A study of alkyl radicals in the matrix of polycrystallinen-alkane γ-irradiated at 77 K

The composition of alkyl radicals (AR) formed by γ-radiolysis (T=77 K) of polycrystallinen-alkanes with different lengths of the carbon chain (C(5), C(7), C(10), C(11), and C(18)) and their polymeric analog (polyethylene) was estimated from the ESR spectra. The ESR spectra of the irradiatedn-alkanes are superpositions of the signals from the H₃CC^.HCH₂− and −CH₂C^.HCH₂− radicals, whose HFS constants with α and β protons as well as the equilibrium conformation are independent of the chain length of then-alkane molecule. A dependence of the concentration of the radicals on the chain length ofn-alkane was found. The absence of the −CH₂C^.H₂ radicals that may arise upon H atom elimination from the Me fragments of then-alkane molecules is most likely related to the transfer of excitation energy from the Me group to the neighboring methylene fragment and the transformation of the −CH₂C^.H₂ radicals into H₃CC^.HCH₂− radicals. With account for this, the concentrations of the AR formed were suggested to be proportional to the number of H atoms at the corresponding C atom. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1034–1037, June, 2000.     

ESR spectra of γ-irradiated samples of iron(iii) binuclear complexes

-Irradiation of -oxo-bridged binuclear iron complexes Fe^III ₂OL _n in a glycerol or dimethylformamide matrix at 77 K affords unstable mixed-valence Fe^IIF^III forms resulting from the transfer of a mobile electron generated by the ionizing radiation. These nonequilibrium forms retain the ligand environment of the original complexes, and their ESR spectra at 77–200 K are characterized by an asymmetric signal with an axially anisotropicg-factor, which is in agreement with the spectra of the Fe^IIFe syu forms obtained by chemical reduction.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 869–871, April, 1996.     

An ESR and Optical Spectroscopic Study of the Mechanism of Photostimulated Reactions in Hydrogen Cyanide γ-Irradiated at 77 K

Changes in the electronic absorption spectra and ESR spectra in the course of photobleaching of radiolyzed solid HCN with light of different wavelengths (236–600 nm) were studied by ESR and optical spectroscopy. Two bands at 270 and 290 nm in the optical spectrum were attributed to the presence of H₂C=N and HC=NH radicals, respectively (the molar absorption coefficients are k ₂₇₀ 2.7 × 10²l mol^–1cm^–1and k ₂₉₀ 1.5 × 10²l mol^–1cm^–1, respectively). Structureless broad bands with maximums at 313 and 465 nm, which were detected after the exposure of a sample to light with 300 nm, can belong to the cyanide ions (CN^–) and H₂C=N⁺cations (the molar absorption coefficients of the ions are k _ion= (0.4–1.0) × 10²l mol^–1cm^–1). In the photobleaching of -irradiated HCN ( = 236–280 nm), H₂C=N⁺radicals were additionally formed by the photoinduced reaction of electron transfer from the CN^–anion to the H₂C=N⁺cation. The amount of these radicals generated in the course of photobleaching is several times greater than that of the same radicals formed in the radiolysis via hydrogen atom addition to the multiple bond of HCN molecules.     

Conformational changes of DNA γ-irradiated in the presence of aliphatic alcohols in solution

The conformational changes of DNA in aqueous solutions containing ethanol and iso- and n-propanol having concentrations of 0–23 wt.% and ionic strength μ = 0.003 NaCl and exposed to γ radiation at doses from 0 Gy to 30 Gy were studied by low-gradient viscosimetry and circular dichroism. The dependence of the specific volume (V _sp) of DNA on the radiation dose changed at different contents of alcohol. There is a critical concentration (C _cr) at which the structure of the mixed solvent is rearranged. The V _sp of DNA decreased as the radiation dose increased at C _alc = 12 wt.% < C _cr and increased at C _alc = 23 wt.% > C _cr. In both cases, the largest V _sp of DNA was observed in water ethanol solutions. The secondary structure of the macromolecule in the systems corresponds to the form. It was shown that the observed conformational changes in irradiated DNA at C _alc < C _cr and C _alc > C _cr could not be explained only by the ability of alcohols to intercept the radicals formed in radiolysis of water.     

