Early effects of radiation radical pairs in a single crystal of bartituric acid

Radical pairs are formed and trapped when barbituric acid is irradiated with electrons at 77 K. The radical pairs were identified and the molecular structure determined in single crystal studies. The two unpaired species, constituting the pair, were found to be identical and equal to the isolated radical previously studied. The radical is formed by a hydrogen abstraction from C ₅, and the pair is formed by two such abstractions in neighbour molecules. The pair formation is quite selective and involve exclusively the nearest neighbour molecules.     

E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C₆. The E.S.R. analysis of the ³⁵Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components a_xx =46·98 MHz, a_yy =-10·98 MHz and a_zz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are g_xx =2·0012, g_yy =2·00862 and g_zz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C₆ with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the ¹⁴N nucleus of nitrogen N₃ (a_xx =9·81 MHz, a_yy =a_zz ? 0 MHz) and with the proton of the hydrogen bonded to N₃. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.     

Radical cations of n-alkanes in irradiated solutions as studied by time-resolved magnetic field effects

The time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs has been measured to study n-alkane radical cations in irradiated solutions at room temperature. The magnetic field effect was recorded as a ratio of fluorescence decay curves in the 0.1 T and zero magnetic fields for solutions of C₈, C₉, C₁₀, C₁₂, and C₁₆ n-alkanes in n-hexane with addition of 3 × 10^?5 M p-terphenyl-d ₁₄. A distinct maximum at 10–30 ns followed by a slowly decaying plateau was observed for all the solutions. Simulation shows that the maximum corresponds to an unresolved ESR spectrum with the peak-to-peak line-width ranging from about 1.6mT to 0.5mT for C₈ to C₁₆ radical cations. The unresolved structure is believed to result from the hyperfine couplings with many protons of the radical cation, the increase in the number of interacting protons compared with low temperature matrices being caused by the methyl group rotation and conformational motion of the carbon chain. With increase in concentration of dissolved n-alkane, the maximum in the curves first moves to longer times and finally disappears; this was attributed to the narrowing of ESR spectrum contour due to degenerate electron exchange.     

Split-shell molecular orbital calculations on the heteronuclear alkali metal diatomics

One of the radicals formed in irradiated 5-iododeoxyuridine is shown unambiguously to be the α-iodo radical RCH₂-?(I)R′ formed by hydrogen atom addition to C₆. The ¹²⁷I hyperfine tensor components, A_x = + 90 G, A_y = (-) 50 G, A_z = (-)40 G are proposed as being characteristic of the coupling to be expected for α-iodo alkyl radicals. Hence a radical recently detected in irradiated iodoacetamide with a maximum hyperfine coupling of 250 G cannot have this structure. Possible alternative structures are discussed.

The way in which the E.S.R. spectra for the α-iodo radica in 5-iododeoxyuridine are modified by the quadrupole interaction from ¹²⁷I is described and hence an estimate of the quadrupole coupling is obtained.     

EPR Spectra of Spin-labeled Fatty Acids in Detergent Solutions: “Immobilized Species” in the Absence of Protein

Electron paramagnetic resonance spectra of 5-doxyl-stearic acid, 3-(2-Dodecyl-1-oxyl-5,5-dimethylpyrrolidin-2-yl)propanoic acid radical (4-proxyl-palmitic acid) and of 3-(1-Oxyl-2,5-dimethyl-5-dodecylpyrrolidine-2-yl)propanoic acid radical (4-azetoxyl-stearic acid) in lipid micelles of nonaethyleneglycol-monododecyl ether (C₁₂E₉) and pentaethyleneglycol-monooctyl ether (C₈E₅) can reveal immobilized components typical of spin-labeled protein-bound fatty acids as well as phospholipids. The occurrence of such species depends on the detergent, the relative orientation of the nitroxide group with respect to the longitudinal axis and the molecular structure of the fatty acid as well as on polarity and ionic strength of the aqueous phase. Whereas 5-doxyl-stearic acid and 4-proxyl-palmitic acid exhibited these highly immobilized components in C₁₂E₉ micelles but not in C₈E₅, only a single relatively mobile component was observed with 4-azetoxyl-stearic acid in both detergents. The immobilized component is lost by the addition of isopropanol but not as much by ethanol and also when distilled water is substituted for the buffer.     

