Pressure effects on the rates of some hydrogen/deuterium transfer reactions between 2,2-Diphenyl-1-picrylhydrazyl and substituted phenols

The volumes of activation for hydrogen transfer reactions between 2,2- diphenyl-1-picrylhydrazyl (dpph) and a series of substituted phenols (2,4,6-tri-t-butylphenol, 2,6-di-t-butylphenol, 2,6-di-t-butyl-4- methylphenol, 2,4,6-trimethylphenol and 2,4,6-triphenylphenol) were measured in toluene at 25�C up to pressures of 2 kbar. The analogous deuterium transfer reactions were studied in the case of the first three phenols. The obtained volumes of activation, ΔV?expt proved to be independent of the nature of the phenol and the extent of deuteration. The overall mean value of ΔV?expt is -13�3 ml mol-1 and can be virtually entirely attributed to intrinsic molecular changes, ΔV?intr. From results of the effect of pressure on the first overtone of the O-H stretching frequency for a number of selected phenols in CS2, and from the lack of influence of pressure on the primary kinetic isotope effect, it is concluded that ΔV?ntr is essentially independent of moderate pressures.