Dynamic NMR study of the mechanisms of double, triple, and quadruple proton and deuteron transfer in cyclic hydrogen bonded solids of pyrazole derivatives

Using dynamic solid state (15)N CPMAS NMR spectroscopy (CP = cross polarization, MAS = magic angle spinning), the kinetics of the degenerate intermolecular double and quadruple proton and deuteron transfers in the cyclic dimer of (15)N labeled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP) and in the cyclic tetramer of (15)N labeled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied in a wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants were measured on a millisecond time scale by line shape analysis of the doubly (15)N labeled compounds, and by magnetization transfer experiments on a second timescale of the singly (15)N labeled compounds in order to minimize the effects of proton-driven (15)N spin diffusion. For DPBrP the multiple kinetic HH/HD/DD isotope effects could be directly obtained. By contrast, four rate constants k(1) to k(4) were obtained for DPP at different deuterium fractions. Whereas k(1) corresponds to the rate constant k(HHHH) of the HHHH isotopolog, an appropriate kinetic reaction model was needed for the kinetic assignment of the other rate constants. Using the model described by Limbach, H. H.; Klein, O.; Lopez Del Amo, J. M.; Elguero, J. Z. Phys. Chem. 2004,218, 17, a concerted quadruple proton-transfer mechanism as well as a stepwise consecutive single transfer mechanism could be excluded. By contrast, using the kinetic assignment k(2) approximately k(3) approximately k(HHHD) approximately k(HDHD) and k(3) approximately k(HDDD) approximately k(DDDD), the results could be explained in terms of a two-step process involving a zwitterionic intermediate. In this mechanism, each reaction step involves the concerted transfer of two hydrons, giving rise to primary kinetic HH/HD/DD isotope effects, whereas the nontransferred hydrons only contribute small secondary effects, which are not resolved experimentally. By contrast, the multiple kinetic isotope effects of the double proton transfer in DPBrP and of the triple proton proton transfer in cyclic pyrazole trimers studied previously indicate concerted transfer processes. Thus, between n = 3 and 4 a switch of the reaction mechanism takes place. This switch is rationalized in terms of hydrogen bond compression effects associated with the multiple proton transfers. The Arrhenius curves of all processes are nonlinear and indicate tunneling processes at low temperatures. In a preliminary analysis, they are modeled in terms of the Bell-Limbach tunneling model.