A dynamic ¹⁵N NMR study of kinetic hydrogen/deuterium isotope and tunnel effects on the triple proton transfer in crystalline 3,5‐dimethylpyrazole

Abstract

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 '

Quantum tunnelling in the hydrogen bond

“…Many of the solid-state systems studied by Limbach's group in Berlin [85][86][87][88][89][90][91][92][93][94][95] have been complexes containing N-HÁ Á ÁN hydrogen bonds with proton transfer rates which are many orders of magnitude slower than those in benzoic acid. The characteristic rate constants are on a timescale similar to 15 N chemical shift interactions so the proton transfer motions are revealed in the evolution of the 15 N NMR lineshapes with temperature; these are the classic effects of chemical exchange on the NMR spectrum.…”

“…One such investigation concerns the cyclic trimer of 3,5-dimethylpyrazole [85,94] for which the 15 N CPMAS NMR spectra have been recorded as a function of temperature. There are three mobile hydrogen atoms and using lineshape analysis with chemical exchange good agreement has been obtained with simulated spectra from which the rate constants for the proton transfer have been extracted.…”

Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

“…Multiple proton transfers in solid pyrazole dimers, trimers and tetramers ( Fig. 1e to 1g) are slower and have been studied by 15 N NMR magnetization transfer and line shape analysis [23][24][25][26][27][28][29][30][31]. The fastest process in Fig.…”

“…Spiess et al [33] have shown by two-dimensional rotor-synchronized 13 C NMR that a very slow tautomerism Hydrogen bonded systems exhibiting multiple proton transfers in the solid state. a N, N -diphenyl-6-aminofulvene-1-aldimine [19], b tropolone [32,33], c benzoic acid dimer [13][14][15], d arylamidine dimer [21,22], e pyrazole dimer, f pyrazole trimer and g pyrazole tetramer [24][25][26][27][28][29][30]. of solid tropolone observed by Bax et al [34] is associated with molecular jumps into neighboring vacant molecular sites.…”

Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy

“…Pyrazoles and particularly 3,5-dimethylpyrazole (1), show a dynamic process in the solid state that consists in the transfer, generally concerted, of several protons between the nitrogen atoms [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. This process (named SSPT, Solid State Proton Transfer) is illustrated in Fig.…”