Time-resolved Magnetic Field Effect Research Articles

Radical ions with nearly degenerate ground state: Correlation between the rate of spin-lattice relaxation and the structure of adiabatic potential energy surface

Paramagnetic spin-lattice relaxation (SLR) in radical cations (RCs) of the cycloalkane series in liquid solution was studied and analyzed from the point of view of the correlation between the relaxation rate and the structure of the adiabatic potential energy surface (PES) of the RCs. SLR rates in the RCs formed in x-ray irradiated n-hexane solutions of the cycloalkanes studied were measured with the method of time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs. Temperature and, for some cycloalkanes, magnetic field dependences of the relaxation rate were determined. It was found that the conventional Redfield theory of the paramagnetic relaxation as applied to the results on cyclohexane RC, gave a value of about 0.2 ps for the correlation time of the perturbation together with an unrealistically high value of 0.1 T in field units for the matrix element of the relaxation transition. The PES structure was obtained with the DFT quantum-chemical calculations. It was found that for all of the cycloalkanes RCs considered, including low symmetric alkyl-substituted ones, the adiabatic PESes were surfaces of pseudorotation due to avoided crossing. In the RCs studied, a correlation between the SLR rate and the calculated barrier height to the pseudorotation was revealed. For RCs with a higher relaxation rate, the apparent activation energies for the SLR were similar to the calculated heights of the barrier. To rationalize the data obtained it was assumed that the vibronic states degeneracy, which is specific for Jahn-Teller active cyclohexane RC, was approximately kept in the RCs of substituted cycloalkanes for the vibronic states with the energies above and close to the barrier height to the pseudorotation. It was proposed that the effective spin-lattice relaxation in a radical with nearly degenerate low-lying vibronic states originated from stochastic crossings of the vibronic levels that occur due to fluctuations of the interaction between the radical and the solvent. The magnitude of these fluctuations, ~100 cm(-1), determines the upper scale of the unperturbed splitting between the vibronic states, for which the manifestation of this paramagnetic relaxation mechanism could be expected. Our estimate for the relaxation rate derived using standard Landau-Zener model of nonadiabatic transitions at the level crossing agrees with the experimental data. This paramagnetic relaxation mechanism can also be operative in paramagnetic species of other types such as linear radicals, radicals with threefold degeneracy, paramagnetic centers in crystals, etc. It looks likely that the proposed SLR mechanism can be quenched by a fast vibrational relaxation in radicals.

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.

The study of radical cations of Me 3С–SiMe 3 and Me 3С–GeMe 3 in alkane solutions using the method of time-resolved magnetic field effect and DFT calculations

Radical cations of Me 3СSiMe 3 and Me 3СGeMe 3 were detected in hexane solution using the method of time-resolved magnetic field effect. The structures and electronic distributions of neutral molecules Me 3CEMe 3 and their radical cations: Me 3СEMe 3 + (E = C, Si, Ge, Sn) were investigated by DFT PBE/TZ2P calculations. The calculated and the experimental hyperfine coupling constants a(H) are in good agreement. The structures of Me 3СSi Me 3 + , Me 3СGe Me 3 + , and Me 3СSn Me 3 + are in essence a tight radical cation pairs of the type CMe 3 / EMe 3 + .

The method of time-resolved magnetic field effect in recombination fluorescence with magnetic field switching

A technique for studying the spin evolution of radical ion pairs is proposed, which is based on switching off the magnetic field with a certain time delay after the ionizing radiation pulse giving rise to the pairs. The spin evolution of radical pairs upon field switching is described by a quasiclassical model. The modeling of magnetic field effect curves, measured for n-decane solutions of deuterated and protonated paraterphenyl, provides fair agreement between theory and experiment.

Detection of branched alkane radical cations in solutions by the method of the time-resolved magnetic field effect

Detection of branched alkane radical cations in solutions by the method of the time-resolved magnetic field effect

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.

Degenerate Electron Exchange Reaction of n-Alkane Radical Cations in Solution

The degenerate electron exchange (DEE) reaction involving radical cations (RCs) of n-nonane, n-dodecane, and n-hexadecane in n-hexane solution was studied over the temperature range 253-313 K using the method of time-resolved magnetic field effect in recombination fluorescence of spin-correlated radical ion pairs. In the dilute solutions the rate constant of DEE was found to be 200 times slower than the diffusion limit. Using n-nonane as an example, we showed that two reasons are responsible for the low value of the RC self-exchange rate: (1) conformational variability of molecules and RCs and (2) the activation barrier of DEE reaction. The calculations of the reaction enthalpy performed by the B3LYP/6-31G(d) method indicated that electron transfer can be effective only upon collision of RC with a neutral molecule either in the all-trans conformation or in the conformation differing from the latter by rotation of the end ethyl fragment. The activation barrier of the DEE reaction was estimated using the reorganization energy of the internal degrees of freedom calculated at the B3LYP level and was found to be about 6 kcal/mol. A possible influence of the interaction between RC and a neutral molecule in an encounter complex on DEE rate constant is also discussed.

