Hydrogen Bond Compression during Triple Proton Transfer in Crystalline Pyrazoles. A Dynamic 15N NMR Study

Using dynamic solid state ¹⁵N CPMAS NMR spectroscopy (CP ≡ cross polarization, MAS ≡ magic-angle spinning), the kinetics of degenerate intermolecular triple proton and deuteron transfers in the cyclic trimers of ¹⁵N-labeled polycrystalline 4-nitropyrazole (4NO₂P) 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 effects of 4NO₂P are similar to those of DMP. These effects indicate a single barrier for the triple proton transfers where all three protons lose zero-point energy in the transition state, as expected for a structure with three compressed hydrogen bonds. At low temperatures, strong deviations from an Arrhenius-behavior are observed which are described in terms of a modified Bell tunneling model and a concerted proton motion. The barrier for the triple proton transfer in 4NO₂P and 4BrP is substantially smaller than in DMP. As there is no correlation with the electronic properties of the substituents, we assign this finding to steric effects where the bulky methyl groups of DMP in the 3- and 5-positions hinder the hydrogen bond compression, in contrast to 4NO₂P and 4BrP exhibiting substitutents in the 4-position. These results lead to a minimum energy pathway of the proton transfer following in the absence of steric hindering the hydrogen bond correlation line q₁ = f(q₂), established previously, where q₁ represents the deviation of the proton from the hydrogen bond center and q₂ the N…N distance. Tunneling occurs at constant N…N distances.