Hidden chemistry in phenoxyl radical (C6H5O•) coupling reaction mechanism revealed

A novel hidden reaction of the phenoxyl radical (C₆H₅O^?) with a specific daughter is found to significantly alter its hitherto accepted coupling reactions' scheme. Transient characterizations and mechanistic evaluations in highly acidic to strongly alkaline aqueous medium reveal this concurrent reaction competing favorably in nanosecond–microsecond time‐scale with the five distinct C₆H₅O^? + C₆H₅O^? reactions, which produce various phenolic end‐products as reported earlier (M. Ye and R. H. Schuler, J. Phys. Chem. 1989, 93, 1898). Presently, only the symmetric 4,4′‐dioxo transient precursor, O?C₆H₅? H₅C₆?O that leads to the stable 4,4′‐biphenol product, gets partially oxidized by a fraction of remaining C₆H₅O^?. The resulting secondary transient ^?C₁₂H₉O₂ radical is generated at diffusion‐controlled rate, k > 5.0 × 10⁹ M^?1 s^?1, and follows an independent chemistry. Consequently, when the previously reported five coupled end product distribution ratios were appropriately updated, the respective fractional values revealed a closer match for the symmetric 2,2′‐ and 4,4′‐biphenols with their suggested coupling reaction branching probabilities based on the atomic spin‐density distributions in the C₆H₅O^? radical (P. Neta, R. W. Fessenden, J. Phys. Chem., 1974, 78, 523). Results also suggest that in the remaining fraction, differential solvation in aqueous medium of various orientation‐related encounter complexes (C₆H₅O…C₆H₅O) formed during coupling favors rearrangement only toward 2,4′‐biphenolic product, at the cost of 2‐ and 4‐phenoxyphenolic species. Copyright © 2009 John Wiley & Sons, Ltd.     

A comparative study of radical cations of thiourea,thiosemicarbazide, and diethylthiourea in aqueous sulphuric acid media employing pulse radiolysis technique

The generation of radical cations of thiourea [(NH₂)₂C=S], thiosemicarbazide [(NH₂―NH)(NH₂)C=S], and diethylthiourea [(C₂H₅―NH)₂C=S] in aqueous sulphuric acid media of low to high acid strengths utilizing the pulse radiolysis technique is reported. In this system, the decay half‐life of radical cations of thiourea and its derivatives observed were longer (ranging from 45 to 100 µs at acid strength pH/H₀ = ?4.1), and the formation times could be altered with the change in acid strengths. By doing so, a wide range in half‐life of radical cations of thiosemicarbazide (0.7 to 100 µs) was observed in contrast to thiourea and diethylthiourea with the variation in acid strengths (pH/H₀) from 0.7 to ?4.1. This perhaps helps in studying the spectral and kinetic properties of transient radical cations comfortably. The general mechanism for the formation of radical cations of the aforementioned compounds in radiolysis has been revisited in which the electron transfer reactions are active. Copyright © 2013 John Wiley & Sons, Ltd.     

Sequence of phase transformations in the formation of superstructures of the M6C5 type in nonstoichiometric carbides

A symmetry analysis has been made of monoclinic and trigonal superstructures of the M₆C₅ type, which are formed in nonstoichiometric cubic carbides MC _y with a B1 structure. Channels of disorder-order transitions MC _y → M₆C₅ have been revealed, and the distribution functions of carbon atoms in the M₆C₅ superstructures have been calculated. The atomic-vacancy ordering in nonstoichiometric cubic carbides of vanadium VC _y and niobium NbC _y has been investigated using neutron diffraction and X-ray diffraction analyses. It has been shown that, as the temperature decreases, the Group V transition metal carbides MC _y can undergo two physically admissible sequences of transformations associated with the formation of M₆C₅ phases.     

