Infrared matrix-isolation spectra of monomeric oxalic acid

Infrared spectra of monomeric oxalic acid-h₂, -hd and -d₂ have been observed using neon matrix-isolation spectroscopy. In addition, spectra of oxalic acid-h₂, vapor were obtained using a heated absorption cell with a 10 m pathlength. All IR active fundamentals are assigned for oxalic acid-h₂, and -d₂, except the low frequency torsion. The spectra are interpreted in terms of the C_2h intramolecularly hydrogen bonded model. Two vibrational assignments are discussed; one involves a very large intensity for the v₈ + v₁₁ (COH torsion) combination band of oxalic acid-h₂. Tentative values for several Raman active fundamentals of oxalic acid-h₂ are suggested using possible combination bands. Few fundamentals of oxalic acid-hd correlating with the Raman modes of the symmetric monomers could be observed. A force constant analysis of the by secular block is presented.     

Infrared spectra of the CF3I dimer: a concurrent application of matrix-isolation spectroscopy and cavity ring-down spectroscopy

We have observed infrared spectra of the CF(3)I dimer produced in a supersonic jet by matrix-isolation Fourier transform infrared spectroscopy and infrared cavity ring-down (IR-CRD) spectroscopy. In the matrix-isolation experiments, the dimer was isolated in an Ar matrix by the pulse-deposition method. The recorded spectral range covers the symmetric (nu(1)) and doubly degenerate (nu(4)) C-F stretching regions. From the concentration dependence of the matrix-isolation spectra we have assigned one dimer band for each fundamental region. It was not easy to identify the dimer band for the nu(4) band because of the multiplet feature of the monomeric nu(4) band caused by the site symmetry breaking. The spectra of (CF(3)I)(2) in the nu(4) band region were thus also measured in the gas phase by IR-CRD spectroscopy, where we detected two dimer bands. Comparing the observed band positions with the results of quantum chemical calculations, we have assigned the observed dimer bands to the head-to-head isomer. The structure of (CF(3)I)(2) and its photochemical implications are discussed, in comparison with methyl iodide dimer reported previously [Ito et al., Chem. Phys. Lett. 343, 185 (2001)].     

Raman amplification spectroscopy using pulsed dye lasers

A picosecond laser system consisting of a mode-locked argon-ion laser synchronously pumping two dye lasers is used for studies of Raman amplification spectra. The two dye laser beams, one kept constant in frequency while the other is tunable, coincide in the Raman sample. Recording the gain or the loss in intensity of one of the lasers as a function of frequency difference produces the Raman spectrum. Good signal to noise ratios have been obtained for a variety of liquids and solids. Fluorescing samples can be studied in the Inverse Raman method where the loss on the higher frequency laser is monitored.     

Intramolecular hydrogen atom tunneling in 2-chlorobenzoic acid studied by low-temperature matrix-isolation infrared spectroscopy

Intramolecular hydrogen atom tunneling in 2-chlorobenzoic acid has been investigated by low-temperature matrix-isolation infrared spectroscopy with the aid of density functional theory calculation. Infrared spectra of two relatively stable syn isomers, SC and ST, were observed in argon and xenon matrixes. When the matrix samples were annealed after deposition, the isomerization from ST to SC occurred around the benzene-carboxyl bond. Two less stable anti isomers, AT, which has an OH...Cl intramolecular hydrogen bond, and AC, which has no OH...Cl bond, were produced from SC and ST upon UV irradiation. When the matrix samples were kept in the dark after UV irradiation, AT and AC changed to ST and SC, respectively, by spontaneous isomerization around the C-O axis in the carboxyl group. The rate constants of isomerization, AT --> ST, in a Xe matrix were estimated from the absorbance changes at various matrix temperatures. The rate constants showed a drastic decrease in deuteration of the hydrogen atom of the carboxyl group. The relationship between the rate constants and the matrix temperature did not follow the Arrhenius law. These findings lead to the conclusion that the isomerization of AT --> ST and AC --> SC in low-temperature rare-gas matrixes proceeds through intramolecular hydrogen atom tunneling.     

