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Radical pair recombination reactions are known to be sensitive to the application of both low and high magnetic fields. The application of a weak magnetic field reduces the singlet yield of a singlet-born radical pair, whereas the application of a strong magnetic field increases the singlet yield. The high field effect arises from energy conservation: when the magnetic field is stronger than the sum of the hyperfine fields in the two radicals, S → T± transitions become energetically forbidden, thereby reducing the number of pathways for singlet to triplet interconversion. The low field effect arises from symmetry breaking: the application of a weak magnetic field lifts degeneracies among the zero field eigenstates and increases the number of pathways for singlet to triplet interconversion. However, the details of this effect are more subtle and have not previously been properly explained. Here we present a complete analysis of the low field effect in a radical pair containing a single proton and in a radical pair in which one of the radicals contains a large number of hyperfine-coupled nuclear spins. We find that the new transitions that occur when the field is switched on are between S and T0 in both cases, and not between S and T± as has previously been claimed. We then illustrate this result by using it in conjunction with semiclassical spin dynamics simulations to account for the observation of a biphasic-triphasic-biphasic transition with increasing magnetic field strength in the magnetic field effect on the time-dependent survival probability of a photoexcited carotenoid-porphyrin-fullerene radical pair. [ABSTRACT FROM AUTHOR]

The article discusses the field of spin chemistry, which focuses on the effects of electron and nuclear spins in chemical reactions and properties of molecules related to magnetic fields. The development of spin chemistry requires collaboration between research communities in physics, chemistry, biology, materials science, and information science. The article introduces a special topic on spin chemistry, which includes 11 papers discussing recent developments in the field. Topics covered include magnetic field effects in living cells, the use of covalent organic frameworks as spin-dependent catalysts, and the potential applications of spin chemistry in areas such as medical diagnostics and crop yields. The article aims to inspire further progress in spin chemistry research and encourage more scientists to contribute to the field. [Extracted from the article]

[Display omitted] • Arbitrary Waveform Generator (AWG) based reaction control is demonstrated for the first time and named AWG-RYDMR. • The periodic chirping technique enhances the spin mixing efficiency by covering the hyperfine structure of the resonance frequencies. • Chirped RF irradiation enhances spin mixing more than single-frequency RF and is as efficient as low static magnetic fields (∼1 mT). Spin-correlated radical pairs (RPs) are crucial for understanding how magnetic fields influence reaction yield and spin polarization. Reaction Yield Detected Magnetic Resonance (RYDMR) enables the observation of oscillating magnetic field effects on reaction yield, linking radical pairs to subsequent reactions. This study introduces the use of an arbitrary waveform generator (AWG) in RYDMR experiments, termed AWG-RYDMR, focusing on low-field RYDMR and its impact on spin-mixing efficiency. It is demonstrated that frequency-modulated RF pulses enhance spin mixing compared to single-frequency pulses by accessing a broader range of resonant frequencies, thus paving the way for advanced electron spin and reaction control techniques. [ABSTRACT FROM AUTHOR]

We have used an efficient new quantum mechanical method for radical pair recombination reactions to study the spin-dependent charge recombination along PTZ•+-Phn-PDI•- molecular wires. By comparing our results with the experimental data of Weiss et al. [J. Am. Chem. Soc. 126, 5577 (2004)], we are able to extract the spin-dependent (singlet and triplet) charge recombination rate constants for wires with n = 2-5. These spin-dependent rate constants have not been extracted previously from the experimental data because they require fitting its magnetic field-dependence to the results of quantum spin dynamics simulations. We find that the triplet recombination rate constant decreases exponentially with the length of the wire, consistent with the superexchange mechanism of charge recombination. However, the singlet recombination rate constant is nearly independent of the length of the wire, suggesting that the singlet pathway is dominated by an incoherent hopping mechanism. A simple qualitative explanation for the different behaviours of the two spin-selective charge recombination pathways is provided in terms of Marcus theory. We also find evidence for a magnetic field-independent background contribution to the triplet yield of the charge recombination reaction and suggest several possible explanations for it. Since none of these explanations is especially compelling given the available experimental evidence, and since the result appears to apply more generally to other molecular wires, we hope that this aspect of our study will stimulate further experimental work. [ABSTRACT FROM AUTHOR]

The standard quantum mechanical expressions for the singlet and triplet survival probabilities and product yields of a radical pair recombination reaction involve a trace over the states in a combined electronic and nuclear spin Hilbert space. If this trace is evaluated deterministically, by performing a separate time-dependent wavepacket calculation for each initial state in the Hilbert space, the computational effort scales as O(Z² log Z), where Z is the total number of nuclear spin states. Here we show that the trace can also be evaluated stochastically, by exploiting the properties of spin coherent states. This results in a computational effort of O(MZ log Z), where M is the number of Monte Carlo samples needed for convergence. Example calculations on a strongly coupled radical pair with Z > 106 show that the singlet yield can be converged to graphical accuracy using just M = 200 samples, resulting in a speed up by a factor of > 5000 over a standard deterministic calculation. We expect that this factor will greatly facilitate future quantum mechanical simulations of a wide variety of radical pairs of interest in chemistry and biology. [ABSTRACT FROM AUTHOR]

We present a new method for calculating the product yield of a radical pair recombination reaction in the presence of a weak time-dependent magnetic field. This method successfully circumvents the computational difficulties presented by a direct solution of the Liouville-von Neumann equation for a long-lived radical pair containing many hyperfine-coupled nuclear spins. Using a modified formulation of Floquet theory, treating the time-dependent magnetic field as a perturbation, and exploiting the slow radical pair recombination, we show that one can obtain a good approximation to the product yield by considering only nearly degenerate sub-spaces of the Floquet space. Within a significant parameter range, the resulting method is found to give product yields in good agreement with exact quantum mechanical results for a variety of simple model radical pairs. Moreover it is considerably more efficient than the exact calculation, and it can be applied to radical pairs containing significantly more nuclear spins. This promises to open the door to realistic theoretical investigations of the effect of radiofrequency electromagnetic radiation on the photochemically induced radical pair recombination reactions in the avian retina which are believed to be responsible for the magnetic compass sense of migratory birds. [ABSTRACT FROM AUTHOR]