Spin trapping of the short-lived free radicals formed in γ-irradiated alcohols

Short-lived free radicals formed in γ-irradiated methanol, ethanol and 2-propanol have been detected by E.S.R. combined with spin trapping method using 2,4,6-tri-t-buthylnitrosobenzene (BNB) as a spin trap. The observed spectra could be identified clearly by the help of deuterated alcohols. The hydrogen spin adduct, which had not been clearly identified in earlier works, was observed together with alkoxy radicals and other radicals. It was revealed that a part of the hydrogen adduct could come from proton transfer to the anion radical of BNB, which was produced by electron capture during irradiation. From the change of ¹CH₂OH/CH₃O¹ ratio with that of added spin trap concentration one may conclude that the methoxy radical and hydrogen atom are primary radicals in radiolysis of liquid phase methanol. This conclusion is also supported by both the change of the ratio with irradiation temperature and the isotope effect on the spin adduct yields.     

Angle-resolved photoelectron spectra of CF3Br and CF2Br2 in the photon energy range 29–115 eV

Photoelectron asymmetry parameters β and partial photoionization cross sections have been measured for ionization from the molecular orbitals of CF₃Br and CF₂Br₂ using synchrotron radiation in the photon energy range from 19 to 115 eV. Cooper minima are observed in the β spectra for ionizations from orbitals with substantial bromine 4p character. The measured positions of the minima have been compared to the results of non-relativistic calculations performed for different elements of the periodic system. Several shape resonances have been observed in β spectra associated with orbitals of predominantly F(2p) character. Ionization from the Br(3d) subshell has also been observed and studied in some detail. Auger transitions involving 3d electrons have been observed at higher photon energies.     

Infrared polarized spectra of hexamethylbenzene above and below the λ point at 116 K

The i.r. spectrum of hexamethylbenzene (HMB) has been investigated above and below the λ point between phase II (stability range 383-116 K) and phase III (below 116 K) in order (i) to get information on the molecular and/or crystal modifications involved in the phase transition and (ii) to assign i.r. absorptions. Spectra of randomly oriented samples and of single crystals having ab and bc as predominant faces were taken at different temperatures. In phase II the number of bands and their polarizations clearly indicate that the benzene like modes feel a potential of symmetry higher than that of the site, whilst the band broadness of the absorptions due to methyl stretchings and bendings does not allow any indication on “peripheral” potential. Changes occurring in the spectrum of the low temperature phase should indicate a more ordered situation in which the molecular symmetry is D_3d or S₆.     

Reactions of hole species in γ-irradiated KCl,KBr and KI with the emulsion of aniline and water

-Irradiated KCl, KBr and KI are dissolved separately in the emulsion of water and aniline. Halogens as radiolytic products are extracted in aniline forming stable halogen-aniline complexes. Hole species from V₂ and V₃ centers and free halogens trapped in crystalline salts are solvated first in water phase during dissolution and immediately form complexes with aniline. A typical vibrational structure of absorption bands around 300 nm is observed. The yields of halogens and thereby complexes are found to depend upon the concentration of F and hole centers and the mesh of the salt.     

Kinetics of the reaction of CF3 with O2 over the temperature range 233–373 K

The rate constant of the reaction CF₃ + O₂ + M → CF₃O₂ + M has been measured as a function of temperature in the fall-off region between 1 and 10 Torr, by laser photolysis and time-resolved mass spectrometry. The values of the fall-off parameters measured at room temperature are in good agreement with previous values and the temperature dependence of k₁₍₀₎ is reasonably well accounted for by the theoretical analysis proposed by Troe and co-workers. With M = N₂k₁₍₀₎ = (1.9 ± 0.2) × 10⁻²⁹ (T/298)^{(−4.7 ± 0.4)} cm⁶ molecule⁻² s⁻¹. Reliable values of k_1(∞) cannot be derived from these low pressure measurements. The rate expression proposed, k_1(∞) = (9 ± 2) X 10⁻¹² (T/298)^(0±1) cm³ molecule⁻¹ s⁻¹, is consistent with a “broadening parameter” of the form F_c = exp(−T/395) and previous values reported at room temperature.     