Longitudinal Electron Spin Relaxation Induced by Degenerate Electron Exchange as Studied by Time-Resolved Magnetic Field Effects

T(1) paramagnetic relaxation of radical ions induced by degenerate electron exchange (DEE) reactions is studied theoretically and experimentally. Our theoretical analysis shows that T(1) relaxation time is well described by the Redfield theory at arbitrary values of the characteristic DEE time tau. Longitudinal relaxation of norbornane (NB) radical cation is studied by means of the time-resolved magnetic field effects (TR-MFE) technique; the rate constant of DEE involving NB(*+) radical cation and NB neutral molecule is obtained. Advantages of the TR-MFE technique and its potential for measuring the short DEE times are discussed in detail.

Spin Dynamics of the Radical Pair in a Low Magnetic Field Studied by the Transient Absorption Detected Magnetic Field Effect on the Reaction Yield and Switched External Magnetic Field

The spin dynamics of the radical pair generated from the photocleavage reaction of (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TMDPO) in micellar solutions was studied by the time-resolved magnetic field effect (MFE) on the transient absorption (TA) and by a novel technique, absorption detected switched external magnetic field (AD-SEMF). Thanks to the large hyperfine coupling constant (A = 38 mT), a characteristic negative MFE on the radical yield was observed at a magnetic field lower than 60 mT whereas a positive effect due to the conventional hyperfine (HFM) and relaxation mechanisms (RM) was observed at higher magnetic field. The negative effect can be assigned to the mechanism "so-called" low field effect (LFE) mechanism and has been analyzed thoroughly using a model calculation incorporating a fast spin dephasing process. The time scale of the spin mixing process of LFE studied by AD-SEMF is shorter than the lifetime of the recombination kinetics of the radical pair. These results indicate that the LFE originates from the coherent spin motion. This can be interfered from the fast spin dephasing caused by electron spin interaction fluctuations.

Spin evolution of radical pair with radical containing two groups of equivalent magnetic nuclei

Analytical solution is obtained for time-resolved magnetic field effects (TR-MFE) on recombination fluorescence of radical-ion pair (RIP) containing radical ion with two groups of magnetically equivalent nuclei. The present theoretical approach is applied to three experimental systems: RIPs containing radical cations of 2,3-dimethylbutane, 2,2,6,6-tetramethylpiperidine, or diisopropylamine and radical anion of p-terphenyl-d14 in nonpolar alkane solutions. Good agreement between theory and experiment is found for all the three systems, hyperfine coupling constants of radical cations are obtained by fitting the experimental TR-MFE traces. The potential of the TR-MFE technique for studying radical ions with nonequivalent nuclei is discussed in detail. The wide applicability of the theoretical model and the experimental technique make them useful for studying short-lived radical species that are often beyond the reach of the conventional electron paramagnetic resonance spectroscopy.

Paramagnetic relaxation of cyclohexane radical cation in solution as measured by the method of time-resolved magnetic field effect

The method of time-resolved magnetic field effect was applied to measure the spin–lattice relaxation time T1 for radical cations of a number of cycloalkanes in n-hexane solution at room temperature. The T1 value for the Jahn–Teller active cyclohexane radical cation was found to be approximately 9 ns which was substantially shorter than typical values for organic radical ions. Based upon the similarity of effects that various factors had on the spin–lattice relaxation rate of the cyclohexane radical cation and the benzene radical anion it was proposed that the spin–orbital interaction made the predominant contribution to the relaxation of both radicals.

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

Download options Please wait... Article information DOI https://doi.org/10.1039/B311315A Article type Paper Submitted 16 Sep 2003 Accepted 07 Jan 2004 First published 03 Feb 2004 Download Citation Phys. Chem. Chem. Phys., 2004,6, 924-928 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Spectroscopic capabilities of the time-resolved magnetic field effect technique as illustrated in the study of hexamethylethane radical cation in liquid hexane V. A. Bagryansky, V. I. Borovkov and Y. N. Molin, Phys. Chem. Chem. Phys., 2004, 6, 924 DOI: 10.1039/B311315A To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Victor A. Bagryansky Vsevolod I. Borovkov Yuri N. Molin

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.

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.