Specific two-stage reversible reaction A + A↔C↔B + B of the decay of species C reactants by two independent transition channels is considered on the basis of the general theory of multistage reactions of isolated pairs of reactants. It is assumed that at the initial instant of time, the reacting system contains only reactants C. The employed general approach has made it possible to consider, in the general case, the inhomogeneous initial distribution of reactants, and avoid application of model concepts of a reaction system structure (i.e., of the structure of reactants and their molecular mobility). Slowing of multistage reaction kinetics as compared to the kinetics of elementary stages is established and physically interpreted. To test approximations (point approximation) used to develop a universal kinetic law, a widely employed specific model of spherical particles with isotropic reactivity diffusing in solution is applied. With this particular model as an example, ultimate kinetics of chemical conversion of reactants is investigated. The question concerning the depths of chemical transformation at which long-term asymptotes are reached is studied. [ABSTRACT FROM AUTHOR]

Chemically Induced Dynamic Nuclear Polarization (CIDNP) is an efficient method of creating non-equilibrium polarization of nuclear spins by using chemical reactions, which have radical pairs as intermediates. The CIDNP effect originates from (i) electron spin-selective recombination of radical pairs and (ii) the dependence of the inter-system crossing rate in radical pairs on the state of magnetic nuclei. The CIDNP effect can be investigated by using Nuclear Magnetic Resonance (NMR) methods. The gain from CIDNP is then two-fold: it allows one to obtain considerable amplification of NMR signals; in addition, it provides a very useful tool for investigating elusive radicals and radical pairs. While the mechanisms of the CIDNP effect in liquids are well established and understood, detailed analysis of solid-state CIDNP mechanisms still remains challenging; likewise a common theoretical frame for the description of CIDNP in both solids and liquids is missing. Difficulties in understanding the spin dynamics that lead to the CIDNP effect in the solid-state case are caused by the anisotropy of spin interactions, which increase the complexity of spin evolution. In this work, we propose to analyze CIDNP in terms of level crossing phenomena, namely, to attribute features in the CIDNP magnetic field dependence to Level Crossings (LCs) and Level Anti-Crossings (LACs) in a radical pair. This approach allows one to describe liquid-state CIDNP; the same holds for the solid-state case where anisotropic interactions play a significant role in CIDNP formation. In solids, features arise predominantly from LACs, since in most cases anisotropic couplings result in perturbations, which turn LCs into LACs. We have interpreted the CIDNP mechanisms in terms of the LC/LAC concept. This consideration allows one to find analytical expressions for a wide magnetic field range, where several different mechanisms are operative; furthermore, the LAC description gives a way to determine CIDNP sign rules. Thus, LCs/LACs provide a consistent description of CIDNP in both liquids and solids with the prospect of exploiting it for the analysis of short-lived radicals and for optimizing the polarization level. [ABSTRACT FROM AUTHOR]

Kinetics of a geminate particle pair with no interparticle interactions controlled by diffusion-limited reversible reactions is investigated in the presence of various possible external fields such as electric or gravitational fields based on continuum theory. Diffusion equations subject to multiple external fields are analytically solved with Green functions and the physical quantities such as the binding and survival probabilities are derived. Particularly, the local binding probabilities at the specific location on the reaction surface depending on the initial distance and orientation and the field directions are calculated. The variations of the binding probabilities due to the change of the field directions are predicted at long times and it shows that the binding probabilities tend to shift along the sum of individual field directions. [ABSTRACT FROM AUTHOR]

In this work, we treat spin-selective recombination of a geminate radical pair (RP) in a spherical "microreactor," i.e., of a RP confined in a micelle, vesicle, or liposome. We consider the microreactor model proposed earlier, in which one of the radicals is located at the center of the micelle and the other one undergoes three-dimensional diffusion inside the micelle. In addition, we suggest a twodimensional model, in which one of the radicals is located at the "pole" of the sphere, while the other one diffuses on the spherical surface. For this model, we have obtained a general analytical expression for the RP recombination yield in terms of the free Green function of two-dimensional diffusion motion. In turn, this Green function is expressed via the Legendre functions and thus takes account of diffusion over a restricted spherical surface and its curvature. The obtained expression allows one to calculate the RP recombination efficiency at an arbitrary magnetic field strength. We performed a comparison of the two models taking the same geometric parameters (i.e., the microreactor radius and the closest approach distance of the radicals), chemical reactivity, magnetic interactions in the RP and diffusion coefficient. Significant difference between the predictions of the two models is found, which is thus originating solely from the dimensionality effect: for different dimensionality of space, the statistics of diffusional contacts of radicals becomes different altering the reaction yield. We have calculated the magnetic field dependence of the RP reaction yield and chemically induced dynamic nuclear polarization of the reaction products at different sizes of the microreactor, exchange interaction, and spin relaxation rates. Interestingly, due to the intricate interplay of diffusional contacts of reactants and spin dynamics, the dependence of the reaction yield on the microreactor radius is non-monotonous. Our results are of importance for (i) interpreting experimental data for magnetic field effects on RP recombination in confined space and (ii) for describing kinetics of chemical reactions, which occur predominantly on the surfaces of biomembranes, i.e., lipid peroxidation reactions. [ABSTRACT FROM AUTHOR]

In a three-site representation, we study a spin polarization transfer from radical pair spins to a nearby electron or nuclear spin. The quantum dynamics of the radical pair spins is governed by a constant exchange interaction between the radical pair spins which have different Zeeman frequencies. Radical pair spins can recombine to the singlet ground state or to lower energy triplet states. It is then shown that the coherent dynamics of the radical pair induces spin polarization on the nearby third spin in the presence of a magnetic field. The spin polarization transfer depends on the difference between Zeeman frequencies, the singlet and triplet recombination rates, and on the exchange and dipole-dipole interactions between the different spins. In particular, the sign of the polarization depends on the exchange coupling between radical pair spins and also on the difference between singlet and triplet recombination rate constants. [ABSTRACT FROM AUTHOR]

