Radical cations of branched alkanes in solutions: time-resolved magnetic field effect and quantum chemical studies

Radical cations of heptane and octane isomers, as well as several longer branched alkanes, were detected in irradiated n-hexane solutions at room temperature by the method of time-resolved magnetic field effect (TR MFE). To identify radical cations, the hyperfine coupling constants as determined by simulation of the TR MFE curves were compared to the constants calculated using the density functional theory (DFT) approach. The g-values of the observed radical cations were close to that of the 2,2,3,3-tetramethylbutane radical cation studied earlier by optically detected electron spin resonance (ESR) and TR MFE techniques. No evidence of the decay of the radical cations of branched alkanes to produce olefin radical cations was found, which was further supported by the observation of positive charge transfer from the observed radical cations to cycloalkane molecules. The lifetimes of the radical cations of the branched alkanes were found to be longer than tens of nanoseconds.     

Radical cations of branched alkanes as observed in irradiated solutions by the method of time-resolved magnetic field effect

Spin dynamics in radical ion pairs formed under ionizing irradiation of n-hexane solutions of two branched alkanes 2,3-dimethylbutane and 2,2,4-trimethylpentane has been studied by the method of time-resolved magnetic field effect in recombination fluorescence. Experimental curves of the magnetic field effect are satisfactorily described by assuming that the spin dynamics is determined by the hyperfine interactions in the radical cation (RC) of branched alkane under study with hyperfine coupling (HFC) constants averaged by internal rotations of RC fragments. The HFC constants determined from the magnetic field effect curves are close to those estimated within DFT B3LYP approach. Analysis of the results indicates that at room temperature the lifetimes of the RC of the studied branched alkanes amount to, at least, tens of nanoseconds.     

Potential energy surface and proton HFI constants of the cyclopentane radical cation

According to the data of UB3LYP/6-31G* and UMP2/cc-pVTZ calculations, the adiabatic potential energy surface of the cyclopentane radical cation is very intricate and combines six types of stationary structures of C _s and C ₂ symmetry. Ten equivalent C _s structures with the totally symmetric electronic state (C _s (² A′)) correspond to global minima. Conformational transitions between the global minima occur along the inversion and pseudorotation coordinates, for each pair of minima the conformational transition occurring in one stage (through the only transition state). The inversion barrier is ～2 kcal/mol; pseudorotation barriers are ～4–8 kcal/mol. The structure of the potential surface provides the interpretation of the EPR data as a result of dynamic averaging over 20 C _s (² A′) and C ₂ (² A) stationary structures.     

MNDO Study of the (Anti)aromaticity of Fluorine-Containing Cyclopentadienyl,Indenyl, and Cyclopenta[b]naphthyl Cations

MNDO calculations were performed to estimate the aromaticity (antiaromaticity) of a series of fluorine-containing cyclopentadienyl, indenyl, and cyclopenta[b]naphthyl cations in terms of the Dewar-Breslow criterion which utilizes the difference in the enthalpies of formation of isomeric cations with closed and open -systems as aromaticity index. The aromaticity is strongly determined by both the structure of the carbon skeleton and the number and position of fluorine atoms. A linear correlation was revealed between the aromaticity index and the energy of the lowest singlet-singlet excitation for cations having a cyclic -system.     

Solvation Dependences of Isotropic Hyperfine Interaction Constants and out-of-Plane Distortions of ortho-Substituted Nitrobenzene Radical Anions

For a number of ortho-substituted nitrobenzene radical anions (RAs) generated in DMF and its binary mixtures with water, it is shown that for radical anions with a substituent of minor effective volume in one ortho position to the nitro group, the dependences of the isotropic hyperfine interaction (ihfi) constants on the mole fraction of water are S-like and dictated by the medium composition and the concerted out-of-plane rotational and pyramidal distortions of the nitro group of the radical anion. The S-like shape of the solvation dependences of the ihfi constants is dictated by the dominant rotational distortions of the nitro group. For most radical anions with two ortho substituents or with one ortho substituent with a large effective volume, the S-like dependences are not observed, and the values of the nitrogen ihfi constants depend on the dominant pyramidal distortion of the nitro group. For the 2-tert-butylnitrobenzene radical anion in water, the nitrogen ihfi constant is a_N=25.62 G, which is typical of nitroaliphatic radical anions. This effect is explained based on the pyramidal structure of the nitro group in the case of its large rotation angles.     

Effect of composition and thermal treatment on the properties and microstructure of Co–SiO<Subscript>2</Subscript> films

The effect of various physical factors (chemical composition, temperature, and thermal treatment) on the macroscopic properties of Co–SiO₂ film composites produced by ion-plasma sputtering of mosaic targets has been investigated. The microstructure parameters have been estimated indirectly by analyzing the magnetization curves and the temperature dependence of magnetization. It has been concluded that the model of homogeneous noninteracting superparamagnetic particles of hexagonal Co is applicable for describing the magnetic properties of the film composites under study over a rather wide range of compositions.