2,6-Dimethylphenylnitrene in low-temperature matrices

Photolysis of 2,6-dimethylphenyl azide in N₂ matrices at 12K gives 2,6-dimethylphenylnitrene; in the presence of CO, the nitrene forms the corresponding isocyanate, while on shorter wavelength irradiation, it undergoes an inefficient rearrangement to an azacycloheptatetraene.     

Photochemistry of acetylacetone isolated in parahydrogen matrices upon 266 nm irradiation

The photochemistry of the chelated enol form of acetylacetone (AcAc) was investigated by UV excitation of the S(2) state at 266 nm in parahydrogen matrices, complemented by experiments in neon and normal hydrogen matrices. Infrared (IR) spectroscopy, combined with theoretical calculations, was used to identify the photoproducts. Isomerization towards various non-chelated forms (no intramolecular H-bond) of AcAc is the dominant channel whereas fragmentation is very minor. The isomerization kinetics is monitored by IR spectroscopy. Among the seven non-chelated conformers of AcAc, only three are formed in parahydrogen matrices, whereas four are observed in normal hydrogen matrices. This difference suggests that an active tunnelling process between conformers occurs in parahydrogen but is quenched in normal hydrogen where guest-host interactions are stronger. Fragmentation and isomerization of excited AcAc are discussed in the light of these new data. The role of the intermediate triplet state in the S(2)→ S(0) relaxation is confirmed, as the importance of phonons in the condensed phase.     

Reactions of hydroxyl radicals with alkenes in low-temperature matrices

The reactions of hydroxyl radicals with a number of stable alkenes have been studied in low-temperature matrices. The reactions were initiated by broad band UV-visible irradiation of matrices containing H2O2, and the alkene under investigation. The hydroxyalkyl radical products were identified principally by comparison of their spectra with the spectra of corresponding stable alcohols. Accordingly, IR spectra were recorded for the following series of alcohols isolated in argon matrices--methanol, ethanol, ethanol-d6, propan-1-ol, propan-2-ol, butan-2-ol, 2-methylpropan-1-ol (iso-butyl alcohol), 2-methylpropan-2-ol (tert-butyl alcohol), 2-methylbutan-2-ol (tert-amyl alcohol), 3-methylbutan-2-ol and 2,3-dimethylbutan-2-ol. The hydroxyalkyl radicals, which appear to be formed from the alkenes studied were as follows--from ethene, 2-hydroxyethyl radical: from cis- or trans-but-2-ene. 1-methyl-2-hydroxypropyl radical; from propene, 1-methyl-2-hydroxyethyl and 2-hydroxypropyl radicals; from but-1-ene. 1-hydroxymethylpropyl and 2-hydroxybutyl radicals; from 2-methylpropene (iso-butene), 1,1-dimethyl-2-hydroxyethyl and 2-methyl-2-hydroxypropyl radicals; the radical products from buta-1,3-diene and isoprene could not be identified. In the cases, where two radical products were possible, i.e. when propene, but-1-ene or 2-methylpropene were the substrates, it was found that the concentration of the secondary or tertiary radical always exceeded that of the primary radical. However, the relative concentration of these radicals appears to be determined by subsequent photolysis to give carbonyl compounds. There seems, therefore, to be little preference for the secondary and tertiary radicals over the primary radicals in the primary addition process. Comments on the mechanism of the transformation from radical to carbonyl compound based upon identification of intermediates within the matrix and isotopic substitution experiments are made. The characterisation of the 2-hydroxyethyl radical has been backed up by experiments utilising isotopic substitution with 13C and D (2H). The other radicals have been identified with varying degrees of certainty. Those radicals, which are observed at the highest concentration and which are, therefore, characterised more certainly are--2-hydroxyethyl (1), from ethene: 1-methyl-2-hydroxypropyl (2), from cis- and trans-but-2-ene; 1-methyl-2-hydroxyethyl (3), from propene; 1-hydroxymethylpropyl (5), from but-1-ene; and 1,1- dimethyl-2-hydroxyethyl (8), from 2-methylpropene.     

