Negative enthalpies of activation and isokinetic relationships in the electron transfer quenching reaction of Pd-tetraphenylporphyrin by aromatic nitro compounds and quinones

Publikations-Art

Zeitschriftenbeitrag

Autoren

Kapinus, E. I.; Rau, H.

Erscheinungsjahr

1998

Veröffentlicht in

J. Phys. Chem. A

Band/Volume

102/28

Seite (von - bis)

5569-5576

The temp. dependence of electron transfer quenching of triplet excited neutral Pd-tetraphenylporphyrin is investigated in a series of isotropic solvents and in a nematic cryst. one. Arom. nitro compds. and quinones of different redox potentials are used as quenchers so that the quenching reaction is exergonic to slightly endergonic. The reactions of quenchers with the largest neg. .DELTA.G0 are diffusion controlled and thus have a pos. enthalpy of activation, the quenchers with moderately neg. .DELTA.G0 values quench in the activation controlled realm with neg. activation enthalpies, and the weakest quenchers again show pos. activation enthalpies. An isokinetic relationship is found only in isotropic solvents, the first case of the isokinetic temp. being well above the temps. of the expts. Reactions with neg. activation enthalpy are generally considered to have a pre-equil., but it is shown that an elementary reaction may exhibit a neg. enthalpy of activation, too, if the reaction entropy is neg. enough. It is conjectured that the neg. enthalpies of activation obsd. in this work are due to an elementary reaction.