed. This model allows to calculate the dimers spectra at various orientation of SiF 4 molecules. On transition from the gas to low temperature matrix the symmetry of (SiF 4) 2 dimer changes from C 2h to D 2d, and R Si–Si distance decreases by 0.3 Å.

Matrix isolation and low temperature solid state FTIR spectroscopic study of α-furil

Alpha-furil [C(4)H(3)O-C(=O)-C(=O)-C(4)H(3)O] has been isolated in argon and xenon matrices and studied by FTIR spectroscopy, supported by DFT(B3LYP)/6-311++G(d,p) calculations. The obtained spectra were fully assigned and revealed the presence in the matrices of three different conformers, all of them exhibiting skewed conformations around the intercarbonyl bond with the two C(4)H(3)O-C(=O) fragments nearly planar. The three conformers differ in the orientation of the furan rings relative to the carbonyl groups: the most stable conformer, I (C(2) symmetry; O=C-C=O intercarbonyl dihedral equal to 153.1 degrees), has both furan rings orientated in such a way that one of their beta-hydrogen atoms approaches the oxygen atom of the most distant carbonyl group, forming two H-C=C-C-C=O six-membered rings; the second most stable conformer, II (C(1) symmetry; O=C-C=O intercarbonyl dihedral equal to 126.9 degrees ), has one furan ring orientated as in I, while the second furan group is rotated by ca. 180 degrees (resulting in an energetically less favourable H-C=C-C=O five-membered ring); in the third conformer, III (C(2) symmetry; O=C-C=O dihedral equal to 106.2 degrees ), both furan rings assume the latter orientation relative to the dicarbonyl group. The theoretical calculations predicted the two higher energy forms being 5.85 and 6.22 kJ mol(-1) higher in energy than the most stable form, respectively, and energy barriers for conformational interconversion higher than 40 kJ mol(-1). These barriers are high enough to prevent observation of conformational isomerization for the matrix isolated compound. The three possible conformers of alpha-furil were also found to be present in CCl(4) solution, as well as in a low temperature neat amorphous phase of the compound prepared from fast condensation of its vapour onto a suitable 10 K cooled substrate. On the other hand, in agreement with the available X-ray data [S. C. Biswas, S. Ray and A. Podder, Chem. Phys. Lett., 1987, 134, 541], the IR spectra obtained for the neat low temperature crystalline state reveals that, in this phase, alpha-furil exists uniquely in its most stable conformational state, I.

Matrix Isolation Fourier Transform Infrared Study of Photodecomposition of Formimidic Acid

The UV isomerization of formamide (HCONH2) trapped in xenon, nitrogen, argon, and neon cryogenic matrices has been monitored by Fourier transform infrared (FT-IR) spectroscopy. Formamide monomer is the only species present in the matrices after deposition; when UV-selective irradiation was carried out at 240 nm, the n --> pi transition allowed us to observe the formation of several isomers of formimidic acid [H(OH)C=NH]. On these latter species, we carried out selective IR irradiation of their OH stretching mode and compared the experimental and theoretical (B3LYP/6-311+G(2d,2p)) sets of bands. This study allowed us to characterize for the first time all the isomers of formimidic acid. We have then studied the vacuum UV photodecomposition (lambda > 160 nm) of this molecule at 10 K in argon and xenon matrices. Several primary photoproducts such as HCN.H2O, HNC.H2O, and HNCO.H2 complexes, yielded by dehydration and dehydrogenation processes, were characterized.

Molecular dynamics simulations of matrix deposition. III. Site structure analysis for porphycene in argon and xenon

Porphycene (1) and porphyrin (2), two constitutional isomers, reveal completely different electronic spectral patterns in argon and xenon matrices. For the former the spectra recorded in the two solidified gases resemble each other, whereas for the latter they are completely different. This difference can be rationalized by molecular-dynamics simulations of the structure of the microenvironment carried out for the two chromophores embedded in argon and xenon hosts. For 1, the structure of the main substitutional site is the same for Ar and Xe and consists of a hexagonal cavity obtained by removing seven host atoms from the [111] crystallographic plane. An analogous structure is obtained for 2 in xenon. However, in argon the porphyrin chromophore environment is shared between several different sites, with the number of replaced host atoms ranging from seven to ten. These results demonstrate that a relatively minor structural alternation may lead to major changes in the spectral pattern of molecules embedded in rare-gas cryogenic matrices.

Infrared spectra of water clusters in krypton and xenon matrices

The infrared absorption spectra of the water molecules and small water clusters, (H(2)O)(n) with n = 2-6, trapped in solid argon, krypton, and xenon matrices have been investigated. The infrared bands of the water clusters with n = 5 and 6 in krypton and n = 3, 4, 5, and 6 in xenon matrices have been identified for the first time in the bonded OH stretching region. The frequency shifts in the bonded OH stretching band of the water dimer and trimer in xenon matrices show fairly large deviations to the red from the empirical correlation between the matrix shifts and the square root of the critical temperatures of the matrix material. The observed anomalous shifts suggest that the water dimer and trimer in solid xenon are trapped in multiple sites, and that the structures of the preferential trapping sites are different from those in argon and krypton matrices.

