Diffusion-controlled kinetics in the reaction of ferroprotoporphyrin IX with carbon monoxide

Abstract

The kinetics of the recombination of ferroprotoporphyrin IX (haem) with carbon monoxide, after photodissociation of the haem-CO complex, have been studied at various pressures (1 to 2760 bar) and temperatures, in two solvents differing greatly in viscosity, viz. ethylene glycol + water (80 % v/v) and glycerol. Photodissociation was effected by a flash from a ruby laser, and a high-pressure cell with spectrophotometric detection was used. The activation parameters at 1 bar in aqueous ethylene glycol are ΔH^{[grephic omitted]} _1b= 7.5 ± 0.8 kcal mol^–1(31 ± 3kJ mol^–1), ΔS^{[grephic omitted]} _1b= 1 ± 3 cal K^–1 mol^–1(4 ± 12 J K^–1 mol^–1), and ΔV^{[grephic omitted]} _1b= 2.0 ± 0.4 cm³ mol^–1. In glycerol at 1 bar they are ΔH^{[grephic omitted]} _1b= 12.2 ± 0.9 kcal mol^–1(51 ± 4 kJ mol^–1) and ΔV^{[grephic omitted]} _1b= 14.0 cm³ mol^–1; these values indicate that in this highly viscous solvent the kinetics are largely diffusion-controlled. The rate constant in glycerol runs parallel with the value calculated from simple diffusion-control theory, at all temperatures and pressures, but is smaller by a factor of about 4.