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 and deuteron 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 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 15N labeled compounds, and by magnetization transfer experiments on a second timescale of the singly 15N labeled compounds in order to minimize the effects of proton-driven 15N 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 ≈ k 3 ≈ k HHHD ≈ k HDHD and k 3 ≈ k HDDD ≈ 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.

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A combination of 13C, 15N magnetic resonance, 14N quadrupole double resonance, and X-ray studies of solid 3,5-dimethylpyrazole between 270 and 350 K has shown that the NH-N hydrogen bond units present in the crystal are dynamically disordered, so that each nitrogen atom is on average attached to half a hydrogen atom. The molecules form discrete hydrogen-bonded cyclic trimers, in which the hydrogen atoms move in a double minimum potential energy surface which is symmetrical, to within experimental error. The experimental evidence in this temperature range is consistent with disorder

Using dynamic solid state I5N CPMAS NMR spectroscopy (CP=cross polarization, MASSmagic angle spinning) the kinetics of the degenerate intermolecular triple proton and deuteron transfer in the cyclic trimers of "N-labeled polycrystalline 3,s-dimethylpyrazole (DMP) have been studied in a wide temperature range. At high temperatures, rate constants of the various isotopic HHH, HHD, HDD, and DDD transfer reactions are obtained in the millisecond timescale by lineshape analysis of partially deuterated doubly I5N-labeled DMP. At low temperatures, the kinetics were followed by magnetization transfer methods in the laboratory frame. In order to suppress artifacts arising from I5N-spin diffusion these experiments were performed on singly I5N labeled DMP, partially diluted in the non-labeled material. As multiple kinetic hydrogen/deuterium isotope effects on triple proton transfer reactions have not yet been studied these effects are modeled theoretically for the single barrier case involving a concerted proton motion and for the triple barrier case where the three protons are transferred stepwise. The experimental kinetic isotope effects obey the rule of the geometric mean, i.e. kHHH/kHHD= kHHD/ kHDD=kHDD/kDDD=3.6 i.e. =kHHH/kDDD=47 at 300K, which is indicative of a single barrier where all hydrons loose zero-point energy in the transition state. At low temperatures, strong deviations from an Arrhenius behavior are observed for all isotopic reactions, indicating incoherent triple hydron tunneling processes, which are described in terms of a modified Bell tunneling model.Ber . Bunsenger. Phys. Chem. 101. 889-901 11997) No. 6 0 VCH Verlagsgesellschaft mbH, 0-69451 Weinheim, 1997 0005-9021/97/06o6-889 $15.00+.25/0 '

~~ 3(5)-Phenyl-and 5(3)-methyl-3(5)-phenylpyrazole have been studied using multinuclear NMR spectroscopy at low temperature to determine the tautomeric equilibrium constants in the slow proton exchange regime by simple signal integration. In order to compare the results in solution with those in the solid state, the X-ray structure of a derivative of the first, namely 4-bromo-3-phenylpyrazole was * Lists of thermal components, hydrogen parameters and bond distances and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). For details of the deposition scheme see 'Instructions for Authors,' J. Chem. Soc., Perkin Trans. 2, 1992, issue 1. * 3-216//3-216 calculations on fully optimized geometries 36 also favours 3a over 3b by 4.2 kJ mol-'.

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