Primary kinetic HH/HD/DH/DD isotope effects and proton tunnelling in double proton-transfer reactions

Abstract

Double proton-transfer reactions are characterized by three primary kinetic H/D isotope effects (k.i.e.) involving a set of rate constants k^HH, k^HD, k^DH and k^DD. We have succeeded in measuring such sets for well defined symmetrical double proton-transfer reactions where k^HD=k^DH for the intramolecular hydrogen migration in meso-tetraphenylporphine (TPP) and for the intermolecular 1 : 1 proton exchange between acetic acid and methanol dissolved in tetrahydrofuran. In this system we also detected a triple proton transfer involving one methanol and two acetic acid molecules. Rate constants k^HHH, k^HHD, k^HDD and k^DDD of this reaction are reported. Additionally, we also observed an intramolecular double proton-transfer reaction in 2,5-dianilino-benzoquinone- 1,4-dianil (azophenine, AP) for which we have measured the HH/HD k.i.e. For the determination of rate constants dynamic n.m.r. spectroscopy was used.

The predictions of the fractionation-factor theory, transition-state theory and different proton-tunnelling theories on the HH/HD/DH/DD k.i.e. of symmetrical double proton-transfer reactions are discussed with special emphasis on the rule of the geometric mean and compared with the experiments. The kinetic and the i.r. results for TPP and AP are well reproduced by the vibrational model of tunnelling as proposed previously by us. TPP is, therefore, the first proton-transfer system with NH-stretching levels not broadened by hydrogen-bond effects. The k.i.e. of the intermolecular proton exchange in the system acetic acid + methanol + tetrahydrofuran are consistent with tunnelling from the OH-stretching ground states. The energy of activation is associated with the enthalpy of formation of the cyclic complex in which the exchange takes place and the excited intermolecular vibrational states within this complex. The tunnelling theories are modified in order to take this enthalpy into account.