New equipment using a static analytic method for the study of vapour–liquid equilibria at temperatures down to 77 K

A new apparatus designed for the study of vapour–liquid equilibria (VLE) of small molecules at temperatures down to 77 K and pressures up to 40 MPa was built, set-up and used up to 2.8 MPa. It is based on the `static-analytic' method using two pneumatic capillary sampling devices, which allow taking small and reliable in-situ samples and performing direct analyses through gas chromatography. Accurate P, x, y isothermal data obtained for the O₂–N₂ and Ar–N₂ mixtures are well represented by the Soave–Redlich–Kwong equation of state (SRK-EoS). The conclusions which can be drawn from the comparison with literature data are the following: maximum deviations on pressure, temperature and vapour phase composition, between literature experimental data and the calculated ones (using our binary interaction parameters) are not larger than 3.3%, 0.12 K and 4.2%. We also point out inconsistencies in several literature data.     

Relative rate study of the kinetics, mechanism, and thermodynamics of the reaction of chlorine atoms with CF3CF═CH2 (HFO-1234yf) in 650-950 Torr of N2 or N2/O2 diluent at 296-462 K

The rate constant of the reaction Cl + CF(3)CF═CH(2) (k(1)) has been measured relative to several reference species using the relative rate technique with either gas chromatographic analysis with flame-ionization detection (GC/FID) or Fourier transform infrared (FTIR) analysis. Cl atoms were generated by UV irradiation of Cl(2)/CF(3)CF═CH(2)/reference/N(2)/O(2) mixtures. At 300-400 K in the presence of >20 Torr O(2), k(1) = 1.2 × 10(-11) e((+1100/RT)) cm(3) molecule(-1) s(-1). In N(2) diluent, k(1) has a sharp negative temperature coefficient resulting from the relatively small exothermicity of the following reactions: (1a) Cl + CF(3)CF═CH(2) ? CF(3)CFClCH(2)(?); (1b) Cl + CF(3)CF═CH(2) ? CF(3)CF(?)CH(2)Cl (reaction 1), which were determined in these experiments to be ～16.5 (±2.0) kcal mol(-1). This low exothermicity causes reaction 1 to become significantly reversible even at ambient temperature. The rate constant ratio for the reaction of the chloroalkyl radicals formed in reaction 1 with Cl(2) (k(2)) or O(2) (k(3)) was measured to be k(2)/k(3) = 0.4 e(-(3000/RT)) for 300-400 K. At 300 K, k(2)/k(3) = 0.0026. The reversibility of reaction 1 combined with the small value of k(2)/k(3) leads to a sensitive dependence of k(1) on the O(2) concentration. Products measured by GC/FID as a function of temperature are CF(3)CFClCH(2)Cl, CF(3)COF, and CH(2)Cl(2). The mechanism leading to these products is discussed. The rate constant for the reaction Cl + CF(3)CFClCH(2)Cl (k(11)) was measured as a function of temperature (300-462 K) at 760 Torr to be k(11) = 8.2 × 10(-12) e(-(4065/RT)) cm(3) molecule(-1) s(-1). Rate constants relative to CH(4) for the reactions of Cl with the reference compounds CH(3)Cl, CH(2)Cl(2), and CHCl(3) were measured at 470 K to resolve a literature discrepancy. (R = 1.986 cal K(-1) mol(-1)).     