The analysis of general (matrix) kinetic equations for the mean survival probabilities of any of the species in a sample (or mean concentrations) has been made for a wide class of the multistage geminate reactions of the isolated pairs. These kinetic equations (obtained in the frame of the kinetic approach based on the concept of "effective" particles in Paper I) take into account various possible elementary reactions (stages of a multistage reaction) excluding monomolecular, but including physical and chemical processes of the change in internal quantum states carried out with the isolated pairs of reactants (or isolated reactants). The general basic principles of total and detailed balance have been established. The behavior of the reacting system has been considered on macroscopic time scales, and the universal long-term kinetics has been determined. [ABSTRACT FROM AUTHOR]

We formulate a multiple-encounter model of the radical pair mechanism that is based on a random coupling of the radical pair to a minimal model environment. These occasional pulse-like couplings correspond to the radical encounters and give rise to both dephasing and recombination. While this is in agreement with the original model of Haberkorn and its extensions that assume additional dephasing, we show how a nonlinear master equation may be constructed to describe the conditional evolution of the radical pairs prior to the detection of their recombination. We propose a nonlinear master equation for the evolution of an ensemble of independently evolving radical pairs whose nonlinearity depends on the record of the fluorescence signal. We also reformulate Haberkorn's original argument on the physicality of reaction operators using the terminology of quantum optics/open quantum systems. Our model allows one to describe multiple encounters within the exponential model and connects this with the master equation approach. We include hitherto neglected effects of the encounters, such as a separate dephasing in the triplet subspace, and predict potential new effects, such as Grover reflections of radical spins, that may be observed if the strength and time of the encounters can be experimentally controlled. [ABSTRACT FROM AUTHOR]

We describe how the semiclassical theory of radical pair recombination reactions recently introduced by two of us [D. E. Manolopoulos and P. J. Hore, J. Chem. Phys. 139, 124106 (2013)] can be generalised to allow for different singlet and triplet recombination rates. This is a non-trivial generalisation because when the recombination rates are different the recombination process is dynamically coupled to the coherent electron spin dynamics of the radical pair. Furthermore, because the recombination operator is a two-electron operator, it is no longer sufficient simply to consider the two electrons as classical vectors: one has to consider the complete set of 16 two-electron spin operators as independent classical variables. The resulting semiclassical theory is first validated by comparison with exact quantum mechanical results for a model radical pair containing 12 nuclear spins. It is then used to shed light on the spin dynamics of a carotenoid-porphyrin-fullerene triad containing considerably more nuclear spins which has recently been used to establish a "proof of principle" for the operation of a chemical compass [K. Maeda, K. B. Henbest, F. Cintolesi, I. Kuprov, C. T. Rodgers, P. A. Liddell, D. Gust, C. R. Timmel, and P. J. Hore, Nature (London) 453, 387 (2008)]. We find in particular that the intriguing biphasic behaviour that has been observed in the effect of an Earth-strength magnetic field on the time-dependent survival probability of the photo-excited C·+PF·- radical pair arises from a delicate balance between its asymmetric recombination and the relaxation of the electron spin in the carotenoid radical [ABSTRACT FROM AUTHOR]

General matrix approach to the consideration of multistage geminate reactions of isolated pairs of reactants depending on reactant mobility is formulated on the basis of the concept of "effective" particles. Various elementary reactions (stages of multistage reaction including physicochemical processes of internal quantum state changes) proceeding with the participation of isolated pairs of reactants (or isolated reactants) are taken into account. Investigation has been made in terms of kinetic approach implying the derivation of general (matrix) kinetic equations for local and mean probabilities of finding any of the reaction species in the sample under study (or for local and mean concentrations). The recipes for the calculation of kinetic coefficients of the equations for mean quantities in terms of relative coordinates of reactants have been formulated in the general case of inhomogeneous reacting systems. Important specific case of homogeneous reacting systems is considered. [ABSTRACT FROM AUTHOR]

We investigate the reversible diffusion-influenced reaction of an isolated pair in the context of the area reactivity model that describes the reversible binding of a single molecule in the presence of a binding site in terms of a generalized version of the Feynman-Kac equation in two dimensions. We compute the corresponding exact Green's function in the Laplace domain for both the initially unbound and bound molecule. We discuss convolution relations that facilitate the calculation of the binding and survival probabilities. Furthermore, we calculate an exact analytical expression for the Green's function in the time domain by inverting the Laplace transform via the Bromwich contour integral and derive expressions for the binding and survival probability in the time domain as well. We numerically confirm the accuracy of the obtained expressions by propagating the generalized Feynman-Kac equation in the time domain. Our results should be useful for comparing the area reactivity model with the contact reactivity model. [ABSTRACT FROM AUTHOR]

Performance of the antisymmetrized product of strongly orthogonal geminal (APSG) ansatz in describing ground states of molecules has been extensively explored in the recent years. Not much is known, however, about possibilities of obtaining excitation energies from methods that would rely on the APSG ansatz. In the paper we investigate the recently proposed extended random phase approximations, ERPA and ERPA2, that employ APSG reduced density matrices. We also propose a time-dependent linear response APSG method (TD-APSG). Its relation to the recently proposed phase including natural orbital theory is elucidated. The methods are applied to Li2, BH, H2O, and CH2O molecules at equilibrium geometries and in the dissociating limits. It is shown that ERPA2 and TD-APSG perform better in describing double excitations than ERPA due to inclusion of the so-called diagonal double elements. Analysis of the potential energy curves of Li2, BH, and H2O reveals that ERPA2 and TD-APSG describe correctly excitation energies of dissociating molecules if orbitals involved in breaking bonds are involved. For single excitations of molecules at equilibrium geometries the accuracy of the APSG-based methods approaches that of the time-dependent Hartree-Fock method with the increase of the system size. A possibility of improving the accuracy of the TD-APSG method for single excitations by splitting the electron-electron interaction operator into the long- and short-range terms and employing density functionals to treat the latter is presented. [ABSTRACT FROM AUTHOR]

