Low-temperature Argon Matrix Research Articles

Structure and infrared spectrum of 2-hydroxyphenyl radical

Infrared spectrum of 2-hydroxyphenyl, C 6H 4-OH, produced from 2-iodophenol in a low-temperature argon matrix upon UV irradiation ( λ>280 nm) was measured with an FT-IR spectrophotometer. This radical was found to be less stable by 120 kJ mol −1 than phenoxyl, C 6H 5-O , by a density functional theory calculation. Two final photoproducts were identified as cyclopentadienylidenemethanone and 4-iodo-2,5-cyclohexadienone in analogy with the photoproducts of 2-chloro and 2-bromophenols. A kinetic analysis shows that the latter is produced via 2-hydroxyphenyl by hydrogen migration and iodine recombination while the former is produced by Wolff rearrangement after elimination of hydrogen iodide.

Mechanism of UV-Induced Conformational Changes among Enol-Type Isomers of (Trifluoroacetyl)acetone Studied by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation

UV-induced conformational changes among chelated and nonchelated enol isomers of (trifluoroacetyl)acetone in a low-temperature argon matrix were investigated by Fourier transform infrared spectroscopy. An infrared spectrum measured after deposition before UV irradiation was assigned to a chelated enol isomer, 1,1,1-trifluoro-4-hydroxy-3-penten-2-one, where no bands for the other chelated enol isomer, 5,5,5-trifluoro-4-hydroxy-3-penten-2-one, were found, the latter being formed from the former by moving of the hydrogen atom in the O−H group to the CO group. Conformations of 3 nonchelated enol isomers among 14 were determined by a comparison of the infrared spectra measured after UV irradiation with the spectral patterns obtained by the density functional theory calculations at the DFT/B3LYP/6-31G* level. This selective isomerization was explained in terms of the inhibition of internal rotation in vibrational relaxation of photoexcited species, which was proposed in our previous studies of acetylacetone and hexafluoroacetylacetone. An infrared spectrum of a transient photoproduct was measured while the matrix sample was continuously exposed to the UV light. It was identified as another nonchelated enol isomer.

Infrared spectrum and molecular structure of lowest electronically excited triplet state of 1,4-dicyano-2,3,5,6-tetrafluorobenzene in low-temperature matrices

An infrared spectrum of the T 1 state for 1,4-dicyano-2,3,5,6-tetrafluorobenzene in a low-temperature argon matrix was measured with a Fourier transform infrared spectrophotometer. The density-functional-theory calculation was performed to assign the observed bands. The optimized geometrical structure in the T 1 state is found to have two stable forms, planar ( D 2 h ) and non-planar ( C 2 v ), the former being more stable than the latter by 35.1 kJ mol −1. The T 1 state is identified as the planar form by a comparison of the observed matrix spectrum with the calculated spectral patterns. This assignment is confirmed by an analysis of the matrix-isolation phosphorescence spectra, where the observed vibronic bands are assigned to the fundamental and/or combination bands of the a g vibrational modes in analogy with those of 1,4-dicyanobenzene, where the T 1 structure belongs to D 2 h symmetry.

UV Photolysis of 1,4-Diaminobenzene in a Low-Temperature Argon Matrix to 2,5-Cyclohexadiene-1,4-diimine via 4-Aminoanilino Radical

UV photolysis of 1,4-diaminobenzene isolated in a low-temperature argon matrix has been investigated by Fourier transform infrared spectroscopy with the aid of the density-functional-theory calculation. Infrared bands of an intermediate produced from 1,4-diaminobenzene upon UV irradiation (λ < 350 nm) are assigned to a semiquinone-type radical, 4-aminoanilino radical. A final product produced from the 4-aminoanilino radical upon shorter-wavelength irradiation (λ < 310 nm) is assigned as 2,5-cyclohexadiene-1,4-diimine. Optimized structures of 1,4-diaminobenzene, 4-aminoanilino radical, and 2,5-cyclohexadiene-1,4-diimine are compared with one another, resulting in changes of the π-conjugated system similar to those of the hydroquinone and 1,4-benzoquinone system. In addition, trans−cis isomerization of 2,5-cyclohexadiene-1,4-diimine upon UV irradiation is observed in photoequilibrium, where the cis/trans population ratio depends on the irradiation wavelength.