Electron spin resonance investigation of carbon-13 hyperfine interactions in nitrobenzene anion radical

Synthesis of carbon-13 enriched nitrobenzene which contains 22·5 mole per cent of nitrobenzene-1-¹³C and an equal amount of nitrobenzene-4-¹³C has been accomplished. We report the carbon-13 hyperfine interactions in the corresponding anion radicals in a variety of solvent media. In hexamethylphosphoramide a _C1 = -7·05 gauss and a _C4 = 6·14 gauss. These couplings change to -9·03 gauss and 5·31 gauss, respectively, when the solvent medium is dimethylformamide containing 0·598 mole fraction of water. The solvent dependence of a _C1 is consistent with the radical remaining planar as opposed to adopting a pyramidal conformation at the nitrogen when hydrogen bonds form between the radical anion and protic solvents. Data reported here provide an experimental means to estimate the spin density distribution in nitrobenzene anion.     

Long- and short-range order in the Pd<Subscript>6</Subscript>B monoclinic superstructure and M<Subscript>6</Subscript>X<Subscript>5</Subscript> and M<Subscript>6</Subscript>X allied superstructures

Symmetry analysis of the Pd₆B monoclinic superstructure (space group C2/c) formed in the cubic (with the B1 structure) solid solution of boron in palladium (PdB _y ) has been carried out. The formation of this superstructure proceeds as a first-order phase transition via the disorder-order channel including nine nonequivalent superstructure vectors of four stars {k ₁₀}, {k ₄}, {k ₃}, and {k ₀}. For the Pd₆B monoclinic super-structure (space group C2/c), the distribution function for boron atoms is calculated and the interval of admissible values of the long-range order parameters is defined. It is shown that the transition channel determined in this way coincides with the channel in which the M₆X monoclinic superstructure (space group C2) is formed; therefore, the Pd₆B superstructure can also be described in space group C2 to the same degree of accuracy. The higher symmetry of the monoclinic model (space group C2/c) suggests that it describes the structure of the Pd₆B phase (Pd₆B□₅), as well as of mutually inverse phases M₆X□₅ and M₆X₅□, more adequately than the model with space group C2. It is shown that superstructures of the M₆X□₅ type (space groups C2/c, C2, C2/m, and P3₁) and inverse superstructures of the M₆X₅□ type with the same space groups have the positions of the nearest surrounding of metal atoms by two types of nonmetallic sublattice sites located in the first and second coordination spheres.     

Synthesis and photophysical properties of polyamides containing in-chain porphyrin and [60]fullerene

Conjugated polyamides containing porphyrin and [60]fullerene (C₆₀) in the main chain were prepared by a direct polycondensation of the 3′H,3″H-dicyclopropa[1, 9:16, 17] [5, 6]fullerene-C₆₀-I _h -3′,3″-dicarboxylic acid and 5,15-bis(4-aminophenyl)-10,20-bis(3,5-dialkoxyphenyl)porphyrin in the presence of triphenyl phosphite and pyridine. Gel permeation chromatography (GPC) analysis of the polyamides showed the weight-average molecular weight was about 23,626–23,736, and the temperature at 5% weight loss determined by thermogravimetric analysis (TGA) was above 216 °C. The transmission electron microscopy (TEM) images displayed the regular one-dimensional linear arrays of the polyamides with lengths exceeded 200 nm. The photoinduced electron transfer from porphyrin to C₆₀ in the polyamides was observed in nanosecond laser-flash photolysis experiments at ambient temperature, which produced a charge-separated state (porphyrin radical cation–C₆₀ radical anion pair) with a lifetime as long as 40 μs. The calculated ratio of k _CS/k _CR was found to be 2.1 × 10⁴. They could have potential applications for photoelectronic devices, organic solar cells and so on.     