Spin-selective reactions of radical pairs are conventionally modelled using an approach that dates back to the 1970s [R. Haberkorn, Mol. Phys. 32, 1491 (1976)]. An alternative approach based on the theory of quantum measurements has recently been suggested [J. A. Jones and P. J. Hore, Chem. Phys. Lett. 488, 90 (2010)]. We present here the first experimental attempt to discriminate between the two models. Pulsed electron paramagnetic resonance spectroscopy has been used to investigate intramolecular electron transfer in the radical pair form of a carotenoid-porphyrinfullerene molecular triad. The rate of spin-spin relaxation of the fullerene radical in the triad was found to be inconsistent with the quantum measurement description of the spin-selective kinetics, and in accord with the conventional model when combined with spin-dephasing caused by rotational modulation of the anisotropic g-tensor of the fullerene radical. [ABSTRACT FROM AUTHOR]

In the presence of an external electric field, the ground-state A+Bq<=>Cq+D diffusion-influenced reversible reaction for a geminate pair, a neutral and a charged particle, is investigated in three dimensions. The probability density functions, the rates of reactions, and the survival probabilities of individual particles are analytically derived in the Laplace domain in terms of series solutions. The long-time kinetics of probability density functions and rates of reactions in rescaled forms shows a kinetic transition behavior from a t-3/2 power law to a t-3/2et increase when the condition D1F12≤D2F22, which depends on the diffusivities of particles and the external electric fields, changes to D1F12>D2F22. In the transition region D1F12=D2F22, the long-time behavior also shows a t-3/2 power law decay but with a different value of the prefactor. The rescaled survival probabilities only exhibit an exponentially increasing behavior at long times with no dependence on the various values of parameters. [ABSTRACT FROM AUTHOR]

This publication, in honour of Robert Kaptein's 80th birthday, contains contributions from colleagues, many of whom have worked with him, and others who admire his work and have been stimulated by his research. The contributions show current research in biomolecular NMR, spin hyperpolarisation and spin chemistry, including CIDNP (chemically induced dynamic nuclear polarisation), topics to which he has contributed enormously. His proposal of the radical pair mechanism was the birth of the field of spin chemistry, and the laser CIDNP NMR experiment on a protein was a major breakthrough in hyperpolarisation research. He set milestones for biomolecular NMR by developing computational methods for protein structure determination, including restrained molecular dynamics and 3D NMR methodology. With a lac repressor headpiece, he determined one of the first protein structures determined by NMR. His studies of the lac repressor provided the first examples of detailed studies of protein nucleic acid complexes by NMR. This deepened our understanding of protein DNA recognition and led to a molecular model for protein sliding along the DNA. Furthermore, he played a leading role in establishing the cluster of NMR large-scale facilities in Europe. This editorial gives an introduction to the publication and is followed by a biography describing his contributions to magnetic resonance. [ABSTRACT FROM AUTHOR]

Sensitivity being one of the main hurdles of nuclear magnetic resonance (NMR) can be gained by polarization techniques including chemically induced dynamic nuclear polarization (CIDNP). Kaptein demonstrated that the basic mechanism of the CIDNP arises from spin sorting based on coherent electron-electron nuclear spin dynamics during the formation and the recombination of a radical pair in a magnetic field. In photo-CIDNP of interest here the radical pair is between a dye and the molecule to be polarized. Here, we explore continuous-wave (CW) photo-CIDNP (denoted CW-photo-CIDNP) with a set of 10 tryptophan and tyrosine analogues, many of them newly identified to be photo-CIDNP active, and we observe not only signal enhancement of 2 orders of magnitude for 1H at 600MHz (corresponding to 10 000 times in measurement time) but also reveal that polarization enhancement correlates with the hydrophobicity of the molecules. Furthermore, the small chemical library established indicates the existence of many photo-CIDNP-active molecules. [ABSTRACT FROM AUTHOR]

In this work we derive conditions under which a level-crossing line in a magnetic field effect curve for a recombining radical pair will be equivalent to the electron spin resonance (ESR) spectrum and discuss three simple rules for qualitative prediction of the level-crossing spectra. [ABSTRACT FROM AUTHOR]

Due to the photobiology of the flavoproteins DNA photolyase and cryptochrome, electron transfer reactions between flavins and tryptophan are of significant biological relevance. In addition, electron transfer across vesicle membranes has also seen much attention. In this work, we study the electron transfer reaction between flavins and tryptophan across lipid bilayer membranes in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine small unilamellar vesicles using time-resolved optical absorption microspectroscopy and magnetically affected reaction yield spectroscopy. We demonstrate that riboflavin tetrabutyrate is embedded in the vesicle bilayer and can undergo electron transfer with tryptophan molecules in either the inner water pool or the bulk solution. Remarkably, flavin mononucleotide encapsulated in the inner water pool can undergo electron transfer across the vesicle bilayer to generate a magnetically sensitive radical pair with tryptophan molecules located in the bulk solution. The observed kinetics suggest that back electron transfer occurs between radical pairs generated by diffusive reencounter, either in the vesicle surface water or via electron hopping through degenerate electron exchange. [ABSTRACT FROM AUTHOR]

