V. I. Borovkov

Spectroscopic capabilities of the time-resolved magnetic field effect technique as illustrated in the study of hexamethylethane radical cation in liquid hexane

The method of the time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs can provide information on the parameters of the EPR spectra of short-lived radical ions. The present paper demonstrates these spectroscopic potentialities using, as an example, hexamethylethane radical cation whose EPR spectrum in solution is determined by isotropic hfc with 18 equivalent protons. The hfc constant is determined from the position of peaks of the time-resolved magnetic field effect of the hexamethylethane and perdeuterated p-terphenyl solution in n-hexane at ambient temperature. The difference in the g-values of (hexamethylethane)˙+ and (p-terphenyl-d14)˙− radical ions is found by analyzing the curve shape of magnetic field effect for various magnetic field strength. Both the hfc constant and the difference in the g-values coincide with the data obtained by the OD EPR method. Independence of the curve shape on hexamethylethane concentration testifies to the low rate of ion-molecular charge transfer. The method of time resolved magnetic field effect is used to determine the times of longitudinal and transverse paramagnetic relaxation. The dependency of the relaxation rates on magnetic field strength is derived and feasible relaxation mechanisms are discussed.

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.

Spin-selective reaction with a third radical destroys spin correlation in the surviving radical pairs.

The goal of this work is to reveal the effect that an irreversible spin-selective reaction of a partner of the spin-correlated radical pair (SCRP) with a third paramagnetic particle has on the spin state of the surviving SCRPs in the absence of spin exchange interaction. As studied SCRPs, we used the geminate (excess electron/radical cation) pairs generated by ionizing irradiation of tetramethyl-para-phenylenediamine solutions in n-dodecane. As a spin-selective reaction, the scavenging of electrons by nitroxide radicals from the bulk of the solution was used. Both the electron scavenging reaction and the spin correlation in the surviving SCRPs were monitored by measuring the recombination fluorescence decays of the irradiated solutions under the same experimental conditions. It was found that the spin-selective electron scavenging results in the acceleration of spin correlation decay in the remaining unreacted SCRPs. In accordance with the suggested theoretical model, the rate of this additional spin correlation decay is revealed to be equal to the scavenging rate.

Pseudorotation in Radical Cations of Low-Symmetric Decalin Molecules

Adiabatic potential energy surfaces (PES) of isomeric decalin cations have been found to be the pseudorotational surfaces due to avoided crossing that is typical for the highly symmetric Jahn-Teller active ions rather than for low-symmetric bicyclic systems. According to the UB3LYP/6-31G* data, the height of the barrier to pseudorotation is less than 2 kcal/mol for the trans isomer and about 9 kcal/mol for the cis isomer. Another peculiarity of the cis isomer PES is that the structure of minimum energy lies beyond the pseudorotation gutter. The calculation results are in fair agreement with the experimental electron spin resonance data.

Structure and Stability of Pentafluoroaniline and 4-Aminononafluorobiphenyl Radical Anions: Optically Detected Electron Paramagnetic Resonance, Time-Resolved Fluorescence, Time-Resolved Magnetic Field Effect, and Quantum Chemical Study.

Radical anions (RAs) are the key intermediates of the selective hydrodefluorination of polyfluoroarenes. We used the techniques of optically detected electron paramagnetic resonance (OD EPR), time-resolved fluorescence, time-resolved magnetic field effect (TR MFE), and the density functional theory to study the possibility of RAs formation from 4-aminononafluorobiphenyl (1) and pentafluoroaniline (2) and estimate their lifetimes and decay channels. To our knowledge, both RAs have not been detected earlier. We have registered the OD EPR spectrum for relatively stable in nonpolar solutions 1(-•) but failed to register the spectra for 2(-•). However, we have managed to fix the 2(-•) by the TR MFE method and obtained its hyperfine coupling constants. The lifetime of 2(-•) was found to be only a few nanoseconds. The activation energy of its decay was estimated to be 3.6 ± 0.3 kcal/mol. According to the calculation results, the short lifetime of 2(-•) is due to the RA fast fragmentation with the F(-) elimination from ortho-position to the amine group. The calculated energy barrier, 3.2 kcal/mol, is close to the experimental value. The fragmentation of 2(-•) in a nonpolar solvent is possible due to the stabilization of the incipient F(-) anion by the binding with the amine group proton.

Unusual proton-transfer complex between the 2,2,6,6-tetramethylpiperidine radical cation and neutral molecule

ISSN 0012-5016, Doklady Physical Chemistry, 2008, Vol. 420, Part 2, pp. 125–127.