Matrix Isolation FTIR Spectroscopic and Theoretical Study of Dimethyl Sulfite

The preferred conformations of dimethyl sulfite and their vibrational spectra were studied by matrix-isolation Fourier transform infrared spectroscopy and theoretical methods (density functional theory (DFT) and Moller-Plesset (MP2), with basis sets of different sizes, including the quadruple-zeta, aug-cc-pVQZ basis). Five minima were found at these levels of theory. At the MP2/6-31++G(d,p) and DFT/B3LYP/aug-cc-pVQPZ levels, the GG conformer (where the O-S-O-C dihedral angles are 73.2 and 70.8 degrees ) resulted in the conformational ground state. At the highest level of theory used, the GT conformer (O-S-O-C = +68.5 and -173.2 degrees ) is 0.83 kJ mol(-1) higher in energy than the GG form, while conformer GG' (O-S-O-C = +85.7 and -85.7 degrees ) has a relative energy of 1.18 kJ mol(-1). The remaining two conformers (G'T and TT) are high-energy forms and not experimentally relevant. In consonance with the theoretical predictions, conformer GG was found to be the most stable conformer in the gaseous phase as well as in the low-temperature matrices. Annealing of the argon matrices first promotes the GG'-->GT isomerization, which is followed by conversion of GT into the most stable conformer. There is no evidence of occurrence of GG'-->GG direct conversion in the low-temperature matrices. On the other hand, during deposition of the xenon matrices conformer GG' totally converts to conformer GT. Two observations demonstrated this fact: no evidence of bands corresponding to GG' were observed in xenon matrices and the GG/GT intensity ratio became similar to the GG/(GT + GG') intensity ratio observed in argon matrices. All these results could be explained by taking into account the relative values of the theoretically predicted energy barriers for the different isomerization processes: GG'-->GT, 1.90 kJ mol(-1); GT-->GG, 9.64 kJ mol(-1); and GG'-->GG, 19.46 kJ mol(-1).

Conformational properties of trimethyl phosphate monomer

Conformational isomerism of trimethylphosphate (TMP) has been studied de novo. TMP monomers were isolated in low temperature xenon matrices at different substrate temperatures and characterized using FTIR spectroscopy. At the lowest temperature, three different conformers, with overall molecular symmetries C 3, C 1 and C s, were shown to co-exist in the matrix. Increase of the sample temperature led to conformational cooling, with the two minor conformers being depopulated successively, following an order of increasing stability. Initially, the less stable C s conformer converted to the C 1 form and, at higher temperatures, the C 1 conformer converted to the global energy minimum, C 3. Interpretation of the experimental data was supported by extensive calculations of the vibrational spectra and barriers to the conformational interconversion.

Matrix isolation FTIR spectroscopic and theoretical study of methyl lactate

Methyl lactate [CH(CH 3)OHC(O)OCH 3] conformational space was investigated by extensive DFT/B3LYP/6-311++G(d,p) and MP2/6-31++G(d,p) calculations and matrix-isolation FTIR spectroscopy in argon and xenon matrices. From the seven different conformers predicted by the calculations, two forms (the most stable conformers yielded by the calculations: SsC and GskC, where the letters refer to the conformations assumed by the HOCC, OCCO and OCOC dihedrals, respectively) were observed and characterized spectroscopically. Conformer interconversion pathways were also theoretically investigated. The low calculated barrier associated with the G′ sk′ C→ GskC conversion (∼1 kJ mol −1) explains the absence of the third most stable form ( G′ sk′ C) of methyl lactate in low-temperature matrices. Deposition of methyl lactate at different temperatures, together with the theoretical data, aided to the full assignment of the observed spectra.

Thioacetylacetone: structural and vibrational assignments.

Thioacetylacetone and its variously deuterated isotopomers have been investigated using electronic and vibrational spectroscopy combined with quantum chemical calculations. Thioacetylacetone is known for its photochromic properties, but the structures of the initial and final forms have been the subject of a long debate. Analysis of the IR spectra recorded in low-temperature argon and xenon matrices, room-temperature solutions, and in the gas phase has allowed us to establish the nature of the photochromic species and of its precursor. Similar to the case of another beta-thioxoketone, monothiodibenzoylmethane, the photo-product has been assigned to the nonchelated SH exo-rotamer of the (Z)-enethiol tautomeric form, whereas the dominant ground-state species corresponds to the chelated (Z)-enol tautomeric form. Detailed vibrational assignments have been proposed for both forms based on quantum chemical calculations and polarization experiments. In the case of the chelated (Z)-enol species prevailing in the ground state, a second-order perturbative anharmonic analysis at the B3LYP/cc-pVTZ level indicated strong anharmonic effects associated with the intramolecular hydrogen bond, leading to a shift of more than 600 cm-1 of the wavenumber of the OH-stretching vibration. A small fraction of the SH endo-rotameric chelated (Z)-enethiol form was also detected under unperturbed conditions. The (Z)-enethiol form can be converted into the (Z)-enol form by irradiation at 290 nm.