Highlights • A new drug-release technology using liposomal nanoparticles with magnetic controls. • Evaluating the drug-release potentials with spin-correlated radical pair mechanisms. • Influence of radical pair-unrelated photoreaction products. • Liposomal membrane packing to generate the pair-unrelated product. Abstract In this paper, we used anti-cancer flutamide (FM) as a radical pair-forming drug to investigate whether magnetic fields can control a liposomal drug release through differences in the physical properties of biomembranes. Irradiations of FM-incorporated liposomal nanoparticles (FM-LNs) were performed using an original air-cooled UV-LED irradiation system. Exposure to a static magnetic field of 0.2 T was performed using a pair of commercially available permanent magnets. For evaluating the drug-release potentials with spin-correlated radical pair mechanisms on the photochemical reactions developing in biomembranes, we prepared various types of FM-LNs equipped with possible magnetic controls. According to the overall result of the prepared FM-LNs, the drug release is deemed not to be caused by only the pair mechanisms. But the tests observing the magnetic field effects on the mechanisms must provide useful information on drug targeting. We strongly propose a new drug-release technology using these liposomal nanoparticles equipped with magnetic controls. [ABSTRACT FROM AUTHOR]

A new diradical having two 4,8,10‐trioxotriangulene (TOT) neutral radical units linked through an m‐phenylene moiety was synthesized and characterized by ESR measurements. An electrochemical study showed that the diradical undergoes two one‐electron reductions to generate corresponding dianion species, suggesting the electronic interaction between two TOT units through the π‐conjugated spacer. A strong intramolecular interaction between the two TOT units gives rise to the spin‐projected small hyperfine couplings in comparison with those of the monomer. Furthermore, the temperature dependent ESR measurement revealed that the dimer behaves as an S=1 species in the ground state with a ferromagnetic interaction of 2 J/kB=+7±3 K. [ABSTRACT FROM AUTHOR]

Formation of nuclei polarization via triplet mechanism has been observed and described, allowing reliable identification of ion-radicals stages of the reaction, estimation of the triplets and ion-radical pairs lifetime from the effects of chemical polarization of the nuclei. The data on relative values of isotropic and anisotropic interactions of unpaired electrons and nuclei have been obtained. [ABSTRACT FROM AUTHOR]

Extremely low-frequency (ELF) magnetic fields have been classified as possibly carcinogenic, mainly based on rather consistent epidemiological findings suggesting a link between childhood leukaemia and 50-60 Hz magnetic fields from power lines. However, causality is not the only possible explanation for the epidemiological associations, as animal and in vitro experiments have provided only limited support for carcinogenic effects of ELF magnetic fields. Importantly, there is no generally accepted biophysical mechanism that could explain such effects. In this review, we discuss the possibility that carcinogenic effects are based on the radical pair mechanism (RPM), which seems to be involved in magnetoreception in birds and certain other animals, allowing navigation in the geomagnetic field. We review the current understanding of the RPM in magnetoreception, and discuss cryptochromes as the putative magnetosensitive molecules and their possible links to cancer-relevant biological processes. We then propose a hypothesis for explaining the link between ELF fields and childhood leukaemia, discuss the strengths and weaknesses of the current evidence, and make proposals for further research. [ABSTRACT FROM AUTHOR]

Radio frequency electromagnetic fields (RF) have been shown to modify the concentrations of the radical O 2 - , H 2 O 2 and cancer cell growth rates at exposure levels below those that cause significant heating. Reactive oxygen species (ROS) are both signaling molecules and species that can do damage, depending on timing, location and concentrations. We briefly look at some mechanisms by which electromagnetic fields can modify the concentrations of ROS and some of the feedback and repair processes that lead to variable biological effects. Of particular interest are the role of radical pairs and their spins, which have received considerable attention recently, and the role of feedback in biological systems, to which less attention has been paid. [ABSTRACT FROM AUTHOR]

Function/location of menaquinone (MQ) was studied in the photosynthetic reaction center of Heliobacterium (Hbt.) modesticaldum (hRC), which is one of the most primitive homodimeric type I RCs. The spin-polarized electron paramagnetic resonance signals of light-induced radical pair species, which are made of oxidized electron donor bacteriochlorophyll g (P800+) and reduced menaquinone (MQ-) or iron-sulfur cluster (FX-), were measured in the oriented membranes of Hbt. modesticaldum at cryogenic temperature. The spectral shape of transient electron spin-polarized signal of P800+FX- radical pair state varied little with respect to the direction of the external magnetic field. It suggested a dominant contribution of the spin evolution on the precursor primary radical pair P800+A0- state with the larger isotropic magnetic exchange interaction J than the anisotropic dipole interaction D. The pure P800+MQ- signal was simulated by subtracting the effects of spin evolution during the electron-transfer process. It was concluded that the J value of the P800+MQ- radical pair is negative with an amplitude almost comparable to |D|. It is in contrast to a positive and small J value of the P700+PhyQ- state in photosystem I (PS I). The results indicate similar but somewhat different locations/binding sites of quinones between hRC and PS I. [ABSTRACT FROM AUTHOR]

The first total synthesis of a chromodorolide marine diterpenoid is described. The core of the diterpenoid is constructed by a bimolecular radical addition/cyclization/fragmentation cascade that unites two complex fragments and forms two C-C bonds and four contiguous stereogenic centers of (-)-chromodorolide B in a single step. This coupling step is initiated by visible-light photocatalytic fragmentation of a redox-active ester, which can be accomplished in the presence of an iridium or a less-precious electron-rich dicyanobenzene photocatalyst, and employs equimolar amounts of the two addends. Computational studies guided the development of this central step of the synthesis and provide insight into the origin of the observed stereoselectivity. [ABSTRACT FROM AUTHOR]

