The relaxation kinetics of the reaction of iodide with cobalamin (vitamin B 12 ) were studied on a high pressure laser temperature jump apparatus in aqueous solution at 25° and ionic strength 0.2 M. Analysis of the pressure dependence of the formation and dissociation rate constants gave their respective volumes of activation to be: ΔV f * = 5.50 ± 0.8 cm 3 mol −1 and ΔV d * = 11.5 ± 1.6 cm 3 mol −1 The positive activation volume for formation of the complex, ΔV f *, after appropriate correction for the volume change due to formation of an outer sphere complex, is consistent with a dissociative type mechanism in which the metal ion – water bond is stretched in the activated complex.
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Brian B. Hasinoff (1974) studied this question.