DYNAMIC NMR-STUDY OF THE KINETIC HH/HD/DD ISOTOPE EFFECTS ON THE DOUBLE PROTON-TRANSFER IN CYCLIC BIS(PARA-FLUOROPHENYL)FORMAMIDINE DIMERS

The H-1 and F-19 NMR spectra of N-15,N-15-bis(p-fluorophenyl)formamidine (DFFA) dissolved in tetrahydrofuran (THF) were measured and analyzed as a function of temperature, concentration, and deuterium fraction in the mobile proton sites. DFFA exists in THF as an s-trans conformer A and an s-cis conformer B. At low concentrations both conformers are present as monomers A1 and B1, which are hydrogen bonded to the solvent. At higher concentrations, A forms cyclic dimers A2. In these dimers, a double proton transfer takes place characterized by kinetic HH/HD/DD isotope effects. The following thermodynamic and kinetic quantities were obtained by NMR line-shape analysis and magnetization transfer experiments in the rotating frame. The equilibrium constant of the isomerization is given by K(B1A1 = exp[(-2 +/- 1 J mol-1 K-1)/R] exp[(1.0 +/- 0.2 kJ mol-1)/RT], 176 K < T < 271 K. The rate constant of the interconversion is given by k(A1B1) = 10(14.3) +/- 1.0 exp[(-62.5 +/- 1.0 kJ mol-1)/RT] s-1, 237 K < T < 302 K. The equilibrium constant of the formation of A2 from A1 is given by K(A2) = exp[(-29 +/- 1 J mol-1 K-1) exp[(5.7 +/- 0.3 kJ mol-1)/RT] L mol-1, 170 K < T < 254 K For the tautomerism in the cyclic dimer the following isotopic rate constants were obtained: k(A2)HH = 10(9.9) +/- 0.3 exp[(-18.9 +/- 0.3 kJ mol-1)/RT] s-1; k(A2)HD = 10(10.7 +/- 0.3) exp[(-26.7 +/- 0.3 kJ mol-1)/RT] s-1; k(A2)DD = 10(11.5 +/- 0.3 exp[(-33.4 +/-0.3 kJ mol-1)/R7] s-1, 164.2 K < T < 260.9 K; with the kinetic isotope effects of k(A2)HH/k(A2)HD = 23 +/- 4, k(A2)HD/k(A2)DD = 10 +/- 2, k(A2)HH/K(A2)DD = 237 +/- 20 at 189.2 K. The Arrhenius curves of all isotopic reactions could be simulated in terms of a concerted proton transfer that proceeds by thermally activated tunneling at low temperatures. This result is in contrast to previous findings for intramolecular degenerate double proton transfer reactions where stepwise reaction pathways have been established. With a three-dimensional model potential surface it is shown, however, that the difference between the concerted and the stepwise reaction pathway disappears when the hydrogen bond length is decreased during the course of the reaction. It follows that the observed kinetic isotope effects are in agreement with both a concerted or a stepwise reaction pathway in a compressed hydrogen-bonded state.