Singlet oxygen ( 1 O 2 ) mediated self-sensitized photo-oxidation in aqueous solution is a relatively uncommon photodegradation pathway that has been reported for some organic pollutants. Such reactions have been modeled by the empirical first-order kinetics, but significant deviation occurs at high substrate concentrations and in the presence of 1 O 2 scavengers and in D 2 O matrices. A mechanistically based kinetic model was developed for 1 O 2 mediated self-sensitized photo-oxidation by accounting for the sensitized generation of 1 O 2 and the heterogeneous reaction between the substrate/sensitizer and 1 O 2 in the photochemical system. The initially formed 1 O 2 is treated in the model to be spatially correlated with its sensitizer, which could simultaneously undergo reaction and dissociation, instead of being evenly distributed in the solution instantaneously once formed. The oxidation of the substrate/sensitizer by 1 O 2 occurs in both the geminate pairs and in solution, while the 1 O 2 in solution is also quenched by the solvent and scavengers present. The model could well describe the solar photodegradation of p -arsanilic acid ( p -ASA) under various initial concentrations, and predict the effect of 1 O 2 scavenger NaN 3 , and D 2 O matrices on degradation rate. The performance of the model was also validated by the kinetics of 1 O 2 mediated self-sensitized photo-oxidation of p -aminobenzoic acid (PABA), l-benzyl-3,4-dihydroisoquinoline, and 3,4-dihydropapaverine reported in the literature. Overall, this kinetic model could help better understand the fundamental processes involved in 1 O 2 mediated self-sensitized photo-oxidation and predict the photochemical fate of organic pollutants that undergo such photochemical transformation in sunlit surface water. [ABSTRACT FROM AUTHOR]

In experimental studies, we could show that the visual threshold of man is influenced by the geomagnetic field. One of the results was that the threshold shows periodic fluctuations when the vertical component of the field is reversed periodically. The maximum of these oscillations occurred at a period duration of 110 s. To explain this phenomenon, we chose the process that likely underlies the navigation of birds in the geomagnetic field: the light reaction of the FAD component of cryptochrome in the retina. The human retina contains cryptpochrome like the bird retina. Based on the investigations of Müller and Ahmad (J Biol Chem 286:21033-21040, 2011) and Solov'yov and Schulten (J Phys Chem B 116:1089-1099, 2012), we designed a model of the light-induced reduction and subsequent reoxidation of FAD. This model contains a radical pair, whose interconversion dynamics are affected by the geomagnetic field. The parameters of the model were partly calculated from the data of our experimental investigation and partly taken from the results of other authors. These parameters were then optimized by adjusting the model behaviour to the experimental results. The simulation of the finished model shows that the concentrations of all substances included show really oscillations with the frequency of the modelled magnetic field. After optimization of the parameters, the oscillations of FAD and FADH* show maximal amplitude at a period duration of 110 s, as was observed in the experiment. This makes it most likely that the signal, which influences the visual system, originates from FADH* (signalling state). [ABSTRACT FROM AUTHOR]

This study focuses on revealing the interaction of sulfur oxides (SOx) and nitrogen oxides (NOj and investigating the application of Fourier transform infrared (FTIR) spectroscopy to quantify SOA. and NOΛ. emissions from gas-phase oxy-combustion systems. The authors aim to contribute to the current state of knowledge by providing speciation data of NOx and SOx species and it elucidates the influence of nitric oxide (NO) on sulfur trioxide (SO3) generation. Detailed kinetic simulations revealed the influence of combustion parameters and the sensitivity analysis confirmed the dominating influence of hydrocarbon fragments on NO reduction. Accompanying experimental analysis exhibited higher reduction of NO to nitrogen (N?) comparing to the predictions by the kinetic simulations. Moreover, the presence of NO in the system was observed to influence the SO3 generation to a variable degree based on the reaction set employed for kinetic simulations. Experimentally, slight decrease in SO3 concentration was observed in the presence of NO and it can be explained by the radical consumption by NO as SO* and NOΛ species share the same radical pool. The oxy-combustion mechanisms available in the literature can be improved further to be able to predict this interaction. [ABSTRACT FROM AUTHOR]

Spin-correlated charge-carrier pairs play a crucial role as intermediate states in charge separation both in natural photosynthesis as well as in solar cells. Using transient electron paramagnetic resonance (trEPR) spectroscopy in combination with spectral simulations, we study spin-correlated polaron pairs in polymer:fullerene blends as organic solar cells materials. The semi-analytical simulations presented here are based on the well-established theoretical description of spin-correlated radical pairs in biological systems, however, explicitly considering the disordered nature of polymer:fullerene blends. The large degree of disorder leads to the fact that many different relative orientations between both polarons forming the spin-correlated pairs have to be taken into account. This has important implications for the spectra, which differ significantly from those of spin-correlated radical pairs with a fixed relative orientation. We systematically study the influence of exchange and dipolar couplings on the trEPR spectra and compare the simulation results to measured X- and Q-band trEPR spectra. Our results demonstrate that assuming dipolar couplings alone does not allow us to reproduce the experimental spectra. Due to the rather delocalized nature of polarons in conjugated organic semiconductors, a significant isotropic exchange coupling needs to be included to achieve good agreement between experiments and simulations. [ABSTRACT FROM AUTHOR]

The nanosecond electronic spectra and kinetics of the radical pairs from various crystalline tetraarylacetones were obtained using transmission laser flash photolysis methods by taking advantage of aqueous nanocrystalline suspensions in the presence of submicellar CTAB, which acts as a surface passivator. After showing that all tetraarylacetones react efficiently by a photodecarbonylation reaction in the crystalline state, we were able to detect the intermediate radical pairs within the ca. 8 ns laser pulse of our laser setup. We showed that the solid-state spectra of the radical pairs are very similar to those detected in solution, with λmax in the 330-360 nm range. Kinetics in the solid state was observed to be biexponential and impervious to the presence of oxygen or variations in laser power. A relatively short-lived component (0.3-1.7 μs) accounts for only 3-8% of the total decay with a longer-lived component having a time constant in the range of 40-90 μs depending on the nature of the substituents. [ABSTRACT FROM AUTHOR]

