Hydrogen bond compression during triple proton transfer in crystalline pyrazoles. A dynamic N-15 NMR study

Using dynamic solid state N-15 CPMAS NMR spectroscopy (CP = cross polarizat ion, MAS = magic-angle spinning), the kinetics of degenerate intermolecular triple proton and deuteron transfers in the cyclic trimers of N-15-labeled polycrystalline 4-nitropyrazole (4NO(2)P) and 4-bromopyrazole (4BrP) have been studied as a function of temperature and are compared to the kinetics of triple proton transfer in bulk solid 3,5-dimethylpyrazole (DMP) studied previously. The results show that the transfer kinetics in the new trimers are much faster than in DMP. However, the kinetic HHH/HHD/HDD/DDD isotope e ffects of 4NO(2)P are similar to those of DMP. These effects indicate a sin gle barrier for the triple proton transfers where all three protons lose ze ro-point energy in the transition stare, as expected for a structure with t hree compressed hydrogen bonds. At low temperatures, strong deviations from an Arrhenius-behavior are observed which are described in terms of a modif ied Bell tunneling model and a concerted proton motion. The barrier for the triple proton transfer in 4NO(2)P and 4BrP is substantially smaller than i n DMP. As there is no correlation with the electronic properties of the sub stituents, we assign this finding to steric effects where the bulky methyl groups of DMP in the 3- and 5-positions hinder the hydrogen bond compressio n, in contrast to 4NO(2)P and 4BrP exhibiting substitutents in the 4-positi on. These results lead to a minimum energy pathway of the proton transfer f ollowing in the absence of steric hindering the hydrogen bond correlation l ine q(1) = f(q(2)), established previously, where q(1) represents the devia tion of the proton from the hydrogen bond center and q(2) the N ... N dista nce. Tunneling occurs at constant N ... N distances.