Infrared absorption of CH3SO2 observed upon irradiation of a p-H2 matrix containing CH3I and SO2

Irradiation with a mercury lamp at 254 nm of a p-H(2) matrix containing CH(3)I and SO(2) at 3.3 K, followed by annealing of the matrix, produced prominent features at 633.8, 917.5, 1071.1 (1072.2), 1272.5 (1273.0, 1273.6), and 1416.0 cm(-1), attributable to ν(11) (C-S stretching), ν(10) (CH(3) wagging), ν(8) (SO(2) symmetric stretching), ν(7) (SO(2) antisymmetric stretching), and ν(4) (CH(2) scissoring) modes of methylsulfonyl radical (CH(3)SO(2)), respectively; lines listed in parentheses are weaker lines likely associated with species in a different matrix environment. Further irradiation at 365 nm diminishes these features and produced SO(2) and CH(3). Additional features at 1150.1 and 1353.1 (1352.7) cm(-1) are tentatively assigned to the SO(2) symmetric and antisymmetric stretching modes of ISO(2). These assignments are based on comparison of observed vibrational wavenumbers and (18)O- and (34)S-isotopic shifts with those predicted with the B3P86 method. Our results agree with the previous report of transient IR absorption bands of gaseous CH(3)SO(2) at 1280 and 1076 cm(-1). These results demonstrate that the cage effect of solid p-H(2) is diminished so that CH(3) radicals, produced via UV photodissociation of CH(3)I in situ, might react with SO(2) to form CH(3)SO(2) during irradiation and upon annealing. Observation of CH(3)SO(2) but not CH(3)OSO is consistent with the theoretical predictions that only the former reactions proceed via a barrierless path.     

Infrared spectroscopy of SO2 aqueous solutions

The chemistry of SO2 solutions was studied with infrared transmission and total internal reflection (ATR) spectroscopy. The cross-sections of the SO2 stretching vibrations v1, v3 and the combination band of v1 + v3 were obtained and found to be slightly different from their gas phase values. Six features we associate with sulfur-containing ions were observed. We present the first infrared evidence for the bisulfite ion HOSO2- through detection of its OH stretching vibration. Other features may be associated with HSO3-, another isomer of the bisulfite ion, and a new compound we propose to be H2S2O6(2-). We found no evidence for sulfurous acid H2SO3.     

Conformational study of monomeric 2,3-butanediols by matrix-isolation infrared spectroscopy and DFT calculations

The FT-IR spectra of two diastereomers of 2,3-butanediol, (R,S) and (S,S), isolated in low-temperature argon and xenon matrixes were studied, allowing the identification of two different conformers for each compound. These conformers were characterized by a +/-gauche arrangement around the O-C-C-O dihedral angle, thus enabling the establishment of a very weak intramolecular hydrogen bond of the O...H-O type. No other forms of these compounds were identified in matrixes, despite the fact that these four conformers had calculated relative energies from 0 to 5.1 kJ mol(-1) and were expected to be thermally populated from 50 to 6% in the gaseous phase of each compound. The nonobservation of additional conformers was explained in terms of low barriers to intramolecular rotation, resulting in the conformational relaxation of the compounds during deposition of the matrixes. The barriers to internal rotation of the OH groups were computed to be less than 4 kJ mol(-1) and are easily overcome in matrixes within the family of conformers with the same heavy atom backbone. The barriers for intramolecular rearrangement of the O-C-C-O dihedral angle in both diastereomers were calculated to range from 20 to 30 kJ mol(-1). Interconversions between the latter conformers were not observed in matrixes, even after annealing up to 65 K. Energy calculations, barriers, and calculated infrared spectra were carried out at the DFT(B3LYP)/6-311++G theory. Additional MP2/6-311++G calculations of energies and vibrational frequencies were performed on the most relevant conformers. Finally, independent estimations of the hydrogen-bond enthalpy in the studied molecules were also obtained based on theoretical structural data and from vibrational frequencies (using well-established empirical correlations). The obtained values for -DeltaH for both diastereomers of 2,3-butanediol amount to ca. 6-8 kJ mol(-1).     