On the verification of different approaches describing spin-selective radical recombination

The so-called “phenomenological” kinetic equation for one -pair density operator for spin-selective reactions is defended. We derive this eq uation from the kinetic equation for density operator of all pairs which are treated as singlet and triplet bosons. There presented some reasons for inconsistency of measurement-like approa ch t the problem. PACS numbers: 82.20.-w

Pseudorotation as a possible origin of fast paramagnetic relaxation in radical ions with a degenerate or quasi-degenerate ground state

In early 1960s, aromatic radical ions with a three-fold or higher symmetry axis were shown to haveanomalously high spin–lattice relaxation rates [1, 2].For example, the paramagnetic relaxation rate in ben-zene and coronene radical anions in solution turned outto be one to two orders of magnitude higher than in rad-icals of lower symmetry. An even higher relaxation ratewas later revealed in the fullerene radical anion [3] andin radical cations of some cycloalkanes [4–6].The observed anomalously fast spin–lattice relax-ation in radical ions of high-symmetry molecules insolution cannot be explained by common relaxationmechanisms, such as modulation of anisotropic hyper-fine coupling (HFC) and g tensor by chaotic molecularmotion or modulation of isotropic HFC due to transi-tions between distorted Jahn–Teller structures, as wellas by spin–rotation interaction [2, 7]. At present, thehypothesis that spin–orbit coupling in such radicals isresponsible for the anomalous relaxation is believed tobe the most probable [2–7]. However, despite numer-ous attempts, no concrete theoretical model of thisrelaxation has been suggestedOne of the factors complicating the creation of thetheory of paramagnetic relaxation of high-symmetryradicals is the absence of experimental information onthe nature of the state in which the unpaired electronspin is efficiently coupled to the other degrees of free-dom. In particular, for the best studied benzene radicalanion, the doubly degenerate ground vibronic state [2],in which the average projection of the orbital momentonto the symmetry axis can be nonzero, is suggested assuch a state. These concepts make it possible to realis-tically estimate the relaxation rate of the benzene radi-cal anion [7]; however, such arguments are untenablefor asymmetric particles, lacking symmetry-relateddegeneracy, with fast paramagnetic relaxation [4–6].It is worth noting that studying the paramagneticrelaxation in high-symmetry molecular structures is notonly of theoretical significance. Symmetric structures(fullerenes, fullerites, nanotubes) are candidates fordesign of nanosized devices of molecular electronicsand spintronics [8]. Therefore, it should be possible tocontrol the states of separate spins in them, and, hence,the paramagnetic relaxation of these states should berather slow.Radical ions exhibiting anomalous paramagneticrelaxation are structurally nonrigid [9], and it is quitelikely that there is a correlation between this relaxationand intramolecular dynamics of such particles. Theo-retical analysis of the adiabatic potential energy surface(PES) of the ground state of radical ions is required tostudy this correlation in detail.In this paper, we briefly describe the results of suchanalysis for radical cations (RCs) of a series of alkyl-substituted cyclohexanes ( R–C

Detection of radical cations of group 14 element organometallics in alkane solutions using the method of time-resolved magnetic field effect

Radical cations of Group 14 element organometallics R4E and R3EER3 (E=Si,Ge,Sn, R=Me,Et) were generated in alkane solutions by X-ray irradiation and studied using the time-resolved magnetic field effect technique. Modeling shows that in alkane solutions the g-factors of Me4E+˙ and Me3EEMe3+˙ are close to those measured in low temperature matrices. At concentrations of organometallics of about 0.1 M the fast electron self-exchange between radical cations Me4E+˙ (E=Si,Ge) and the corresponding neutrals takes place. In the case of R3EER3 the electron self-exchange has not been observed. Short times of phase relaxation (∼10 ns) are found for all the studied radical cations. Possible contributions to relaxation rates are discussed. For solutions of Me4E a peculiarity manifested as a positive magnetic field effect in strong magnetic field is observed in spin dynamics at short times. This peculiarity is assigned to the radical cations of olefins arising upon radiolysis of an alkane solvent.

Solvent Radical Anions in Irradiated Aliphatic Ketones and Esters as Observed Using Time-Resolved Magnetic Field Effects in the Recombination Fluorescence

Abstract It has been found that addition of alcohols (~0.1 M) to some liquid ketones and esters results in well-pronounced oscillations in the decay of the delayed fluorescence intensity from irradiated solutions. The analysis of the time-resolved magnetic field effects (TR MFEs) in the recombination fluorescence has shown that these oscillations are a manifestation of singlet-triplet transitions in spin-correlated radical ion pairs (RIPs) created by irradiation. Comparison with literature data indicates that the transitions are due to hyperfine couplings (HFCs) in the solvent radical anion (RA), stabilized due to the presence of alcohol molecules. In acetone, this stabilization effect has been observed for methanol, ethanol, 2- propanol, and, to a smaller extent, for tert-butanol. Similar effects have also been observed in diethyl ketone, ethyl acetate, and methyl propionate but not in methyl tert-butyl ketone and ethyl trimethylacetate. The results obtained indicate that the interaction between the radical anions (RAs) of carbonyl compounds and alcohol molecules is of importance in pulse radiolysis studies of organic liquids and their mixtures.