Photochemistry of cyclohexene oxide radical cations in freonic matrices at 77 K

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Benzoyl peroxide—tetraalkylammonium iodide system as an initiator of the low-temperature oxidation of cumene

The initiating effect of the benzoyl peroxide—tetraalkylammonium iodide system in the low-temperature liquid-phase oxidation of cumene at 313 K is studied. The reaction proceeds by a nonbranching-chain radical mechanism. At the initial stage, the kinetics of the process is described by a first-order equation. The parameters of the kinetic equation are determined at various substrate concentrations and component proportions in the initiating system. Addition to the reaction mixture of benzoic acid, cumene hydroperoxide, and iodine in amounts that can accumulate during the reaction produces only a slight effect on the oxidation rate.Translated from Kinetika i Kataliz, Vol. 45, No. 6, 2004, pp. 821–828.Original Russian Text Copyright © 2004 by Opeida, Zalevskaya, Turovskaya.     

Far-infrared studies of ethanols and fluoroethanols trapped in low-temperature matrices

Ethanol, its deuterated derivatives (C₂H₅OD, C₂D₅OD) and fluoroethanols (CFH₂CH₂OH, CF₃CH₂OH) have been isolated in low temperature matrices and investigated in the far-infrared region. From the concentration dependency of the observed bands and from studies of the pure alcohols in the gaseous, liquid, and solid phase it was found that the hydrogen bond stretching frequencies ν_σ associated with the hydrogen bonded system OH?O appear in the 100 – 160 cm^?1 range. At higher M/A ratios the OD(OD) torsion modes τ_OH are dominating and were identified in the 200 – 300 cm^?1 region. The influence of various matrix materials like argon, krypton, xenon, nitrogen and methane on the low frequency spectra of ethanol have also been studied. It was found, that nitrogen and methane matrices produce significant changes in the far infrared spectra.     

Physicochemical properties of tetraalkylammonium tetraphenylborates and tetraalkylammonium dodecylsulfates

Physicochemical parameters (composition, conventional solubility products, and thermal stability) of ionic associates of tetraalkylammonium dodecylsulfates and tetraalkylammonium tetraphenylborates are determined. The usefulness of these compounds as electrode-active components of membranes in potentiometric sensors that selective to tetraalkylammonium salts is assessed.     

Photochemical reaction of sulfur hexafluoride with water in low-temperature xenon matrices

Sulfur hexafluoride SF(6) is a very stable molecule with which very few reactions with other molecules have been reported. Here, we report a photochemical reaction of SF(6) with water molecules using a matrix-isolation technique, where SF(6) and H(2)O were co-condensed in Xe matrices, and the products were observed using infrared spectroscopy. Irradiation at 193 nm from an ArF excimer laser caused the simultaneous decomposition of SF(6) and H(2)O, which resulted in the production of novel species. Infrared spectra and molecular orbital calculations of the species showed that the product was a SF(4)?HF?HOF complex, which consists of hydrogen bonds and charge transfer interaction between S and F atoms. The assignment of the species was confirmed by isotope shifts using D and (18)O isotope substitutions.     

Infrared-induced conformational interconversion in carboxylic acids isolated in low-temperature rare-gas matrices