Solidification of small p-H2 clusters at zero temperature

We have determined the ground-state energies of para-H(2) clusters at zero temperature using the diffusion Monte Carlo method. The liquid or solid character of each cluster is investigated by restricting the phase through the use of proper importance sampling. Our results show inhomogeneous crystallization of clusters, with alternating behavior between liquid and solid phases up to N = 55. From there on, all clusters are solid. The ground-state energies in the range N = 13-75 are established, and the stable phase of each cluster is determined. In spite of the small differences observed between the energy of liquid and solid clusters, the corresponding density profiles are significantly different, a feature that can help to solve ambiguities in the determination of the specific phase of H(2) clusters.     

Molecular complexes of cyclopropane with hydrogen halides and water studied by matrix-isolation vibrational spectroscopy

Infrared spectra of cyclopropane-hydrogen halide and cyclopropane-water complexes show similar perturbations of the cyclopropane modes, suggesting that all have the proton donor “hydrogen-bonded” to the ring edge. The spectrum of the cyclopropane-water complex is consistent with water forming a bifurcated hydrogen bond to the ring.     

Conformational study of eugenol by density functional theory method and matrix-isolation infrared spectroscopy

The B3LYP/6-311++G(2d,2p) study of the potential energy surface of eugenol (4-allyl-2-methoxyphenol, 2-methoxy-4-pro-2-emyl-phenol) was performed with the aim of finding all possible conformers of the molecule. Twelve conformers were found belonging to one of three groups differing in the relative orientation of the OH and OCH 3 moieties: SA (syn-anti), AA (antianti) and AG (antigauche). The lowest-energy conformers of eugenol (SAA+, SAA- and SAS) stabilized by the intramolecular hydrogen bond differ only in the arrangement of the allyl group with respect to the aromatic ring. The calculated abundance of all three SA species equals 99.8% whereas the remaining AA and AG show the negligible population of 0.2%. In consonance with theoretical predictions, only syn-anti conformers are present in the low temperature matrices studied. The presented FTIR results allow, for the first time, unequivocal identification and spectral characterization of three SA conformers of the eugenol molecule isolated in solid argon and xenon. The performed studies reveal that conformational cooling (upon increasing the substrate temperature during deposition) takes place in the studied matrices and that the less stable SAA- and SAS species convert into SAA+. This observation appears to be consistent with the theoretically predicted energy barriers of 6.70 and 10.45 kJ/mol for the SAA- --> SAA+ and SAS --> SAA+ interconversions which are low enough to be surpassed during deposition at higher temperatures.     

Characterisation of historic plastics using terahertz time-domain spectroscopy and pulsed imaging

Terahertz (THz) time-domain spectroscopy and 3D THz pulsed imaging have been explored with regard to polymer materials, both commodity and historic polymers. A systematic spectroscopic study of a wide range of different polymer materials showed significant differences in their spectra. Polyolefins and polystyrenes generally exhibit lower absorption than other examined polymers, various cellulose derivates, poly(vinyl chloride), poly(methyl methacrylate), polyamide, hard rubber and phenol formaldehyde resin, the last of these exhibiting the most intense absorption over the entire range, 0.15–4.2 THz. It was also examined how the presence of plasticisers in poly(vinyl chloride), the presence of fillers in polypropylene, and the degree of branching in polyethylene and polystyrene affect the spectra; inorganic fillers in polypropylene affected the absorption most. With 3D THz pulsed imaging, features in polymer objects were explored, appearing either as integral parts of the material (coatings and pores in foams) or as a consequence of physical deterioration (cracks, delamination). All of these features of various complexities can be successfully imaged in 3D. Terahertz technology is thus shown to have significant potential for both chemical and structural characterisation of polymers, which will be of interest to heritage science, but also to the polymer industry and development of analytical technologies in general.     

簇合物(p-H2)N-HCCCN的结构和能量

采用两体作用势模型和遗传算法对簇合物(p-H2)NHCCCN的极小能量结构和能量进行了理论研究.结果表明,20个para-H2分子形成HCCCN周围的第一个溶剂层,第一个溶剂层包含三个溶剂环,每个溶剂环都有6个para-H2分子,第19和20个para-H2分子分别聚集在HCCCN分子的N、H原子末端.进一步计算了(p-H2)N-HCCCN的化学势,发现化学势随para-H2分子个数的增加呈震荡变化.     