The diphenylphosphino radical, (C6H5)2˙P

The diphenylphosphino radical, (C₆H₅)₂P, has been observed at room temperature in X-irradiated single crystals of triphenylphosphine oxide and diphenylphosphine sulphide and in ultra-violet irradiated polycrystalline diphenylphosphine at 77 K. The isotropic ³¹P coupling constant is deduced to be +252 MHz and the anisotropic components are approximately 504, -252, -252 MHz. The largest value of the g tensor lies perpendicularly to the direction of the largest hyperfine coupling constant, the values in diphenylphosphine sulphide being 2·0094, 2·0035 and 2·0021. X-irradiated triphenylphosphine oxide also contains the (C₆H₅)₂PO radical.     

Flame structure of laminar premixed anisole flames investigated by photoionization mass spectrometry and photoelectron spectroscopy

Two laminar, premixed, fuel-rich flames fueled by anisole-oxygen-argon mixtures with the same cold gas velocity and pressure were investigated by molecular-beam mass spectrometry at two synchrotron sources where tunable vacuum-ultraviolet radiation enables isomer-resolved photoionization. Decomposition of the very weak O–CH₃ bond in anisole (C₆H₅OCH₃) by unimolecular decomposition yields the resonantly-stabilized phenoxy radical (C₆H₅O). This key intermediate species opens reaction routes to five-membered ring species, such as cyclopentadiene (C₅H₆) and cyclopentadienyl radicals (C₅H₅). Anisole is often discussed as model compound for lignin to study the phenolic-carbon structure in this natural polymer. Measured temperature profiles and mole fractions of many combustion intermediates give detailed information on the flame structure. A very comprehensive reaction mechanism from the literature which includes a sub-scheme for anisole combustion is used for species modeling. Species with the highest measured mole fractions (on the order of 10^?3–10^?2) are CH₃, CH₄, C₂H₂, C₂H₄, C₂H₆, CH₂O, C₅H₅ (cyclopentadienyl radical), C₅H₆ (cyclopentadiene), C₆H₆ (benzene), C₆H₅OH (phenol), and C₆H₅CHO (benzaldehyde). Some are formed in the first destruction steps of anisole, e.g., phenol and benzaldehyde, and their formation will be discussed and with regard to the modeling results. There are three major routes for the fuel destruction: (1) formation of benzaldehyde (C₆H₅CHO), (2) formation of phenol (C₆H₅OH), and (3) unimolecular decomposition of anisole to phenoxy (C₆H₅O) and CH₃ radicals. In the experiment, the phenoxy radical could be measured directly. The phenoxy radical decomposes via a bicyclic structure into the soot precursor C₅H₅ and CO. Formation of larger oxygenated species was observed in both flames. One of them is guaiacol (2-methoxyphenol), which decomposes into fulvenone. The presented speciation data, which contain more than 60 species mole fraction profiles of each flame, give insights into the combustion kinetics of anisole.     

Simulation analysis of ESR spectrum of polymer alkyl-C60 radicals formed by photoinitiated reactions of low-density polyethylene

Polymer alkyl-fullerene (P-C₆₀) radical adducts produced by ultraviolet (UV) photoinitiated reactions between low-density polyethylene (LDPE) and C₆₀ in the presence of benzophenone (BP) as a photoinitiator have been detected and identified for the first time by electron spin resonance (ESR) and confirmed by simulation analysis of the spectrum. A well-resolved ESR spectrum was recorded in situ upon UV irradiation of the LDPE/BP/C₆₀ sample in the molten state (413 K). Detailed analysis of hyperfine structures shows that the observed spectrum is composed of three components: a broad singlet atg = 2.0025 from the C₆₀ radical anion; an innermost pair of¹³C satellites; and a 12-line spectrum superposed on the broad singlet. The simulation analysis of the spectrum shows that the 12-line spectrum is due to the overlapping of two kinds of radical adducts of tertiary carbon-C₆₀ (A) and secondary carbon-C₆₀ (B), which have slightly differentg-values and almost the same integral intensity I_A/I_B (48.4/51.6). The spectrum simulated on the basis of the¹H and¹³C hyperfine interaction parameters is in good agreement with the observed spectrum. These results provide positive evidence that the C₆₀-bound LDPE materials can be obtained directly by a simple method of BP-photoinitiated reaction of the LDPE/C₆₀ system.     