Charge carrier dynamics in organolead iodide perovskites is analyzed by employing time-resolved photoluminescence spectroscopy with several ps time resolution. The measurements performed by varying photoexcitation intensity over five orders of magnitude enable separation of photoluminescence components related to geminate and nongeminate charge carrier recombination and to address the dynamics of an isolated geminate electron-hole pair. Geminate recombination dominates at low excitation fluence and determines the initial photoluminescence decay. This decay component is remarkably independent of the material structure and experimental conditions. It is demonstrated that dependences of the geminate and nongeminate radiative recombination components on excitation intensity, repetition rate, and temperature, are hardly compatible with carrier trapping and exciton dissociation models. On the basis of semiclassical and quantum mechanical numerical calculation results, it is argued that the fast photoluminescence decay originates from gradual spatial separation of photogenerated weakly bound geminate charge pairs. [ABSTRACT FROM AUTHOR]

The dynamics of the ultraviolet (UV) photoexcitation of methyl nitrite in weakly interacting perfluoromethylcyclohexane solution are investigated using transient absorption spectroscopy. UV excitation in the structured S 1 ← S 0 absorption band induces dissociation, with geminate recombination and vibrational cooling of syn and anti-conformers of methyl nitrate on a timescale of ∼56 ps. Solvent-induced vibrational cooling favours relaxation to the higher-energy anti-conformer on entropic grounds, and subsequent inter-conversion to the lower-energy syn-conformer is prevented by a 3500 cm –1 barrier. UV excitation to the S 2 state produces a transient electronic absorption band resembling the absorption spectrum of NO 2 . [ABSTRACT FROM AUTHOR]

The effect of geminate recombination on the photocurrent kinetics is modeled by Monte-Carlo numerical simulation in organic bulk heterojunction. Non-correlated Gaussian distribution of hopping centers is considered. Non-stationary polarization of geminate pairs by external electric field has a considerable influence on transient photocurrent at the time scale of the order of 0.1 µs after generation pulse, and even inversion of the photocurrent sign is possible. Increase of an energetic barrier at donor-acceptor interface weakens this effect and increases a characteristic time. [ABSTRACT FROM AUTHOR]

Photoinitiated subnanosecond electron transfer within covalently linked electron donor-acceptor molecules can result in the formation of a spin-correlated radical pair (SCRP) with a well-defined initial singlet spin configuration. Subsequent coherent mixing between the SCRP singlet and triplet ms = 0 spin states, the so-called zero quantum coherence (ZQC), is of potential interest in quantum information processing applications because the ZQC can be probed using pulse electron paramagnetic resonance (pulse-EPR) techniques. Here, pulse-EPR spectroscopy is utilized to examine the ZQC oscillation frequencies and ZQC dephasing in three structurally well-defined D-A systems. While transitions between the singlet and triplet ms = 0 spin states are formally forbidden (Δms = 0), they can be addressed using specific microwave pulse turning angles to map information from the ZQC onto observable single quantum coherences. In addition, by using structural variations to tune the singlet-triplet energy gap, the ZQC frequencies determined for this series of molecules indicate a stronger dependence on the electronic g-factor than on electron-nuclear hyperfine interactions. [ABSTRACT FROM AUTHOR]

Persistent π-radicals such as MV+• (MV refers to methyl viologen, i.e., N,N'-dimethyl-4,4'-bipyridinum) engage in weak radical-radical interactions. This phenomenon has been utilized recently in supramolecular chemistry with the discovery that MV+• and [cyclobis(paraquat-p-phenylene)]2(+•) (CBPQT2(+•)) form a strong 1:1 host-guest complex [CBPQT MV]3(+•). In this full paper, we describe the extension of radical-pairing-based molecular recognition to a larger, square-shaped diradical host, [cyclobis(paraquat-4,4'-biphenylene)]2(+•) (MS2(+•)). This molecular square was evaluated for its ability to bind an isomeric series of possible diradical cyclophane guests, which consist of two radical viologen units that are linked by two ortho-, meta-, or para-xylylene bridges to provide different spacing between the planar radicals. UV-Vis-NIR measurements reveal that only the m-xylylene-linked isomer (m-CBPQT2(+•)) binds strongly inside of MS2(+•), resulting in the formation of a tetraradical complex [MS m-CBPQT]4(+•). Titration experiments and variable temperature UV-Vis-NIR and EPR spectroscopic data indicate that, relative to the smaller trisradical complex [CBPQT MV]3(+•), the new host-guest complex forms with a more favorable enthalpy change that is offset by a greater entropic penalty. As a result, the association constant (Ka = (1.12 ± 0.08) x 105 M-1) for [MSm-CBPQT]4(+•) is similar to that previously determined for [CBPQT MV]3(+•). The (super)structures of MS2(+•), m-CBPQT2(+•), and [MS m-CBPQT]4(+•) were examined by single-crystal X-ray diffraction measurements and density functional theory calculations. The solid-state and computational structural analyses reveal that m-CBPQT2(+•) is ideally sized to bind inside of MS2(+•). The solid-state superstructures also indicate that localized radical-radical interactions in m-CBPQT2(+•) and [MS m-CBPQT]4(+•) disrupt the extended radical-pairing interactions that are common in crystals of other viologen radical cations. Lastly, the formation of [MS m-CBPQT]4(+•) was probed by cyclic voltammetry, demonstrating that the radical states of the cyclophanes are stabilized by the radical-pairing interactions. [ABSTRACT FROM AUTHOR]

The relations have been analytically derived for recombination yields of radical pairs with different precursor states and by different reaction channels (singlet and/or triplet) in arbitrary magnetic and microwave fields. Recombination of two particles from a radical pair mostly with anisotropic reactivity is assumed to take place in a narrow reaction zone, and relative translation motion including rotation of reactants may be an arbitrary stochastic process. In addition, the approach may take into account the time-dependent spin Hamiltonian. The novel method for the calculation of recombination yields is suggested, and general expressions for recombination yields are derived for strong singlet-triplet dephasing. Some of the relations and expressions are the basis of the results given in the literature and their generalization to the case of non-model reacting systems that does not use simplification of reactants structure and reactants motion in solutions. [ABSTRACT FROM AUTHOR]

