Effects of magnetic field on chemical reactions

Abstract

Abstract-The effect of the magnetic field on CIDNP can be found in many chemical reactions. It is considered to result from the fact that the probability of singlet-triplet transitions in a radical pair is determined by magnetic interactions in the pair and is field strength dependent. In the present communication we wish to demonstrate that for the same reason the external magnetic field can also markedly affect the rates of chemical reactions involving free radicals.WE INVESTIGATED the reactions of substituted benzyl chlorides with n-butyllithium.? The major reaction products, phenylpentane (the unsymmetrical product 1) and diphenylethane (the symmetrical product 2), are formed from the pair of benzyl and butyl radicals in the singlet state. The unsymmetrical product is derived mainly from the cage, whereas the symmetrical product 2 results from the radicals which have escaped from the cage. The probability of a radical escaping from the cage depends on singlet-triplet mixing in the original radical pair. Since the CIDNP effects are significantly affected by the magnetic field,l one can expect an influence of this field on the rate of singlet-triplet transitions and consequently on the ratio of the products derived from the cage to those derived from the radicals which have escaped from the cage.The reactions were carried out in a 15 mm sample tube in the earth's magnetic field and in a magnetic field of about 15,000 Oe. To obviate the start of the reaction outside the magnetic field of the instrument, benzyl chloride was added to a frozen solution of n-BuLi; then the frozen reaction mixture was placed in the magnetic field. The reaction mixture was kept for 15mins in either the earth's or the strong magnetic field at the temperature of boiling hexane. Under these conditions the reactions proceed to completion. The reaction products were analysed using the 19F and IH NMR techniques. Figure 1 presents the ratios of the products 1 and 2 obtained from the integral intensities of the 19F signals in the para-position. The data presented in this figure indicate the product ratio 4/12 in all the experiments carried out in the strong field to be higher than that for the reactions run in the weak field.? The mean value of the effect studied was more than 30 %. Figure 2 shows the results of the product ratio measurements for the reaction of benzyl chloride with n-BuLi. The product ratio was determined from the integrated signals of the cx-CH,-groups in the lH NMR spectra of unsymmetrical * Paper presented at the International Symposium on Chemically 3 This result is in good agreement with gas-liquid chromatography measurements. and symmetrical products. One can see that this reaction is less affected by the magnetic field.Let us estimate the influence of the magnetic field on the total probability of singlet-triplet transitions in a radical pair. For simplicity, consider a system of two radicals, with two magnetic nuclei with I = +. The Hamiltonian of this system comprises terms for the Zeeman energies of electrons and n...

Paramagnetic spin catalysis of a radical recombination reaction

“…The radical pair mechanism is common for a large variety of MFE effects (CIDNP and CIDEP [16], RYDMR [17,18], magnetic isotope effect on radical recombination reaction yield [19] and paramagnetic additives effects on the RRR [32]). All depend on the existence of the spin-correlated RP.…”

“…Any perturbation which affects this crucial spin-mixing stage might affect the yield of the RRR. This also depends on optimising the re-encounter rate in the diffusion process within the geminate RP solvent cage [16,50].…”

“…These are chemically induced dynamic nuclear and electron polarization (CIDNEP) [16], reaction yield detected magnetic resonance (RYDMR) [17,18], magnetic field effect (MFE) and magnetic isotope effect on radical recombination reaction yield [ 191. Recombination of two single-topic [4,15] radicals in the triplet state is forbidden because of a destabilizing exchange interaction.…”

Outer sphere complexes as intermediates in coordination chemistry

“…A typical illustration of the type of effects observed is provided by the work of Sagdeev et al [100,101] on the reaction of benzyl chloride with nbutyllithium. The mechanism of this reaction is summarized in the scheme of Figure 4.…”

“…If there is interstate crossing to the triplet state the radicals can diffuse apart and eventually react to give symmetric products. Sagdeev et al [101] found for the reaction of pentafluorobenzyl chloride with n-butyl lithium that the ratio of the amount of unsymmetrical pro-RC1 + n-BuLi Square brackets indicate outer sphere complex [101].…”