Conformational Behavior of Cyanoacetic Acid: A Combined Matrix Isolation Fourier Transform Infrared Spectroscopy and Theoretical Study

This work reports the first experimental study of the cyanoacetic acid (CAA) monomer. Samples of CAA were isolated in low-temperature argon, krypton, and xenon matrixes and characterized using FTIR spectroscopy. Annealing experiments revealed the presence of different conformers in the matrixes. The direct interconversion of the gauche−cis (gc) into the cis−cis (cc) conformer was observed upon annealing of matrixes before formation of aggregates. The use of different matrix host gases enabled separation of the observed conformational effects from the site-splitting of IR absorptions. The assignment of the experimental spectra was based on both the annealing results and the comparison of the experimental data with results of theoretical calculations performed at the B3LYP/aug-cc-pVTZ level of theory. The relative energies of the observed conformers, as well as the transition states separating these forms, were calculated at the MP2, MP4, QCISD, and CCSD levels of theory with the aug-cc-pVxZ (x = D, T, and Q) basis sets. The planar cc conformer was found to be the lowest energy form of CAA. The second conformer, with a nonplanar backbone, gc, is doubly degenerated by symmetry and was predicted to be 1.3 kJ mol-1 (ZPVE corrected CCSD/aug-cc-VTZ energies) less stable than the cc form. In matrixes a fraction of the gc conformer undergoes geometrical distortion toward the planar trans−cis (tc) structure, which in the gaseous phase corresponds to the saddle point between the two mirror gc forms.

Proton-Transfer Processes in Thiourea: UV Induced Thione → Thiol Reaction and Ground State Thiol → Thione Tunneling

Thiourea monomers isolated in a low-temperature argon matrix adopt exclusively the thione tautomeric form. Upon UV (λ > 300 nm) irradiation a photoreaction converting the initial isomer of the compound into its thiol tautomer occurred. Two structures of the photoproduct (with syn and anti orientation of the imino CN−H group with respect to the C−S bond) were identified in the matrix. Subsequently, a back reaction transforming the thiol form into the initial thione tautomer was observed for the matrix kept at 10 K and in darkness. The molecules with the anti conformation underwent this process, whereas those in the syn form remained unchanged. The only possible mechanism of the ground-state thiol → thione transformation at low temperature is proton-tunneling through the very high energy barrier of 9030 cm-1 (108 kJ mol-1) (calculated at the MP2/6-31++G(d,p) level). The experimentally obtained time constant of this process was 52 h. The structure of the photoproduced species was identified by comparing the ...

Infrared and UV–visible absorption spectra of hexafluoroacetylacetone in a low-temperature argon matrix. I. Structure of a non-chelated enol-type isomer

Photoisomerization of hexafluoroacetylacetone was investigated by a low-temperature matrix-isolation technique with an aid of the density functional theory (DFT) calculation. A less stable isomer produced by UV irradiation ( λ>260 nm) from the most stable enol was identified by its IR and UV–visible absorption spectra. This isomer was found to be non-chelated enol with cis, trans, and trans conformations around the CO–CH, CHC, and C–OH bonds, respectively, by a comparison with the spectral patterns of seven possible isomers with non-chelated enols derived from DFT simulations.

Infrared and UV–visible absorption spectra of hexafluoroacetylacetone in a low-temperature argon matrix. II. Detection of the nπ * transition by monitoring IR spectral changes due to photoisomerization

The chelated enol-type isomer was found to isomerize to a non-chelated enol isomer by UV irradiation between 260 and 300 nm, while backward isomerization was observed by that between 320 and 340 nm although no electronic absorption band was detectable in this region. Wavelength dependence of the isomerization rate constants, measured by monitoring the infrared spectral changes, shows that the isomerization by UV radiation is caused by the ππ * transition for the chelated enol located around 280 nm and by the ππ * and nπ * transitions for the non-chelated enol around 260 and 340 nm, respectively.

Photoisomerization and Tunneling Isomerization of Tetrachlorohydroquinone in a Low-Temperature Argon Matrix

Torsional isomerization of tetrachlorohydroquinone in a low-temperature argon matrix has been investigated by Fourier transform infrared spectroscopy with the aid of density-functional-theory calculations. Infrared bands of the more stable isomer, trans, were observed in an argon matrix at 16 K. Those of the less stable isomer, cis, were observed during UV irradiation but disappeared immediately after the irradiation was discontinued. The isomerization from trans to cis occurred in the dark despite the high torsional barrier, 19.1 kJ mol-1, when the matrix temperature was increased from 16 to 30 K. The enthalpy difference between the two isomers was estimated from the observed population changes to be 0.46 ± 0.10 kJ mol-1 (39 ± 8 cm-1) by a least-squares fitting. This isomerization in low-temperature matrixes is ascribed to the matrix-induced hydrogen-atom tunneling.

Conformational isomerization of formic acid by vibrational excitation at energies below the torsional barrier.

