The spin mixing process of a radical pair in low magnetic field observed by transient absorption detected nanosecond pulsed magnetic field effect

The spin mixing process of the radical pair in the sodium dodecyl sulfate (SDS) micelle is studied by using a novel technique nanosecond pulsed magnetic field effect on transient absorption. We have developed the equipment for a nanosecond pulsed magnetic field and observed its effect on the radical pair reaction. A decrease of the free radical yield by a reversely directed pulsed magnetic field that cancels static field is observed, and the dependence on its magnitude, which is called pulsed MARY (magnetic field effect on reaction yield) spectra, is studied. The observed spectra reflect the spin mixing in 50-200 ns and show clear time evolution. Theoretical simulation of pulsed MARY spectra based on a single site modified Liouville equation indicates that the fast spin dephasing processes induced by the modulation of electron-electron spin interaction by molecular reencounter affect to the coherent spin mixing by a hyperfine interaction in a low magnetic field.     

CIDEP of micellized radical pairs in low magnetic fields

We report the first experimental study of chemically induced electron spin polarization (CIDEP) processes in low magnetic fields for spin-correlated radical pairs (SCRPs) in micellar environments. Photoexcitation of (2,4,6-trimethylbenzoyl) diphenylphosphine oxide (TMBDPO) leads to the radical pair comprised of acyl radical 1 and phosphonyl radical 2. The spin polarization, which is very strong in free solution even at zero field, was detected using L-band time-resolved electron paramagnetic resonance (TREPR) spectroscopy with specially modified resonators. The mechanism of formation and decay of low field CIDEP in SCRPs is presented and discussed. The prominent difference between low and high field spectra in micelles is the absence of anti-phase structure for radical 2 with HFI a > B0. This feature is consistent with the proposed polarization mechanism and theoretical predictions.     

Influence of dipolar interactions on radical pair recombination reactions subject to weak magnetic fields

Monte Carlo simulations of the effects of weak magnetic fields on the recombination of interacting radical pairs undergoing free diffusion in solution have been performed, with the aim of determining the influence on the low field effect of the magnetic dipolar coupling between the radicals. The suppression of singlet-triplet interconversion in the radical pair by the dipolar interaction is found to be pronounced at magnetic field strengths comparable to the hyperfine interactions in the radicals, to the extent that the low field effect is completely abolished. The averaging of the dipolar coupling by the translational diffusion of the radicals around one another is relatively efficient in the presence of strong magnetic fields but becomes ineffective in weak applied fields where the strength of the dipolar interaction is independent of the orientation of the inter-radical vector. Low field effects are only likely to be observed if the motion of the radical pair is restricted in some way so as to increase the likelihood that, having separated to the large distance required for the dipolar interaction to have a negligible effect, the radicals subsequently encounter and have the opportunity to recombine.     

Direct observation of f-pair magnetic field effects and time-dependence of radical pair composition using rapidly switched magnetic fields and time-resolved infrared methods

A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.     

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.     

Radio frequency magnetic field effects on a radical recombination reaction: a diagnostic test for the radical pair mechanism

The photoinduced electron-transfer reaction of chrysene with isomers of dicyanobenzene is used to demonstrate the sensitivity of a radical recombination reaction to the orientation and frequency (5-50 MHz) of a approximately 300 muT radio frequency magnetic field in the presence of a 0-4 mT static magnetic field. The recombination yield is detected via the fluorescence of the exciplex formed exclusively from the electronic singlet state of the radical ion pair Chr*+/DCB*-. Magnetic field effects are simulated using a modified version of the gamma-COMPUTE algorithm, devised for the simulation of magic angle spinning NMR spectra of powdered samples. The response of a chemical or biological system to simultaneously applied radio frequency and static or extremely low-frequency magnetic fields could form the basis for a diagnostic test for the operation of the radical pair mechanism that would not require prior knowledge of the nature and properties of the radical reaction.     

Spin relaxation of a short-lived radical in zero magnetic field

A short-lived radical containing only one I = 1/2 nucleus, the muoniated 1,2-dicarboxyvinyl radical dianion, was produced in an aqueous solution by the reaction of muonium with the dicarboxyacetylene dianion. The identity of the radical was confirmed by measuring the muon hyperfine coupling constant (hfcc) by transverse field muon spin rotation spectroscopy and comparing this value with the hfcc obtained from DFT calculations. The muon spin relaxation rate of this radical was measured as a function of temperature in zero magnetic field by the zero field muon spin relaxation technique. The results have been interpreted using the theoretical model of Fedin et al. (J. Chem. Phys., 2003, 118, 192). The muon spin polarization decreases exponentially with time after muon implantation and the temperature dependence of the spin relaxation rate indicates that the dominant relaxation mechanism is the modulation of the anisotropic hyperfine interaction due to molecular rotation. The effective radius of the radical in solution was determined to be 1.12 ± 0.04 nm from the dependence of the muon spin relaxation rate on the temperature and viscosity of the solution, and is approximately 3.6 times larger than the value obtained from DFT calculations.     

Coherent laser excitation of Rydberg states in a weak static magnetic field

We study one-photon excitation of atomic Rydberg- and continuum states close to a photoionization threshold in the presence of a weak static external magnetic field. A semiclassical closed orbit representation for the atomic transition amplitudes is derived, which exhibits the connection between quantum mechanics and the classical dynamics of the excited electron whose motion under the combined influence of the Coulomb field of the ionic core and the magnetic field is chaotic.     