In recent work from this group (J. H. Klein et al. J. Am. Chem. Soc. 2015, 137, 11011), the magnetic field dependent charge recombination kinetics in donor/Ir-complex/acceptor triads has been determined with outstanding accuracy and reproducibility. The field-dependent kinetics has been analyzed in terms of a classical reaction scheme including the field-independent rate parameters of singlet recombination (rate constant kS) and S/T0 mixing (rate constant kST0) and the field-dependent rate constant k±( B) connecting central and outer Zeeman levels. In the present work, the extraction of k± from the experimental data is more precisely defined and the appearance of a 'coherent' and 'incoherent' regime of spin motion in a double log plot of k± vs. B is confirmed. The experimental decay curves have been reproduced by a full quantum dynamical model based on the stochastic Liouville equation, which was solved numerically, taking into account isotropic hyperfine coupling with five nuclear spins (1 N on donor radical, 4 H on acceptor radical) and anisotropic hyperfine coupling with the nitrogen nucleus at the donor radical. The results of the quantum calculations serve as a rigorous basis of interpreting the classical parameter k±. Furthermore, it is demonstrated that the incoherent part of spin motion is essential for a full understanding of the charge recombination kinetics even in the 'coherent' regime. [ABSTRACT FROM AUTHOR]

In the present work, the reaction operator for radical pairs of the main equation of spin chemistry has been modified on the basis of an exactly solvable model. The model contains both the reversible and irreversible transitions. For this model, expression for the reaction operator is more complex due to the expanded basis of spin states. The reaction operator of a phenomenological equation is a particular case of a more general approach and only in the limiting case of the completely irreversible recombination process, these operators coincide. [ABSTRACT FROM AUTHOR]

While it has been argued that field-dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT-type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p-DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV-characteristics as a function of intensity and charge-extraction-by-linearly-increasing-voltage-type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness ( Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor-acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor-acceptor interface. This effect is absent in the p-DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s-shape of the JV-curves and the concomitant additional decrease of the FF with increasing layer thickness. [ABSTRACT FROM AUTHOR]

Since the advent of mechanically interlocked molecules (MIMs), many approaches to templating their formation using various different noncovalent bonding interactions have been introduced and explored. In particular, employing radical-pairing interactions between BIPY.+ units, the radical cationic state of 4,4′-bipyridinium (BIPY2+) units, in syntheses is not only a convenient but also an attractive source of templation because of the unique properties residing in the resulting catenanes and rotaxanes. Herein, we report a copper-mediated procedure that enables the generation, in the MIM-precursors, of BIPY.+ radical cations, while the metal itself, which is oxidized to CuI, catalyzes the azide-alkyne cycloaddition reactions that result in the efficient syntheses of two catenanes and one rotaxane, assisted by radical-pairing interactions between the BIPY.+ radical cations. This procedure not only provides a fillip for making and investigating the properties of Coulombically challenged catenanes and rotaxanes, but it also opens up the possibility of synthesizing artificial molecular machines which operate away from equilibrium. [ABSTRACT FROM AUTHOR]

The structures, properties, and spectroscopic signatures of oxidized water clusters,(H2O)+n=6-21, are examined in this work, to provide fundamental insight into renewable energy and radiological processes. Computational quantum chemistry approaches are employed to sample cluster morphologies, yielding hundreds of low-lying isomers with low barriers to interconversion. The ion-radical pair-separation trend, however, which was observed in previous computational studies and in small-cluster spectroscopy experiments, is shown to continue in this larger cluster size regime. The source of this trend is preferential solvation of the hydronium ion by water, including effects beyond the first solvation shell. The fundamental conclusion of this work, therefore, is that the initially formed ion-radical dimer, which has served as a prototypical model of oxidized water, is a nascent species in large, oxidized water clusters and, very likely, bulk water. [ABSTRACT FROM AUTHOR]

The radical-pair mechanism is understood to underlie the magnetic navigation capability of birds and possibly other species. Experiments with birds have provided indirect and in cases conflicting evidence on the actual existence of this mechanism. We here propose a new experiment that can unambiguously identify the presence of the radical-pair magnetoreceptor in birds and unravel some of its basic properties. The proposed experiment is based on modulated light excitation with a pulsed laser, combined with delayed radio-frequency magnetic field pulses. We predict a resonance effect in the birds’ magnetic orientation versus the rf-pulse delay time. The resonance's position reflects the singlet–triplet mixing time of the magnetoreceptor. [ABSTRACT FROM AUTHOR]

This study reports the results of experiments that probed how solvents affect the recombination efficiency (FcP) of geminate radical cage pairs. The macroviscosity of solvents has traditionally been used to make quantitative predictions about FcP, but experiments reported here show that FcP varies dramatically for solvent systems with identical macroviscosities. Experiments show that FcP correlates with the solvent microviscosity: five different solvent systems (consisting of a solvent and a structurally similar viscogen) were examined, and FcP was the same for all five solvent systems at any particular microviscosity. The translational diffusion coefficient of the radicals (measured by DOSY) in the solvent system was used to define the microviscosity of the solvent system. [ABSTRACT FROM AUTHOR]

Photosolvation of hexachloroplatinate(IV) in acetonitrile with the formation of the Pt IV Cl 5 (CH 3 CN) − complex was studied in the time range from hundreds of femtoseconds to seconds. The primary Pt(III) intermediate was recorded and interpreted as the primary Adamson radical pair [Pt III Cl 5 2‐ (C 4v )⋯Cl ], which then accepts an electron from a solvent molecule. Two successive Pt(III) intermediates recorded in time range from microseconds to tens of milliseconds were interpreted as Pt III Cl 5 2− (C 4V ) and Pt III Cl 4 − complexes. Pt III Cl 4 − was proposed to be a chain carrier in the chain photosolvation process. Dependencies of quantum yield of photosolvation on the initial complex concentration and excited light intensity were fitted in the framework of the proposed chain mechanism. Rate constants of the reactions of intermediates were determined. [ABSTRACT FROM AUTHOR]