SOME autocatalytic reactions may exhibit propagating reaction fronts if a small amount of autocatalyst is introduced into the unstirred, autocatalyst-free system1-10. The velocity of the front propagation depends on the diffusion constants of the reacting species, on their initial concentrations, on the kinetics and mechanism of the reaction and on any electric11,12 and gravitational forces13 that influence the migration of the reactive species. The cobalt(n)-catalysed auto-oxidation of benzaldehyde in glacial acetic acid14, which involves radicals as well as the transformation of paramagnetic Co(n) into diamagnetic Co(in), is an example of a propagating-front reaction15. It is well known16,17 that magnetic fields may alter the rate constants of some radical reactions by ∼5-30%. Here we present results which show that, in this system, even a weak magnetic interaction may cause a change of several orders of magnitude in the velocity of the propagating front, as a result of the paramagnetic-diamagnetic transformation involved in the process.
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