Finite-temperature properties of the muonium substituted ethyl radical CH2MuCH2: nuclear degrees of freedom and hyperfine splitting constants

The finite-temperature (T) properties of the muonium substituted ethyl radical CH₂MuCH₂ have been theoretically studied by Feynman path integral quantum Monte Carlo (PIMC) simulations. To derive the ensemble averaged expectation values we have combined the PIMC formalism with an efficient tight-binding (TB) Hamiltonian and a density functional operator of the B3LYP type in the EPRIII basis. The TB operator has been used to calculate the potential energy surface (PES) of the ethyl radical in the doublet ground state, the harmonic and anharmonic vibrational wave numbers as well as several probability density functions of the nuclei. The harmonic linear response approximation, which makes use of the Feynman centroid density, has been adopted to evaluate the anharmonic wave numbers. The large anharmonicities in the nuclear potential lead to bond lengths in thermal equilibrium which exceed the vibrationless parameters at the PES minimum. This enhancement is particularly strong for the C–Mu bond. It is responsible for the suppression of the intramolecular rotation for temperatures below room temperature. In C₂ H₅ the rotation is allowed down to 10?K. The dissimilar rotational dynamics for H₂MuCH₂ and C₂ H₅ has been studied with the help of TB-based probability density functions. The nuclear configurations of CH₂MuCH₂ and C₂ H₅, which are populated in thermal equilibrium, have been used to evaluate the isotropic and anisotropic hyperfine splitting (hfs) constants under explicit consideration of the nuclear vibrations and the internal rotation. The hfs constants have been determined with the help of the B3LYP-EPRIII Hamiltonian. The hindered low-temperature rotation in the Mu isomer is responsible for roto-vibrational corrections to the isotropic hfs constants which are smaller than the corrections in C₂ H₅. The shortcomings of single-configuration approaches for the evaluation of isotropic hfs constants have been demonstrated for both radicals. The ensemble corrections to the isotropic hfs parameters are correlated with fluctuations in the atomic spin densities. Differences in the absolute values of the isotropic hfs parameters in CH₂MuCH₂ and C₂ H₅ can be traced back to differences in the nuclear degrees of freedom. The ensemble shift for each isotropic hfs parameter can be explained by characteristic nuclear motions. For this discussion we make use of the distribution functions of the isotropic hfs constants. Roto-vibrational corrections to the anisotropic hfs constants are rather small. PIMC simulations have been performed between 25 and 1000?K, i.e. in a T interval that is large enough to consider nuclear effects beyond zero-point motions. The TB and B3LYP-EPRIII based physical quantities of CH₂MuCH₂ and C₂ H₅ have been compared with experimental findings whenever possible.     

有机分子超薄膜的结构对摩擦的影响

采用分子动力学方法, 模拟了由脂肪酸C_nH_2n+1COOH}和C₁₇H₃₁COOH (n=12,13,14,15,16,17)组成的混合单层Langmuir-Blodgett(LB)膜间的摩擦特性, 探究了膜结构的变化对超薄膜的摩擦的影响. 结果显示. 在滑动过程中, 随着n的增加, 膜内分子的运动受到邻近分子的约束逐渐增加, 膜结构的稳定性也逐渐增加, 其剪切压逐渐减小, n=17时的剪切压最小. 在两单层膜之间无氢键形成; 而混合膜内的分子之间形成的氢键是单层膜结构稳定的主要因素, 其中n=16时形成的氢键最稳定, 但全部由相同C₁₇H₃₁COOH分子组成的单层膜的滑动效果最好. 分子的弯曲形变能对剪切压影响非常小.     

