Structural features of 1,3,5-trifluorobenzene radical anion,as studied by optically detected ESR spectroscopy and quantum chemistry

The ESR spectrum of 1,3,5-trifluorobenzene radical anion was obtained for the first time using optically detected ESR spectroscopy. Experimental data and results of a quantum chemical study of the potential energy surface (density functional theory B3LYP and BHHLYP methods) are indicative of fast interconversions between nonplanar conformations of this radical anion.     

Long-lived 1-adamantaneimminyl free radical as studied by ESR in 1-cyanoadamantane

The products of γ-irradiation of 1-cyanoadamantane were studied as a function of temperature by ESR spectroscopy. 1-adamantyl, 1-cyanoadamantyl and 1-adamantaneimminyl free radicals are formed in proportions depending on temperature. The low-temperature major product, 1-cyanoadamantyl transforms upon heating into the imminyl free radical which can be used as a spin label for studying molecular dynamics in glassy crystalline 1-cyanoadamantane.     

ESR方法检测竹红菌素的半醌负离子自由基

从顺磁共振波谱检测到通过竹红菌素的光诱导还原和基态竹红菌素跟脂肪胺的电子转移两条途径所得到的自由基信号, 并由电化学还原得到同一自由基, 确认它为半醌负离子自由基。竹红菌素溶液中加入芳香胺观察不到ESR信号。它的3,10位上与醌基相邻近的羟基离解以后, 无论在基态还是在激发态都观察不到ESR信号。此外, 还从吸收光谱观察到半醌负离子自由基吸收以及竹红菌素跟脂肪胺之间的相互作用。     

Molecular structure of N-chlorosuccinimide studied by gas-phase electron diffraction and quantum-chemical methods

The molecular structure of N-chlorosuccinimide has been studied by GED method at the nozzle temperature of 116 °C. Vibrational corrections to the r _a parameters, Δ(r _a − r _e), have been calculated using the scaled quadratic and cubic force constants from B3LYP/6-31G(df,p) calculations. The force field scaling has been carried out using the IR and Raman spectra of the solid N-chlorosuccinimide. The molecular skeleton and the bond conformation around nitrogen were found to be planar within large experimental errors. The equilibrium geometrical parameters derived from the experimental data assuming C _2v molecular symmetry and those from MP2(fc)/6-311G(3df,2pd) calculations are in a good agreement.     

Reactions of the primary radical cations of carboxylic acids as studied by ESR in freonic matrices

Radical products of radiolysis of frozen solutions of propionic and butyric acids were studied in the matrices of Freon-11, Freon-113, and Freon-113a at 77 K. It was shown that the primary radical cations generated by radiation were not trapped in the freonic matrices (in contrast with the corresponding freonic solutions of acetic acid). The radical cations of propionic and butyric acids decay in concurrent processes of rearrangements yielding terminal-type and ylide-type distonic radical cations and intramolecular proton transfer in the dimeric radical cations resulting in acyloxy radicals. The latter species undergo decarboxylation to yield ethyl and propyl radicals for propionic and butyric acids, respectively. According to mass-spectrometric data, the terminal-type distonic radical cations undergo the McLafferty rearrangement.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 2, 2005, pp. 97–104.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.This revised version was published online in April 2005 with a corrected cover date.     

Temperature change of OD ESR and freon matrix ESR of the radical cations of 9,10-octalin and cyclohexene

The optical detection (OD) ESR and Freon matrix ESR spectra were observed for the radical cations of 9,10-octalin and cyclohexene from 77 to 300 K. The observed spectral change was attributed to ring inversion in both radical cations based on MO calculations and an analysis by the modified Bloch equation. The activation energy for the inversion of the radical cation of octalin was calculated as 18.8 kJ mol⁻¹.     

Ion—molecular charge transfer as studied by the method of optically detected ESR of radical pairs

Effects of ion—molecular charge transfer upon optically detected (OD) ESR spectra of radical pairs have been studied experimentally and theoretically. Theoretical analysis has shown that under certain conditions (often realized in experiments) OD ESR spectra are analogous to standard ESR ones. Broadening and collapse of the hyperfine structure (exchange broadening) have been observed experimentally depending on the concentration of neutral acceptors. Ion—molecular charge transfer rate constants have been calculated for some radical-anions from the experimental data.     

An ESR study of various anthrasemiquinone radical anions obtained by in situ electrochemical method

Electron spin resonance spectra of seven derivatives of 9,10-anthrasemiquinone and 1,4-anthrasemiquinone are studied. These radicals have been obtained by in situ electrochemical reduction of the parent molecules.The compounds studied are: 9,10- (1), 2-chloro-1,4-dihydroxy-9,10- (2), 2,3-dichloro-1,4-dihydroxy-9,10-(3), 1,4-diacethoxy-6,7-dimethyl-9,10- (4), 1,4-dichloro- (5), 1,4-dihydroxy-9,10-anthrasemiquinone (6) and 9-chloro-10-methoxy-1,4-anthrasemiquinone (7).The hyperfine coupling constants (hfs) and the corresponding g factor are given. The assignment of these hfs is justified.     