Origin of the doublet at 1360 and 1340 cm−1 in the Raman spectra of tryptophan and related compounds

Raman spectra of indole, skatole, tryptophan and its derivatives including C₄-monodeuterated tryptophan were studied with special attention to the origin of the band around 1350 cm⁻¹. The band is observed as a doublet or triplet depending on the compound and solvent, which was ascribed to Fermi resonance between one fundamental and one or two combination bands of the out-of-plane bending vibrations. Removal of the interaction is seen for C₄D-tryptophan and some solutions of skatole. The assignments of the component vibrations were made based on the shifts in frequencies upon C₄-deuteration. In tryptophan derivatives the doublet (or triplet) peaks exhibit steep changes in relative intensity at the pK_a points of the dissociable groups, which is ascribed as arising from slight frequency shifts of a pK_a-sensitive component vibration. Relations between the relative intensity and environment of indole were obtained and their possible applicability to the analyses of Raman spectra of proteins is suggested.     

Vibrational spectra and force constants of symmetric tops—XLII. Rovibrational spectra of CF3I in the ν2, ν3, 2ν3 and ν2 + ν3 regions

The gas phase i.r. spectrum of CF₃I has been investigated in the ν₂, ν₃, 2ν₃ and ν₂ + ν₃ region with a resolution of 0.04 cm⁻¹. Rotational J clusters have been resolved, and several vibrational and rovibrational parameters of ν₂, 743.364(8) cm⁻¹ and ν₃, 286.303(3) cm⁻¹, have been determined by polynomial methods and by band contour simulation.     

Deposition of radiation energy in solids as visualized by the distribution,structure and properties of alkyl radicals in γ-irradiated n-alkane single crystals

This paper summarizes results obtained earlier from E.S.R. studies of γ-irradiated n-alkane single crystals. It also contains some new experimental results that serve to give a more complete picture of the deposition of radiation energy in solid alkanes. The experiments performed with solid n-alkanes have thus far provided structural data that permit the nature and even the conformation of alkyl radicals to be clearly understood. Two types of radical exist, namely, one where the unpaired electron is located next to the end methyl group and one with the unpaired electron in the interior of the chain. The first type has a conformation which differs from that of the undamaged molecule. Microwave saturation data show that there is a difference in relaxation properties of these radicals which can be understood in terms of a difference in mobility. Relative yield measurements give the distribution of isomeric alkyl, the result differing from that obtained using product analysis in liquids. For protiated n-alkanes n-alkyl is lacking and the 2-alkyl concentration is higher than expected. For deuterated n-alkanes the E.S.R. spectrum is mainly that of radicals with the unpaired electron located in the interior of the carbon chain. This isotope effect is again contrary to observations in liquid n-alkanes.The broad lines observed in protiated alkanes irradiated at 77 K and deuterated alkanes irradiated at 4.2 K are not believed to arise from strong spin-spin interactions. They are thought instead to arise from distorted crystal and radical structures relating to the damage regions of the crystals.Radical pairs exist with different stability, yield and structure. Our estimate, in deuterated alkanes, that as much as 40% of the radicals are formed in pairs or clusters at 4.2 K shows that radical pair formation is an important radiolytic process in solid n-alkanes. Energy transfer from deuterated to protiated molecules has also been experimentally verified. This energy transfer is temperature dependent and occurs efficiently at 77 K, but less efficiently at 4.2 and 273 K. A relationship also exists for deuterated alkanes between the amount of radical pairs formed at 4 K and the long-range energy transfer at 77 K. This can be readily accounted for by an exciton transfer mechanism analogous to that in aromatic crystals. It might also be qualitatively described by a hydrogen abstraction process. In this case, however, the properties of the deuterium atoms are not understood.     

Analysis of the hyperfine structures and A doubling of high-resolution LMR spectra of ~(15)N~(16)O (X~2Π)

From the theories of the nuclear hyperfine structure (HFS) and A doubling of diatomic molecules, several brief algebraic equations for interpretation of HFS and A doubling of transitions of diatomic molecule have been developed. A few important parameters of HFS and A doubling of 15N16O have been efficiently and accurately obtained from the analysis of the high resolution spectra of 15N16O (X2Π) observed in our experiments with these equations. This method can provide an effective approach to obtain important hyperfine parameters of novel radicals from their high resolution laser magnetic resonance spectra.