Magnetic field effects on spin-rephasing in a photochemically produced radical pair investigated by a double-pulse technique using a nitrogen laser

The effect of an external magnetic field on the yield of the benzophenone ketyl radical has been investigated by a new, double-pulse technique. The technique and its variants can be used not only for studying geminate recombination of photochemically produced radical pairs but also for continuous photoelectric recording of fluorescence spectra of the free radicals which can be generated by nitrogen laser irradiation.     

Electron spin dynamics as a probe of molecular dynamics: temperature-dependent magnetic field effects on charge recombination within a covalent radical ion pair

The electron spin-spin exchange interaction, 2J, in radical pairs (RPs) is exquisitely sensitive to the details of molecular structure and can thus serve as an important probe of structural dynamics in RPs of potential interest to photonic and electronic devices. Photoinitiated ultrafast two-step charge separation produces (1)(MeOAn(+)(*)-6ANI-NI(-)(*)), where MeOAn = p-methoxyaniline, 6ANI = 4-(N-piperidinyl)naphthalene-1,8-dicarboximide, and NI = naphthalene-1,8:4,5-bis(dicarboximide). Radical pair intersystem crossing subsequently produces (3)(MeOAn(+)(*)-6ANI-NI(-)(*)), and the total RP population decays with approximately 10 ns lifetime at 140 K, which increases to nearly 30 ns at 300 K in toluene. The activation energy observed for this process is negative and can be explained by a mechanism involving a conformational preequilibrium of the RP followed by charge recombination. Over the same temperature range, the magnetic field effect (MFE) on yield of the triplet recombination product, MeOAn-6ANI-(3)()NI, yields the magnitude of 2J, which directly monitors the superexchange electronic coupling for charge recombination. A single resonance in the MFE plot is observed at 300 K, which splits into two resonances at temperatures below 230 K, suggesting that there are two distinct groups of RP conformations at low temperature. The magnitude of 2J for the lower field resonance (10 mT) at 140 K is 5 times smaller than that of the high field resonance. At 300 K the equilibrium is shifted almost entirely to the set of conformers with the stronger electronic coupling. The motion that couples these two groups of conformations is the motion that most effectively gates the donor-acceptor electronic coupling.     

Magnetic field effects and radical pair mechanisms in enzymes: a reappraisal of the horseradish peroxidase system

Reinvestigation by stopped-flow spectrophotometry of the previously observed influence of a static magnetic field on the horseradish peroxidase (HRP)-catalyzed reduction of hydrogen peroxide by Taraban et al. (J. Am. Chem. Soc. 1997, 119, 5768) did not reproduce the originally observed effects. No magnetic field effect was observed for static fields of up to 75 mT. Field-induced changes in both k1 and k2 reported in the original work were found to produce equal and opposite effects on the shape of the observed kinetic decay of the 418 nm spectroscopic signal as a result of the difference in the relative absorbances of Native HRP and Compound II.     

Effect of a weak magnetic field on hematite sol in stationary and flowing systems

The effect of a weak magnetic field on the aggregation state and electrophoretic mobility of hematite sol was studied in flowing (dynamic) systems as a function of time and electrolyte concentration (0–60 mmol/dm³ KCl) and compared with the effect of the field in stationary (static) systems and flow in the absence of the field. During the entire treatment period, the pH remained almost constant (4.06–4.24). Conductance varied with KCl concentration, but except for minor fluctuations appeared to be unaffected by any form of treatment. While aggregation of hematite was observed during dynamic magnetic treatment (change in turbidity, scattered light intensity, and photon correlation spectroscopy), little effect on aggregation state was observed for the static systems or for the flowing systems in the absence of the field. Mobility also increased during the first 30 min of static and dynamic magnetic treatment. After longer treatment periods (90–120 min), the mobility decreased, but in almost all cases remained larger than in the case of untreated systems. Changes in both mobility and particle aggregation state also showed a significant dependence on electrolyte concentration. These effects are discussed in terms of a magnetohydrodynamic interaction between the magnetic field and the charged colloidal particles, which results only when the particles are made to pass rapidly through the field.     

Effect of a weak magnetic field on the reaction between neutral free radicals in isotropic solution

Radical recombination following the photodissociation of 2-hydroxy-4'-(2-hydroxyethoxy)-2-methylpropiophenone in isotropic solution was monitored using time-resolved infrared spectroscopy. The rate of radical recombination was determined to decrease in the presence of a magnetic field of greater than 5 mT and to increase in the presence of a magnetic field smaller than 5 mT.     

Hyperfine quantum beat spectroscopy of a polyatomic molecule in a weak magnetic field with polarized light

Resolved Zeeman splittings in molecular quantum beats are used for the first time to assign hyperfine components of a triplet sublevel of a polyatomic     

External magnetic field effect on the decay rate of benzophenone ketyl radical in a micelle

The magnetic field effect upon the decay rate of the benzophenone ketyl radical in micellar solution was studied via nanosecond laser photolysis. Its decay is represented by a combination of two exponentials. The rate constant of the slow component was found to decrease from 1.6 × 10⁶ s^?1 at zero field to 9 × 10⁵ s^?1 at 700 G.     

Resonance effect of a high-frequency magnetic field on the recombination probability of radical pairs in a liquid

Expressions for a resonance dependence of geminate recombination probability of radical pairs on the intensity of an external steady magnetic field in the presence of a high-frequency magnetic field normal to it have been obtained Numerical calculations of the above dependence have been performed for the cases involving Δg and hyperfine coupling mechanisms of spin conversion.