Investigation of the dimerization of acridine radical anions by electrochemical methods

The electrochemical reduction of acridine (dimethylformamide (DMF) and Bu₄NClO₄) was investigated by differential cyclic voltammetry and controlled-potential electrolysis. The experimental data (dimerization rate constant and yield of the 9,9 dimer) agreed with the theoretically calculated ones.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1284–1286, June, 1990.     

The molecular structure of N-methylsuccinimide studied by gas-phase electron diffraction (GED) and quantum-chemical methods

The molecular structure of N-methylsuccinimide was studied by the GED method at a nozzle temperature of 69–73°C. Anharmonic vibrational corrections to thermal-average r _a bond lengths, Δ(r _a − r _e), were calculated using the quadratic and cubic force constants from B3LYP/6-31G(df, p) calculations. The molecular skeleton was found to be planar within measurement errors. Some structural effects were likely caused by the conjugation of the N atom with two C=O bonds. The equilibrium geometric parameters derived from the experimental data and those from MP2/cc-pVTZ(seg-opt) calculations were in close agreement.     

ESR spectra of endohedral metallofullerene Ce@C82 radical anions in dimethylformamide and pyridine

Endohedral metallofullerene Ce@C₈₂ dissolved in dimethylformamide or pyridine is reduced to the radical anion. Analysis of hyperfine coupling with the topologically different ¹³C nuclei indicates the electronic structure with bivalent cerium and the paramagnetic carbon framework Ce²⁺@C₈₂ ^·3–. The ESR spectra of the radical anions of the functionalized Ce@C₈₂ derivatives are detected.     

Micelle-polymer complexes as studied by the ESR spin probe technique

Complexes between sodium dodecyl sulfate micelles and the water-soluble polymers poly(N-vinylpyrrolidone), poly(ethylene oxide) and a copolymer of poly (vinyl alcohol) and poly(vinyl acetate) have been studied in aqueous solution by the electron spin resonance (ESR) technique using di-tert-butyl nitroxide as a spin probe. The effective rotational correlation times reveal lowering of the critical micelle concentration and decreased headgroup packing in the micelle upon interaction with the polymers.     

Intramolecular dynamics and conformational transition in proteins studied by biophysical labelling methods. Common and specific features of proteins from thermophylic micro-organisms

A general survey is carried out on the theoretical grounds for methods of spin, luminescence and M?ssbauer labels, as well as their application in the study of protein intramolecular dynamics. When combined, these methods allow the protein dynamics to be investigated within a wide range of correlation times (tau c = 10(2) - 10(-10) s) and amplitudes. The purposeful application of the methods to various proteins at different temperatures (30-330 K), water content, substrate addition, etc., revealed a number of dynamical processes and conformational transitions in proteins. The experiments indicated correlations between the local segmental mobility of protein globules in a nanosecond temporal scale and biochemical reactions, such as long-distance electron transfer, hydrolysis and photoreactions. The biophysical labelling methods results were analysed together with the data on dynamics obtained using complementary physico-chemical methods and theoretical calculations. Special emphasis is given to recent results on proteins from thermophylic micro-organisms. The mechanisms of protein intramolecular dynamics and their role in the stability and functions of proteins and enzymes are discussed.     

Mechanism of radiation-induced degradation of poly(methyl methacrylate) as studied by ESR and electron spin echo methods

The role of radical species in the degradation of poly(methyl methacrylate) (PMMA) induced by γ-irradiation has been studied by means of electron spin resonance and electron spin echo spectroscopy. The major radical species generated initially at 77 K are assigned to main chain ? CH ? and side chain ? COOCH₂ radicals, and ? COOCH anion radical. Only the ? COOCH₂ radical converts to the scission-type ? CH₂ ? C(CH₃)COOCH₃ radical on warming the sample of >180 K. A part of the ? CH ? radical disappears on warming the sample of >265 K. It is concluded that the scission of PMMA main chain occurs by the intramolecular process from the ? COOCH₂ radical as the precursor state.     

Polymorphism and dynamics of MBBA as studied by NMR

We report proton NMR experiments on the liquid crystal material N-(p-methoxybenzylidene) p-n-butylaniline (MBBA) at 100MHz in the temperature range 110-350 K. The phase diagram was investigated by means of second moment and spin-lattice relaxation measurements in order to establish connections between dynamics and phase transitions. The results show that in a slow cooling experiment, two processes contribute to the relaxation, a slow ethyl group motion together with reorientation of the methyl groups. For the glassy nematic state, as well as for the phases observed after reheating a quenched sample, only methyl rotation is observed. The correlation times of these various mechanisms were determined and the results compared with those obtained by previous NMR and dielectric analysis.