III. Temperature coefficients of iodine atom recombination

Abstract

The temperature coefficients of iodine atom recombination rates have been measured in eight gases having widely different efficiencies as third bodies or chaperons. If the rate constant is expressed in Arrhenius form , the activation energies, which are negative, decrease as the chaperon efficiency increases and it is the exponential term which principally determines the chaperon efficiency. A priori statistic al calculations based on intermediate complex formation predict values of the pre-exponential term which are in reasonably good absolute and relative agreement with experiment. The heat of formation of the complex is much greater than that which could result from normal vander Waals type interactions and the recombination is interpreted in terms of charge transfer complex formation between chaperon and iodine atom . Allowance for the two effects described in part II (Christie, Harrison, Norrish & Porter 1955) namely, the high efficiency of I 2 as a third body and the disturbances caused by temperature inhomogeneity, have reconciled the differences between rate constants determined by different schools and there is now excellent agreement on the values for iodine atom recombination in the presence of the inert gases. Further theories of atom recombination have been proposed (Keck 1958; Bunker & Davidson 1958; Husain & Pritchard 1959; Jepson & Hirschfelder 1959; Keck i960; Bunker 1960) most of which predict absolute rate constants in reasonable agreement with those measured experimentally for recombination in the presence of inert gases.