K.A. McLauchlan

Electron spin resonance of spin-correlated radical pairs

Abstract Shortly after their formation, transient free radicals trapped in micelles exhibit electron spin resonance (ESR) spectra with “antiphase” lineshapes, i.e. lines with their low-field halves in emission and high-field halves in absorption. It is demonstrated that this hitherto unexplained effect is consistent with the detection of ESR transitions in geminate radical pairs before the radicals have had time to diffuse apart.

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 ...

The spin-correlated radical pair as a reaction intermediate

The concept of the spin-correlated radical pair has been used for some time in the interpretation of the results of magnetic polarization experiments in NMR and in ESR (CIDNP and CIDEP), and of the effects of magnetic fields on chemical reactions. It has, however, a much wider general significance as a reaction intermediate in all radical reactions, including photochemistry. Here the nature of the pair is introduced, and the evidence for it reviewed, and it is further shown how it can be treated theoretically, before some of the consequences of its existence are pointed out. Its recognition, and in particular the understanding of the processes which occur within it, notably the interplay of spin-mixing and diffusion, allow the design of new experiments in the absence or presence of an external magnetic field, and the optimization of reaction yields.

Electron spin-lattice relaxation times from the decay of E.S.R. emission spectra

Chemically induced electron polarization provides a new method for the direct determination of electron spin-lattice relaxation times of radicals in liquids. A rapid-response time-resolved E.S.R. spectrometer has been used to observe the decay of an emission spectrum into the corresponding absorption, and the effective relaxation time extracted by fitting an exponential curve to the early part of the decay, and then obtaining the true T 1 at any point of the spectrum by extrapolation to zero microwave power. The decay is monitored on resonance in the dispersion mode and at the points of maximum slope in the absorption mode : both results converge to T 1 at zero power with a gradient determined by T 2. Results for T 1 and T 2 on the benzophenone ketyl radical in liquid paraffin over a viscosity range 0·3 poise to 30 poise are reported. In this preliminary investigation the deuterated radical was used, and no attempt was made to resolve the hyperfine lines. The values of T 1 and T 2 for the modulation broad...

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.

Chemically induced electron spin polarization in electron-transfer reactions

Chemically induced electron polarization (CIDEP) has been observed for the durosemiquinone radical anion generated in the flash photolysis of solutions of duroquinone in the presence of various amines. The initial polarization has been measured directly by using a fast response time-resolved E.S.R. spectrometer. The magnitude of polarization is shown to depend on amine concentration and identity, and the solvent medium. Conventional nanosecond flash photolysis has been used to measure duroquinone triplet lifetimes under various conditions. The results are discussed in terms of the triplet mechanism and the radical pair mechanism.

Reaction yield detected magnetic resonance in exciplex systems

Reaction yield detected magnetic rescnance (RYDMR) experiments are reported from separate solutions of pyrene in the presence of different electron acceptors, the three isomers of dicyanobenzene, and the electron donors, N,N-dimethyl- and N,N-diethyl-aniline. For the first time a complete set of RYDMR B 0 and B 1 spectra are provided from radical pairs in low viscosity organic solvents. The data are of high quality, due to apparatus development, and provide detailed insight into the processes occurring within the radical pair. The results have been interpreted using a stochastic Liouville equation to account for the simultaneous action of spin and molecular dynamics within the spin correlated radical pair. Two different models of the latter have been used, a simple exponential one, which allowed the influence of the electron spin exchange interaction to be investigated, and a full diffusional one, with and without a potential between the radicals. In the dicyanobenzene systems, the RYDMR behaviour is show...

The influence of ST−1 mixing in spectra which exhibit electron spin polarization (CIDEP) from the radical pair mechanism

Abstract The effect of ST −1 coupling on the intensities in the electron spin resonance spectra of spin-polarized radicals is discussed, and demonstrated in spectra with many hyperfine components for the first time. Both hyperfine-dependent and hyperfine-independent contributions to the polarization occur; the latter could easily be mis-assigned to the action of the triplet mechanism. The effects are demonstrated in radicals produced on photolysis of acetone, both in the presence of diethyl phosphite and in propan-2-ol solution at low temperatures.

Radiofrequency magnetic field effects on chemical reaction yields

Abstract The influence of radiofrequency magnetic fields (1–80 MHz) on the yields of radical recombination reactions is investigated. A decrease in the recombination probability of radical ion pairs (monitored via exciplex fluorescence) is detected when the radiofrequency matches energy-level splittings arising from hyperfine interactions in the radicals. The dependence of the oscillating magnetic field effect (OMFE) spectrum on radiofrequency field strength and hyperfine coupling constants is presented for several reaction partners. The experimental studies are supported by theoretical simulations of recombination yields based on the radical pair mechanism (RPM).

Measurement of spin-lattice relaxation times of triplet states in fluid solution

Abstract A technique is reported for measuring triplet spin-lattice relaxation times in fluid solution. It is based on the observation of chemically induced electron spin polarization, in the presence of a triplet quencher. Spin-lattice relaxation times of the order of 10 ns are reported for the duroquinone triplet in various solvents.