An overview of our recent studies dealing with infrared-induced conformational interconversion of carboxylic acids isolated in rare-gas matrices is presented. Extensive rotational photoisomerization studies have been performed on formic acid, which is the simplest organic acid enabling this kind of processes. Formic acid has two conformers and interconversion between them can be induced by vibrational excitation. As such, it is an ideal model system to study the conformational dynamics of the carboxylic group. Formic acid molecules were found to be isolated in different local environments within the rare-gas matrices, as shown by the site splitting of the vibrational bands. Narrowband tunable infrared (IR) radiation was used to induce site-selective isomerization processes. The induced changes in the IR absorption spectra allowed for a detailed analysis of the vibrational properties of both conformers of formic acid isolated in solid argon. In particular, derived from the intermode coupling constants the local environment was shown to affects the intramolecular potential energy surface. Tunneling is involved in the rotamerization of formic acid, with the tunneling rate being affected by the local environment. Additionally, formic acid exhibits isomer-selective photodissociation where narrowband IR excitation can control the conformer-dependent photodissociation channels. Tunable IR radiation was also used to promote rotamerization in a series of matrix-isolated dicarboxylic acids (ethanedioic, propanedioic, and 2-butenedioic acids) by exciting the first overtone of the OH stretching mode or a suitable combination mode at similar energies. Efficient isomerization involving rotation around the CO bond was observed in most cases whereas the internal rotation around the CC bond was found to be constrained for ethanedioic and (Z)-2-butenedioic acids.     

Photoinduced deprotonation of cyclopentene oxide radical cations in low-temperature Freon matrices

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Trihalogenomethane - base complexes studied by vibrational spectroscopy in low-temperature matrices

Hydrogen-bonded complexes formed by weak proton donors such as the trihalogenomethanes have received little attention. As a precursor to the study of such complexes, infrared and Raman spectra of trifluoromethane and trichloromethane were examined in argon, nitrogen and other matrices. The spectra of these trihalogenomethanes mixed with water or ammonia in argon or nitrogen matrices showed evidence of complex formation, the complexes with trichloromethane being stronger than those with trifluoromethane.     

Photochemistry of trimethylene oxide and trimethylene sulfide radical cations in freonic matrices at 77 K

It was shown that trimethylene oxide (oxetane) radical cations were converted at 77 K into either distonic radical cations ·CH₂CH₂CH=OH⁺ or 2-oxetanyl radicals, depending on the freonic matrix used, by the action of light at λ = 546 nm and trimethylene sulfide radical cations transformed into distonic radical cations CH₂CHSH⁺CH ₂ ^· under 436-nm irradiation. The quantum yields of the photochemical reactions were determined. Quantum-chemical calculations on the structure and HFC constants of the radical cations and possible paramagnetic products of their transformation were performed. The reasons behind the observed difference in reactivity between the radical cations under the action of light are discussed.     

Photochemistry in the charge transfer and neutral excited states of HCl in Xe and Kr matrices

HCl-doped Xe and Kr films are irradiated with wavelength dispersed synchrotron radiation in the wavelength range from 200 to 130 nm. The growth of H, Cl, Xe2H+, XeH2, HXeCl, Kr2H+, and HKrCl as well as the decomposition of HCl are recorded by a combination of UV, VIS, and IR spectroscopy. A turnover in the formation of Xe2H+ and Kr2H+ by a predominant two-step reaction on neutral surfaces at low energies to a one-step formation on ionic surfaces is determined at 172 and 155 nm in Xe and Kr, respectively. A potential energy diagram for neutral and ionic states is derived that is consistent with a DIIS calculation, with new UV fluorescence bands from Xe+HCl- centers, with the turnover energies and with a deconvolution of the absorption spectra in neutral and ionic contributions. The cage exit of charged as well as of neutral H, the latter via a harpoon reaction, is discussed for the ionic surfaces. The self-limitation of HCl decomposition on the neutral surfaces due to absorption by H and Cl fragments is treated quantatively. Dissociation efficiencies phi(e), together with absolute absorption cross sections sigma(H) and sigma(Cl) of the fragments, are derived. sigma(H) and sigma(Cl) are of the order of 10(-16) cm(2) compared to 10(-18) cm(2) for sigma(HCl). Dissociation is accompanied by many excitation cycles of the fragments, which leads to light-induced migration of H and recombination. phi(e) therefore represents a product of the cage exit probability phi that was treated theoretically and the survival probability concerning geminate and nongeminate recombination.     