Photoinduced conformational isomerization of formic acid has been studied in a low-temperature argon matrix. It is found that conformational isomerization occurs when the photon energy is below the energy barrier for this process. The quantum yield for the process near the top of the barrier is comparable with the quantum yield above the barrier and drops at lower energies. The isomerization takes place via a tunneling mechanism.

Conformations of nitro-substituted spiropyran and merocyanine studied by low-temperature matrix-isolation infrared spectroscopy and density-functional-theory calculation

Infrared spectrum of a spiropyran, 1 ′,3 ′-dihydro-1 ′,3 ′,3 ′-trimethyl-6-nitrospiro[2H-1-benzopyran-2,2 ′-[2H]indole], in a low-temperature argon matrix was measured with a Fourier transformed infrared spectrophotometer. The conformation of this spiropyran was determined by comparison of the observed spectrum with a spectral pattern obtained by the density-functional-theory calculation. This conformation differs from the optimized geometry derived from an HF/6-31G* calculation. The conformations of a merocyanine produced from the spiropyran upon UV irradiation were also determined by the same method.

Photochemical Double-Proton-Transfer Reactions in 2,6-Dithiopurine. A Matrix Isolation Study

It was found that 2,6-dithiopurine isolated in low-temperature argon and nitrogen matrixes adopts only one tautomeric form: dithione-N7H. The UV (λ > 345 nm) induced reactions converting this tautomer into dithiol-N3H and dithiol-N7H isomers were observed in Ar and N2 matrixes. The first of these reactions represents a novel type of photoinduced double-proton transfer, whereas the second is analogous to that previously observed for 2,4-dithiouracil (Lapinski, L.; Nowak, M. J.; Kolos, R.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta A 1998, 54, 685). The assignment of the form initially present in the matrix to the dithione-N7H isomer and of the photoproducts to the dithiol-N3H and dithiol-N7H isomers was based on the good agreement between the experimental IR matrix spectra and the spectra theoretically simulated at DFT(B3LYP)/6-31G(d,p) level.

Conformational analysis of dichloromethyl formate by the combined use of infrared and matrix-infrared spectroscopy and ab initio calculations

The conformations of dichloromethyl formate (HCOOCHCl2) was studied in the gas, liquid, and crystalline phases and in a low-temperature argon matrix by infrared spectroscopy. The experimental results, supported by RHF/and MP2/6-311++G** ab initio calculations, show that the same single conformer, Z,quasi-sp, is present in each of these states. The Z,quasi-sp conformer belongs to the C1 point group and the two dihedral angles, OC–O–C and C–O–C–Halkyl, are locked in planar Z and near-planar orientations, respectively, calculations providing a value of 23° for the latter angle. Altogether, a total of four conformers, Z,quasi-sp; Z,ap; E,sc; and E,ap, were calculated to represent local minima on the potential energy surface, but consideration of their relative energies implies that it is only the Z,quasi-sp conformer which would give rise to observable spectral lines. The complete assignment of the fundamental infrared bands of this conformer has been made based on the potential energy distributions obtained from normal coordinate calculations performed on both dichloromethyl formate and its dideuterated derivative.

Hydrogen-Atom Tunneling of 2,5-Dichloro-3,6-dihydroxy-1,4-benzoquinone in a Low-Temperature Argon Matrix Studied by FTIR Spectroscopy

Torsional isomerization around the two C−O bonds of 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone in a low-temperature argon matrix has been investigated by FTIR spectroscopy with an aid of the density functional theory calculations. The infrared spectrum of the most stable isomer, which has two intramolecular hydrogen bonds of CO···H−O, was observed, while that of a less stable isomer, which has CO···H−O and C−Cl···H−O hydrogen bonds, was observed as a transient species during UV−visible irradiation. Isomerization from the less stable isomer to the most stable one occurred immediately after the irradiation was stopped. This isomerization is ascribed to the effect of hydrogen-atom tunneling.

Molecular structure and infrared spectra of dimethyl fumarateElectronic supplementary information (ESI) available: Definition of internal symmetry coordinates, calculated frequencies, IR intensities, Raman activities, potential energy distributions and optimized conformers I, II and III. See http://www.rsc.org/suppdata/cp/b2/b203246p/

Infrared spectra of dimethyl fumarate isolated in low-temperature argon and xenon matrixes and of the compound in the solid amorphous and crystalline states have been studied. Theoretical calculations, carried out at the MP2/6-31G** and DFT(B3LYP)/6-31G** levels, predict three planar conformers of low internal energy, all of them exhibiting the methyl ester moieties in the cis (C–O) configuration: the conformational ground state (conformer I), with the two OC–CC dihedrals equal to 0°, and forms II and III, where one or both OC–CC dihedrals are 180°. In the spectra of the matrix isolated compound, characteristic bands of all three conformers were identified. During annealing of the xenon matrix up to 60 K, conversion of the less stable conformers, II and III, into the most stable conformer, I, was observed. In the amorphous solid, these three conformers could also be identified spectroscopically. The IR and Raman spectra of the crystalline phase clearly show that in the crystal only form I is present, since no bands ascribable to other conformers could be observed.