簇合物(p-H2)N-HCCCN的结构和能量

采用两体作用势模型和遗传算法对簇合物(p-H2)N-HCCCN的极小能量结构和能量进行了理论研究.结果表明,20个para-H2分子形成HCCCN周围的第一个溶剂层,第一个溶剂层包含三个溶剂环,每个溶剂环都有6个para-H2分子,第19和20个para-H2分子分别聚集在HCCCN分子的N、H原子末端.进一步计算了(p-H2)N-HCCCN的化学势,发现化学势随para-H2分子个数的增加呈震荡变化.     

Photolysis of regioisomeric diazides of 1,2-diphenylacetylenes studies by matrix-isolation spectroscopy and DFT calculations

A series of diazides of 1,2-diphenylacetylenes was photolyzed in matrices at low temperature and transient photoproducts were characterized by using IR, UV/vis methods combined with ESR studies. Theoretical calculations were also used to understand the experimental findings. The introduction of phenylethynyl groups on phenyl azides has little effect on the photochemical pathway. Thus, upon photoexcitation, (phenylethynyl)phenyl azides afforded the corresponding triplet nitrene, which is in photoequilibrium with the corresponding azacycloheptatetraene. In marked contrast, azidophenylethynyl groups exhibited a dramatic effect not only on the photochemical pathway of phenyl azides but also on the electronic and molecular structure of the photoproducts. The patterns of the effect depended upon the relative position of azide groups in the diphenylacetylene unit. Whenever two azide groups were situated in a conjugating position with respect to each other, as in p,p'-, o,o'-, and p,o'-bis(azides), the azides always resulted in the formation of a quinoidal diimine diradical in which unpaired electrons were extensively delocalizedin the pi-conjugation. The situation changed rather dramatically when azide groups were introduced in the meta position. Thus, the formation of azacycloheptatetraene was noted in the photolysis of the m.m'-isomer. ESR studies indicated the generation of a quintet state that was shown to be a thermally populated state with a very small energy gap of ca. 100 cal mol(-1). The m,p'-isomer was shown to be an excellent precursor for the high-spin quintet dinitrene. The IR spectra of the photoproduct showed no bands ascribable to azacycloheptatetraene. The observed spectra were in good agreement with that calculated for the quintet state. Strong EPR signals assignable to the quintet state were observed, along with rather weak signals due to mononitrenes. Moreover, the quintet bis(nitrene) was rather photostable under these conditions.     

Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding by matrix-isolation infrared spectroscopy

Experimental evidence for intramolecular blue-shifting C-H...O hydrogen bonding is presented. Argon matrix-isolation infrared spectra of 1-methoxy-2-(dimethylamino)ethane exhibit a band at 3016.5 cm-1. Spectral behavior with annealing indicates that this band is assigned to the most stable conformer, trans-gauche-(trans|gauche'), with an intramolecular C-H...O hydrogen bond. Density functional calculations show that this band arises from the stretching vibration of the C-H bond participating in the formation of the C-H...O hydrogen bond. The C-H bond is shortened by 0.004 A, and the C-H stretching band is blue-shifted by at least 35 cm-1 on the formation of the hydrogen bond. The (C)H...O distance is calculated as 2.38 A, which is shorter than the corresponding van der Waals separation by 0.3 A.     

Diminished cage effect in solid p-H2: Infrared absorption of CH3S observed from photolysis in situ of CH3SH, CH3SCH3, or CH3SSCH3 isolated in p-H2 matrices