Vibrational spectroscopy of cyclohexylaminoglutethimide in low-temperature matrices, solutions and the solid state

The vibrational spectra of cyclohexylaminoglutethimide (CHAG) molecules were studied in low-temperature Ar and N₂ matrices and in CCl₄, CHCl₃, CS₂ and CH₃CN solutions (at different concentrations). It was found that increasing the solute concentration in CCl₄, CS₂ and CHCl₃ solutions leads to the formation of dimers. On the other hand, in CH₃CN solution only the solute–solvent complexes occur.

The molecular structure and theoretical IR and Raman spectra of CHAG were predicted with the use of ab initio RHF/6-31G^** and density functional B3LYP/6-31G^** quantum mechanical methods. A comparative analysis of theoretical and experimental spectra of monomeric (isolated) species has led to the assignment of most of the absorption bands in terms of CHAG normal modes.

The IR and Raman spectra of crystalline samples (solid film and KBr pellet) were recorded as well. A reliable assignment of the IR and Raman spectra of crystalline CHAG was obtained with the help of theoretically simulated spectra and the results of our earlier investigations of aminoglutethimide and glutethimide vibrational spectra.     

Formation of trimer and dimer radical cations of methyl-substituted benzenes in gamma-irradiated low-temperature matrices

Dimer and trimer radical cations of benzene, toluene, and xylenes were produced selectively after gamma-irradiation in low-temperature 2-methylpentane matrices with electron scavengers: oxygen (O(2)) and sec-butyl chloride (sec-BuCl). The charge resonance (CR) band of the trimer radical cation (M(3)(+)) produced via the corresponding dimer radical cation (M(2)(+)) is clearly seen in the solution containing O(2) as the temperature increases over a range from 80 to 90 K. In o-xylene solution, a fairly strong and distinct M(3)(+) CR absorption is observed; this is due to the large M(3)(+)/M(2)(+) relative extinction coefficient. All benzene derivatives show an equilibrium between dimer and trimer radical cations at approximately 90 K; however, the equilibrium constants of toluene and the xylenes are considerably lower than that of benzene. Formation of the trimer radical cation is inhibited in sec-BuCl, which has commonly been used as a low-temperature optical matrix for producing cationic species. An ab initio DFT method is applied to predict the geometry of M(3)(+), giving "slipped sandwich" (for benzene, m-xylene, and p-xylene) and "slipped fan-shaped" (toluene and o-xylene) structures as the most plausible geometries. The experimentally observed spectroscopic parameters reflect well those predicted by TD-DFT calculation based on geometry, suggesting strong dependence of the geometry of M(3)(+) on substitution patterns. This is the first report not only of direct spectroscopic observation of aromatic trimer radical cations in the condensed phase but also on the quantitative analysis of their equilibria.     

Synthesis of tetraalkylammonium thiolates

A convenient method has been developed for the preparation of tetraalkylammonium thiolates by the reaction of tetraalkylammonium hydroxides with various thiols in benzene.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2146–2148, September, 1991.     

The structure of the radical cation of ethylene oxide (oxirane) in low-temperature matrices

Based on analysis of ESR spectra, it is suggested that the initial species formed by electron loss from oxirane (¹³C) in CFCl₃ at 77 K is the ring-closed o(n) radical, rather than the ring-opened “allylic” radical.     

Ibrational spectra of maleimide, N-deuterated maleimide and maleic anhydride in low-temperature matrices

Infrared and Raman spectra are reported for maleimide, N-deuterated maleimide and maleic anhydride in argon and nitrogen matrices and in the solid phase at 20 K. The data enabled a revised vibrational assignment to be made for the monomers of maleimide and N-deuterated maleimide, while the assignment for the dimers was mainly in agreement with previous work. The interaction of maleimide with nitrogen, water and hydrogen chloride was also investigated. Water and hydrogen chloride form hydrogen-bonded complexes with a carbonyl group of the maleimide, whereas nitrogen interacts with the NH group.