First experimental evidence of the third conformer of glycolic acid: combined matrix isolation, FTIR and theoretical study

The FT-IR spectra of monomeric glycolic acid, GA, isolated in argon and xenon matrices (at 10 and 20 K) were studied, enabling identification of three different conformers of GA. The most stable SSC conformer involves a hydrogen bond between the carbonyl group and the hydrogen atom of the alcohol hydroxyl group. The minor GAC and AAT forms involve intramolecular hydrogen bonds between alcohol and carboxyl OH groups, with alcohol and carboxylic groups serving as proton donors, respectively. The GAC conformer, predicted by theoretical calculations (at the DFT/B3LYP and MP2 levels with the 6-31++G(d,p) and aug-cc-pVDZ basis sets) as the second most stable form, was now experimentally identified for the first time.

Rotational isomers of lactic acid: first experimental observation of higher energy forms

Lactic acid {2-hydroxypropionic acid [CH(CH3)OHCOOH]} monomer was studied by matrix isolation FT-IR spectroscopy and molecular orbital calculations undertaken at both DFT(B3LYP)/6-311++G(d,p) and MP2/6-31G(d,p) levels of approximation. The theoretical calculations predicted the conformer (SsC) with the carboxylic group and both the CCOH(alcohol) and OCCO moieties in a cis configuration as the most stable form. In this conformer, the α-hydroxy hydrogen atom is involved in an intramolecular hydrogen bond with the carbonyl oxygen atom. The second most stable conformer (GskC) also shows a cis carboxylic group and differs from SsC in the CCOH(alcohol) and OCCO dihedral angles, which are equal to 43.3° and 156.8° respectively. These angles are equal to −51.5° and −149.9° in the third most stable conformer (G′sk′C). These forms are characterized by showing a relatively weak intramolecular H(alcohol)⋯O(acid) hydrogen bond. In the AaT conformer, the carboxylic group is trans and the CCOH(alcohol) and OCCO dihedral angles are 162.2° and 174.1°, respectively. This conformer shows a relatively strong H(acid)⋯O(alcohol) hydrogen bond. In consonance with the theoretical results, the matrix isolation experiments confirmed the predominance of conformer SsC in argon and xenon matrices, and provide the first experimental evidence of conformers GskC and AaT. Since the barrier for interconversion G′sk′C ↔ GskC is only ∼2 kJ mol−1, these two conformers are in equilibrium in the matrices and, at low temperature, the population of the less stable G′sk′C form is too small to enable its observation. Full assignment of the observed spectra was undertaken on the basis of comparison with the theoretical data and temperature variation studies. Water doping of matrices enabled the identification of spectral features due to weakly bound complexes of lactic acid with water.

Molecular dynamics simulations of matrix deposition. I. Site structure analysis for porphyrin in argon and xenon

Classical molecular dynamics methodology was applied for the simulation of rare gas matrix deposition and for the structural analysis of the environment around porphyrin embedded in argon or xenon. The results confirm the experimental observations of several trapping sites in argon, as opposed to one dominant location in xenon. The main site in xenon has been assigned to a symmetrical hexagonal cavity, obtained after removing seven host atoms from the {111} crystallographic plane. Differences between argon and xenon matrices are due to the fact that, for the former, a larger number of host atoms have to be removed from the crystal in order to accommodate the porphyrin molecule. This results in less stable and more perturbed site structure in argon with respect to xenon.

Spectroscopic studies of 13CO 2 trapped in rare gas matrices

Low temperature (5 K) high resolution (0.15 and 0.03 cm −1) absorption spectra of 13CO 2 have been recorded in neon, argon, krypton, and xenon matrices, in the ν 3 and ν 2 regions. Diffusion experiments have been performed in krypton and xenon in order to identify vibrational traps which could be responsible for the decrease and shortening of the emission observed after laser excitation: high-frequency structures in the ν 3 region are assigned to dimers and a doubling of the monomer line is due to a site effect. In neon, only a double substitutional site, with a splitting of the degenerate ν 2 vibration, is observed. In argon, as previously reported, a single and a double site are characterized. In krypton and xenon, where ν 2 is not split, only single sites would be predicted. As one of them exhibits a ν 3 line highly sensitive to temperature, we expect a large coupling with the lattice and a fast vibrational relaxation. This site is very likely the vibrational trap we are looking for.