We report the infrared absorption spectrum of the methylthio (or thiomethoxy) radical, CH(3)S (X (2)E(3/2)), produced via photodissociation in situ of three precursors CH(3)SH, CH(3)SCH(3), and CH(3)SSCH(3) isolated in solid p-H(2). The common absorption features observed with similar intensity ratios in each experiment are assigned to CH(3)S. The wavenumbers of these features agree satisfactorily with those predicted with a spin-vibronic Hamiltonian accounting for the anharmonic effects and the Jahn-Teller effects to the quartic term [A. V. Marenich and J. E. Boggs, J. Chem. Theory Comput. 1, 1162 (2005)]. In addition to an absorption line at 724.2?cm(-1), corresponding to a transition of 3(1) previously determined to be 727?cm(-1) from fluorescence spectra of gaseous CH(3)S, we identified fundamental transitions 6(1)(a(1)) at 771.1, 6(1)(e) at 1056.6, 5(1)(a(1)) at 1400.0, 4(1)(a(1)) at 2898.4?cm(-1), and several combination and overtone transitions. In contrast, photolysis of CH(3)SSCH(3) isolated in solid Ar produces mainly H(2)CS, CH(3)SH, and CS(2), but no CH(3)S. These results demonstrate the feasibility of using photolysis in situ of precursors isolated in solid p-H(2) to produce free radicals by taking advantage of the diminished cage effect of the matrix.     

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.     

Dynamics of intramolecular hydrogen-atom migrations in 2-hydroxy-3-nitropyridine studied by low-temperature matrix-isolation infrared spectroscopy and density functional theory calculation

Intramolecular hydrogen-atom migrations in 2-hydroxy-3-nitropyridine have been investigated by low-temperature matrix-isolation infrared (IR) spectroscopy with the aid of density functional theory (DFT) calculation. An IR spectrum measured after deposition was assigned to an enol isomer, the conformation of which is anti in relation to OH versus N in the pyridine ring. When the matrix sample was exposed to UV and visible light (lambda>350 nm), an IR spectrum consistent with a keto product was observed. During the irradiation, an IR spectrum of a transient species, a photoreaction intermediate between anti-enol and keto, was observed, which was assigned to syn-enol. The bands of syn-enol disappeared completely when the irradiation was stopped, while those of the original isomer, anti-enol, reappeared. No reverse isomerization was observable in the corresponding deuterated species. This led to the conclusion that the isomerization from syn to anti occurs through hydrogen-atom tunneling. On the other hand, an aci-nitro form was produced by UV irradiation (lambda=365+/-10 nm) without visible light. The conformation around the aci-nitro group was determined to be cis-cis by comparison with the spectral patterns obtained by the DFT/B3LYP/6-31++G** calculation. The dynamics of the hydrogen-atom migrations between anti- and syn-enols, syn-enol and keto, and anti-enol and aci-nitro are discussed in terms of the potential surfaces obtained by the DFT calculation.     

Analysis of reaction products in the oxidation reactions of hydrocarbons by means of matrix-isolation FTIR spectroscopy

Summary The application of matrix isolation-FTIR spectroscopy for the product analysis of reaction systems in atmospheric chemistry is described. A mixture of Ar and O₂ with a molar composition of 80% and 20%, respectively, is used as carrier gas. The Ar/O₂ matrix deposited at 7±1 K exhibits an O₂-doublet absorption band centred at 1550 cm^–1 and 1551.5 cm^–1 which can be used as an internal reference for calibration purposes. The reaction products isolated in the Ar/O₂ matrix give sharp and well-defined absorption bands which are suitable for quantitative analysis. Various procedures and associated problems in the calibration are discussed in detail. The matrix isolation-FTIR technique was applied to the ozonolysis of simple alkenes and the photooxidation of biacetyl.     

Structures of methyl halide dimers in supersonic jets by matrix-isolation infrared spectroscopy and quantum chemical calculations

The CH₃Cl and CH₃Br dimers produced by supersonic-jet expansion were directly deposited on a cold plate using a standard matrix-isolation technique. Dependence of the relative intensities of the observed infrared bands on the stagnation pressure was used to assign the dimer bands appearing near the monomer bands. By a comparison of the wavenumber shifts from the monomer bands with the corresponding values obtained by quantum chemical calculations, DFT/B3LYP/6-311++G(3pd,3df) and MP2/LanL2DZ+fdp, the structures of CH₃Cl and CH₃Br dimers were determined to be a head-to-tail isomer, which is common to the CH₃F and CH₃I dimers determined previously by the same method. The remaining dimer bands, which could not be assigned to the head-to-tail isomer, were tentatively assigned to a head-to-head isomer in analogy with CH₃I dimer.