Molar extinction coefficients of solutions of some organic compounds

Molar extinction coefficients of aqueous solutions of some organic compounds, viz. formamide (CH₃NO),N-methylformamide (C₂H₅NO),NN-dimethylformamide (C₃H₇NO),NN-dimethylacetamide (C₄H₉NO), 1,4-dioxane (C₄H₈O₂₄), succinimide (C₄H₅NO₂) and solutions of acetamide (C₂H₅NO) and benzoic acid (C₇H₆O₂) in 1,4-dioxane (C₄H₈O₂) have been determined by narrow beam γ-ray transmission method at 81, 356, 511, 662, 1173 and 1332 keV. The experimental values of mass attenuation coefficients of these compounds have been used to calculate effective atomic numbers and electron densities. The additivity rule earlier used for aqueous solution has been extended to non-aqueous (1,4-dioxane) solutions.     

MRCI方法研究CSe(X1Σ+)自由基的光谱常数和分子常数

采用内收缩多参考组态相互作用方法在0.08—2.5 nm的核间距范围内计算了CSe(X¹Σ⁺)自由基的势能曲线.为确保势能曲线的计算精度,C原子使用较大的相关一致基aug-cc-pV5Z,Se原子使用最大的相对论赝势基aug-cc-pV5Z-pp.对CSe(X¹Σ⁺)自由基的势能曲线进行了拟合,并进行了同位素识别,得到了该自由基6个主要同位素分子(¹²C^{74< 关键词： 同位素识别 势能曲线 光谱常数 分子常数}     

Theoretical study of the nuclear degrees of freedom in the isotropic and anisotropic hyperfine coupling constants of the C2H5 radical: a Feynman path integral–density functional approach

The isotropic and anisotropic hyperfine coupling (hfs) constants of the C₂H₅ radical have been theoretically studied under the conditions of thermal equilibrium, i.e. under the explicit consideration of the nuclear degrees of freedom. For this purpose the Feynman path integral quantum Monte Carlo (PIMC) formalism has been combined with an electronic Hamiltonian of the B3LYP–EPRIII type. The density functional operator has been used to derive both the distribution functions for the isotropic and anisotropic hfs constants of the ethyl radical as well as the thermal mean values. The electron paramagnetic resonance (EPR) timescale enables only the measurement of the thermal averages. The underlying distribution functions of these mean values, however, offer insight into the nature and strength of the nuclear degrees of freedom contributing to the observable thermal averages. The EPR parameters of C₂H₅ have been studied between 25 and 1000?K. This temperature (T?) window is large enough to consider nuclear fluctuations beyond zero-point effects. The deviations between the thermally averaged hfs constants and the values at the minimum of the potential energy surface (PES) are caused by (i) enlargements of the bond lengths in thermal equilibrium under the influence of anharmonicities in the internuclear potential, and (ii) by the intramolecular methylene rotation. The latter degree of freedom leads to a planar CH₂ unit in thermal equilibrium. At the minimum of the PES the methylene fragment exhibits a certain pyramidalization. The ensemble corrections as well as the T dependence of the isotropic hfs constants are larger than the ensemble shifts and T dependence of the anisotropic parameters. The non-validity of the crude Born–Oppenheimer approximation for the theoretical evaluation of physically meaningful isotropic hfs constants of the ethyl radical has been explained on the basis of specific nuclear degrees of freedom. Theoretical results of the ensemble averaged Monte Carlo type as well as single-nuclear configuration data are compared with experiment whenever available.     

ESR of γ-irradiated DI-, TRI- and poly-glycin

The effects of γ radiation on glycylglycine, triglycine, and polyglycine have been studied using ESR spectroscopy. The radiation exposures have been performed under vacuum at room temperature. All those irradiated substances show a doublet absorption signal often ascribed to a radical formed on the α carbon of the peptide chain. The magnetic susceptibility has been found to depend on temperature according to the Curie-Weiss law. The radio-sensitivity parameter G_r , the hyperfine splitting constant A_h and the spectroscopic factor g nave been determinated as well as θ which is the temperature corresponding to the Curie temperature of the irradiated substance. The results on C_r , θ and A_h for all peptides studied here agree with the assumption of radiation induced free radicals located at the α carbon of the peptide chain, leading to conclude that the degree of delocalization of the unpaired electron increases for increasing number of glycine units in the peptide. Finally, from negative θ values obtained, a conclusion has been reached on the antiferromagnetic nature of the interaction between unpaired spins.