Brian Brocklehurst

EFFECTS OF WEAK MAGNETIC FIELDS ON FREE RADICAL RECOMBINATION REACTIONS

The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Magnetic fields and radical reactions: recent developments and their role in nature

Recombination of pairs of radicals is exceptional in being affected by magnetic fields. The mechanism has been known for some thirty years, but recently new applications have appeared and research has been extended to very high fields (up to 30 Tesla). Claims that low electromagnetic fields damage health have led to extensive medical, chemical and physical research: no firm evidence of hazards has emerged; on the other hand, migrating birds orient themselves in the earths field (50 microT): radical pairs may provide the mechanism.

The effects of weak magnetic fields on radical recombination reactions in micelles

PURPOSE To demonstrate the effects of weak magnetic fields (> approximately 1 mT) on chemical reactions involving free radicals, in the context of possible effects of environmental electromagnetic radiation on biological systems. MATERIALS AND METHODS Transient absorption, flash photolysis experiments have been performed to study the kinetics and yields of radical reactions. The triplet state of benzophenone has been used as a convenient source of radical pairs, whose identity is largely immaterial to the investigation of the so-called Low Field Effect. Hydrogen abstraction from surfactant molecules in micelles yields a pair of neutral radicals, one large and one small, in a region of restricted translational and rotational motion. RESULTS In alkyl sulphate and sulphonate micelles a weak field increases the concentration of free radicals that escape from the micelle to an extent that depends on the structure, dynamics and volume of the space in which the radical pairs are confined. The effect (up to 10%) is typically largest at 1-2 mrT. Smaller effects are found for Brij and TX100 micelles. CONCLUSIONS Low Field Effects depend strongly on the local environment of the radical pair. Larger effects than observed here might be expected for radicals formed from singlet (rather than triplet) precursors, as would be the case in biological reactions.Purpose : To demonstrate the effects of weak magnetic fields (> ˜ 1 mT) on chemical reactions involving free radicals, in the context of possible effects of environmental electromagnetic radiation on biological systems. Materials and methods : Transient absorption, flash photolysis experiments have been performed to study the kinetics and yields of radical reactions. The triplet state of benzophenone has been used as a convenient source of radical pairs, whose identity is largely immaterial to the investigation of the so-called Low Field Effect. Hydrogen abstraction from surfactant molecules in micelles yields a pair of neutral radicals, one large and one small, in a region of restricted translational and rotational motion. Results : In alkyl sulphate and sulphonate micelles a weak field increases the concentration of free radicals that escape from the micelle to an extent that depends on the structure, dynamics and volume of the space in which the radical pairs are confined. The effect (up to 10%) is typically largest at 1-2 mT. Smaller effects are found for Brij and TX100 micelles. Conclusions : Low Field Effects depend strongly on the local environment of the radical pair. Larger effects than observed here might be expected for radicals formed from singlet (rather than triplet) precursors, as would be the case in biological reactions.

The influence of very small magnetic fields on radical recombination reactions in the limit of slow recombination

Abstract A magnetic field can modify the recombination probability of pairs of free radicals even though it may be weaker than the local magnetic (hyperfine) interactions of the unpaired electrons. For a radical pair born in an electronic singlet state, there is a remarkably simple and general relation between the (singlet) recombination yield ΦS in zero field and in the presence of a weak field, when the recombination is very slow. The magnitude of ΦS at zero field in excess of its statistical value of one quarter is simply three times that in a weak field. The origin of this relation and the conditions under which it is obeyed are explored.

Yields of excited states from geminate recombination of hydrocarbon radical ions

Abstract A theory similar to the radical pair model for CIDNP, etc., is used to describe the rate of loss of spin correlation in geminate pairs of radical ions produced by radiolysis. The ratio of the yields of triplet and singlet excited states and the effects of magnetic fields are discussed.

Model calculations of magnetic field effects on the recombination reactions of radicals with anisotropic hyperfine interactions

The effects of anisotropic hyperfine interactions on the recombination reactions of spin correlated radical pairs in a weak applied magnetic field are discussed in the context of the radical pair mechanism (RPM). Model calculations are presented for radical pairs containing a single spin-1/2 nucleus with an axial or rhombic coupling to one of the unpaired electrons. The so-called low field effect (LFE) and various resonances in the magnetic field effect (MFE) are calculated. Approximate analytical expressions are given for the field positions of the resonances which are shown to arise from energy level crossings.

Spin correlation and magnetic field effects in radiolysis

Abstract When pairs of radicals meet at random the reaction probability is limited to one quarter by spin considerations, unless spin relaxation is exceptionally fast. Conversely, dissociation or ionisation from a singlet state can always be followed by geminate recombination, providing the initial spin correlation is retained. The spin correlation must decay eventually because of spin relaxation but usually a faster process intervenes—spin evolution due to hyperfine coupling. This gives rise to magnetic field effects: so too does electronic fine structure (zero-field splitting) in processes involving pairs of triplet excited states. The role of these processes in radiation chemistry is reviewed and the possible future applications of magnetic studies are discussed.

Model calculations on hydrocarbon radiolysis. Part 1.—Spin correlation effects in pure alkanes

A computer model is described which uses Monte Carlo methods to simulate the processes occurring in spurs produced by the radiolysis and VUV photolysis of liquid hydrocarbons. The independent-pairs approximation is used to simplify the treatment of the dynamics. Electron spin is treated explicitly: it is assumed that the overall spin state of the spur is singlet initially and remains so. For reactions between radicals and radical ions, radical quenching of triplet states, triplet–triplet annihilation etc., the spin state of the reactants is evaluated and used to choose the products formed. The model is applied to formation of radicals, alkenes and excited states in alkanes: it is shown that spin correlation plays a major role.

Magnetic field effect on the pulse shape of scintillations due to geminate recombination of ion pairs

Abstract Scintillation pulse shapes from solutions of para-terphenyl in decalin, squalane and benzene can be changed by magnetic fields. A striking effect of deuteration on the evolution of the effect in time is observed, confirming the role of electron—nuclear hyperfine interaction in the solute radical ions. The size of the effect varies with the solvent.

Ion-recombination luminescence in alkanes excited by synchrotron radiation in the range 10–40 eV

Abstract Time-resolved single-photon counting has been used to study ion-recombination luminescence in squalane solutions of paraterphenyl. The synchrotron storage ring at Daresbury provided excitation pulses in the photon energy range 10–40 eV. Changes in luminescence pulse shape and in the effect of applied magnetic field were observed. The results are related to the behaviour of multi-ion pair spurs produced by fast electrons or beta-particles.