Dynamic NMR Study of the Mechanisms of Double, Triple, and Quadruple Proton and Deuteron Transfer in Cyclic Hydrogen Bonded Solids of Pyrazole Derivatives
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- 作者：Oliver Klein ; Francisco Aguilar-Parrilla ; Juan Miguel Lopez ; Nadine Jagerovic ; José ; Elguero ; and Hans-Heinrich Limbach
- 刊名：Journal of the American Chemical Society
- 出版年：2004
- 出版时间：September 22, 2004
- 年：2004
- 卷：126
- 期：37
- 页码：11718 - 11732
- 全文大小：416K
- 年卷期：v.126,no.37(September 22, 2004)
- ISSN：1520-5126
文摘
Using dynamic solid state 15N CPMAS NMR spectroscopy (CP = cross polarization, MAS =magic angle spinning), the kinetics of the degenerate intermolecular double and quadruple proton anddeuteron transfers in the cyclic dimer of 15N labeled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP)and in the cyclic tetramer of 15N labeled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied ina wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants weremeasured on a millisecond time scale by line shape analysis of the doubly 15N labeled compounds, and bymagnetization transfer experiments on a second timescale of the singly 15N labeled compounds in order tominimize the effects of proton-driven 15N spin diffusion. For DPBrP the multiple kinetic HH/HD/DD isotopeeffects could be directly obtained. By contrast, four rate constants k1 to k4 were obtained for DPP at differentdeuterium fractions. Whereas k1 corresponds to the rate constant kHHHH of the HHHH isotopolog, anappropriate kinetic reaction model was needed for the kinetic assignment of the other rate constants. Usingthe 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 transfermechanism could be excluded. By contrast, using the kinetic assignment k2 
k3 kHHHD kHDHD and k3 kHDDD kDDDD, the results could be explained in terms of a two-step process involving a zwitterionicintermediate. In this mechanism, each reaction step involves the concerted transfer of two hydrons, givingrise to primary kinetic HH/HD/DD isotope effects, whereas the nontransferred hydrons only contribute smallsecondary effects, which are not resolved experimentally. By contrast, the multiple kinetic isotope effectsof the double proton transfer in DPBrP and of the triple proton proton transfer in cyclic pyrazole trimersstudied previously indicate concerted transfer processes. Thus, between n = 3 and 4 a switch of the reactionmechanism takes place. This switch is rationalized in terms of hydrogen bond compression effects associatedwith the multiple proton transfers. The Arrhenius curves of all processes are nonlinear and indicate tunnelingprocesses at low temperatures. In a preliminary analysis, they are modeled in terms of the Bell-Limbachtunneling model.
k3 kHHHD kHDHD and k3 kHDDD kDDDD, the results could be explained in terms of a two-step process involving a zwitterionicintermediate. In this mechanism, each reaction step involves the concerted transfer of two hydrons, givingrise to primary kinetic HH/HD/DD isotope effects, whereas the nontransferred hydrons only contribute smallsecondary effects, which are not resolved experimentally. By contrast, the multiple kinetic isotope effectsof the double proton transfer in DPBrP and of the triple proton proton transfer in cyclic pyrazole trimersstudied previously indicate concerted transfer processes. Thus, between n = 3 and 4 a switch of the reactionmechanism takes place. This switch is rationalized in terms of hydrogen bond compression effects associatedwith the multiple proton transfers. The Arrhenius curves of all processes are nonlinear and indicate tunnelingprocesses at low temperatures. In a preliminary analysis, they are modeled in terms of the Bell-Limbachtunneling model.