Paramagnetic relaxation of radical cations in alkane solutions as measured by time-resolved magnetic field effects

Spin-lattice relaxation times of radical cations were measured by the decay of the time-resolved magnetic field effect in the recombination fluorescence of radical ion pairs in liquid hydrocarbons at high concentrations of solvent holes acceptor. The dependences of spin-lattice relaxation time on the magnetic field strength and solvent viscosity were studied. The results could not be explained in terms of the model of ion-molecular charge transfer involving monomeric or dimeric radical cations. The paramagnetic relaxation observed in a weak magnetic field is attributed to internal reorganizations of aggregates that originate from radical cations at high acceptor concentration.

Identification of Radical Cations of n -Alkanes in Irradiated Solutions by Time-Resolved Magnetic Field Effect Methods

Alkane radical cations are the key transient species determining the composition of the products of radiolysis of alkane solutions. As probed by pulse radiolysis, the lifetimes of n -alkane radical cations under common conditions range from several nanoseconds to several tens of nanoseconds [1]. The same lifetime range for these species was obtained in studying stationary magnetic field effects by radioluminescence methods [2]. Short lifetimes of n -alkane radical cations render routine ESR methods impracticable for studying these species in solutions. Only such a highly sensitive ESR technique as optically detected ESR of spin-correlated radical ion pairs made it possible to observe a line with a width of 0.27 mT upon irradiation of pentadecane; it was assigned to the pentadecane radical cation [3]. No signals were observed for other alkanes [4]. We pioneered in obtaining ESR spectra of some n -alkane radical cations in solution by means of another method applicable to investigation into spin chemistry, namely, the method of magnetic field effects in recombination fluorescence of spin-correlated radical ion pairs [5, 6]. In nonpolar alkane solutions, fluorescence decay caused by recombination of radical ion pairs is determined by the relationship

Study of a fraction of spin-correlated pairs in radiation spurs by the methods of time-resolved magnetic field effects and quantum beats

The fractions Θ β and Θ X of spin-correlated singlet radical-ion pairs in alkane solutions irradiated by β-particles and X-rays were got from experiments on magnetic field effects and quantum beats in recombination fluorescence. The ratio Θ β / Θ X values of 1.3−2.8 for the solvent series studied were found both from magnetic effect and quantum beats. The Monte Carlo calculations were made to explain the variations of Θ β / Θ X in different solvents. The high value of Θ β / Θ X = 2.8 for isooctane is probably due both to large separation between ions in pair and to high concentration of neutral radical in the spur.

Time-Resolved magnetic field effects in the recombination products of geminate triplet pairs for electron phototransfer reactions

We studied the influence of the additives of tetracyanoethylene (TCE) (radical-anion acceptor) and the variations in exciplex concentration on the magnitude of time-resolved magnetic field effect in a delayed excimer fluorescence in polar exciplex system pyrene-N,N-diethylaniline. The nonmonotonous dependence of the magnetic field effect on tetracyanoethylene concentratration was found. The magnitude of the magnetic field effect was brought close to the predicted maximum limit of about 300 ns by decreasing the exciplex concentration. We conclude that the yield of the triplets, resulting from intercombination transitions in a singlet exciplex, is rather small. The upper limit for the rate constant of these transitions has been determined to be less than 4.6 × 106 s−1.

Electron hopping and magnetic field dependent spin dynamics of radical ions in solution

The role of hopping on the geminate recombination of radical ions (N,N-dimethylaniline cation and anthracene anion) in acetonitrile is studied via the nanosecond time-resolved magnetic field effect on the triplet yield and the influence of donor concentration thereon. Increasing donor concentration leads to lifetime broadening of the magnetic field dependence of the triplet yield. Responsible for this effect is the perturbation of the coherent spin motion caused by hopping of the electron spin between donor sites of different nuclear spin configuration. Comparison of experimental results with calculations based on the semiclassical theory of spin motion yields an estimate of the hopping rates. Deuteration of both radicals influences the halfwidth of the magnetic field effect: at long probing times and low donor concentrations the halfwidth measured for protonated radical ions exceeds the one for the deuterated species: at short delay times and large donor concentrations, i.e. high hopping rates, this isotopic effect is reversed.

Nanosecond time-resolved magnetic field effect on radical recombination in solution

The time dependence of the magnetic field effect on radical recombination in solution has been analyzed experimentally and theoretically. For the geminate recombination of anthracene anions and dimethylaniline cations in a polar solvent, the effect originates from a magnetic field dependent production of triplet states in an initially singlet phased radical pair, induced by hyperfine interaction of the unpaired electrons with the nuclei. The magnetic field dependence of the triplet yield shows a lifetime broadening of the energy levels of the radical pair if a short delay-time between radical production and triplet observation is chosen. The agreement of this delay-time dependent broadening effect with the theoretical results proves directly the coherence of the spin motion in the radical pairs.