Matrix isolation FTIR and molecular orbital study of E and Z acetaldoxime monomersElectronic supplementary information (ESI) available: Internal coordinates for the Z and E isomers, calculated barriers for the rotation of methyl groups and the relative energies of the relevant stationary points for acetaldoxime. See http://www.rsc.org/suppdata/cp/b2/b203240f/

The molecular structures and vibrational spectra of the monomers of the Z and E acetaldoxime isomers were investigated by ab initio MP2/6-311++G** and DFT (B3LYP)/6-311++G** calculations. Experimentally, acetaldoxime was studied using FTIR spectroscopy combined with the technique of isolation in low temperature argon matrixes. Significant dependence of the ratio of E and Z isomers in the matrix on the method of sample preparation was observed. Matrixes with high predominance of the Z form were deposited, as well as such with dominating E form or with both forms in nearly the same amount. That allowed separation of the infrared spectra of both isomers. An assignment of the vibrational spectra of the two isomeric forms was based on comparison with the spectra theoretically calculated at the DFT and MP2 levels. The vibrational data were correlated with some important structural parameters, in particular the different methyl torsional barriers present in the two isomers.

Molecular structure and infrared spectra of dimethyl oxalateElectronic Supplementary Information available. See http://www.rsc.org/suppdata/cp/b1/b107232n/

Infrared spectra of dimethyl oxalate isolated in low-temperature argon matrix and of the compound in the solid amorphous and crystalline state are reported. The experimental observations are interpreted in terms of a large amplitude, low frequency vibration along the torsional coordinate OC–CO. Assumption of the large amplitude vibration allows consistent explanation of the spectra observed in matrixes and in the gas phase, and the differences between these and the spectrum of the compound in the crystalline state, in which the planar trans structure is fixed. IR spectra of crystalline dimethyl oxalate clearly show that in the relaxed crystal lattice only the trans form is populated. Davydov splitting, due to the presence of two molecules per unit cell, was observed for several bands in the IR spectrum of the crystalline compound.

UV-Induced Photoisomerization of Acetylacetone and Identification of Less-Stable Isomers by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation

UV-induced photoisomerization of acetylacetone in low-temperature argon matrixes has been studied by Fourier transform infrared spectroscopy. Identifications of the species produced by UV irradiation (λ > 280 nm) were carried out with the aid of the density functional theory (DFT) calculation, in which the 6-31G* basis set was used to optimize the geometrical structures. By comparison of the observed infrared bands with the calculated spectral patterns, it was found that cis−trans isomerization around the C−C, CC, and C−O bonds occurs to produce less-stable enol isomers, 2-hydroxy-2-penten-4-one. Shorter-wavelength irradiation (λ < 280 nm) induced hydrogen-atom migration of the enol isomers to produce a keto isomer, 2,4-pentanedione.

Infrared Spectra and Density Functional Theory Calculations of Iminoxy Radicals Produced by Visible-Light-Induced Reaction between 1-Phenyl-1-Propyne and NO2 in Low-Temperature Argon Matrixes

Visible-light-induced reaction between 1-phenyl-1-propyne and NO2 in low- temperature argon matrixes has been studied by Fourier transform infrared spectroscopy with an aid of density functional theory (DFT) calculation. Infrared bands observed upon 580-nm irradiation are assigned to acetyl phenyl iminoxy radical, which is produced by recombination of acetylphenylmethylene, a ketocarbene intermediate, with a reactive coproduct, NO. Conformations around the OC−CN and CN axes of acetyl phenyl iminoxy radical are determined by a comparison of the observed and calculated wavenumbers. A small amount of methylphenylketene is produced from ketocarbene by migration of the methyl group. The branching ratio of the migration against the recombination is estimated to be 0.072 ± 0.006 by an analysis of absorption-growth behavior for the infrared bands of methylphenylketene and acetyl phenyl iminoxy radical.

Formation of Nitrile Ylide by Addition of Carbene with Acetonitrile in a Low-Temperature Argon Matrix

Formation of Nitrile Ylide by Addition of Carbene with Acetonitrile in a Low-Temperature Argon Matrix