Triple proton transfer in crystalline 3,5-dibromo-1H-1,2,4-triazole and 3,5-dichloro-1H-1,2,4-triazole studied by variable-temperature15N NMR andab initio calculations

Abstract

Results are reported of variable-temperature 15N cross-polarization magic angle spinning (CP/MAS) NMR experiments performed on [15N2]-labelled polycrystalline 3,5-dibromo-1H-1,2,4-triazole (2b) and 3,5-dichloro-1H-1,2,4-triazole (3b), the synthesis of which is also described. According to the crystal structures these compounds form cyclic trimers in the solid state. The molecular and hydrogen bond structures were compared with those derived from ab initio calculations. The 15N CP/MAS NMR spectra show temperature-dependent lineshapes which were analysed in terms of near-degenerate triple proton transfer processes. The equilibrium constants are slightly different from unity as observed by high-temperature line splittings. The populations of the two quasi-degenerate tautomers were calculated from the internal angles at N1 and N2 and compared with those obtained from solid-state NMR; both methods agree fairly well. By lineshape analysis rate constants of the triple proton transfer processes were obtained at different temperatures. The proton transfer kinetics of 2b and 3b were compared with those of 3,5-dimethylpyrazole, 4-nitropyrazole and 4-bromopyrazole, studied previously, which also form cyclic trimers in the solid state, exhibiting more or less concerted, degenerate triple proton transfer processes proceeding by tunnelling at low temperatures. It is shown that the triazoles behave in a similar way to the pyrazoles. The barrier heights of the triple proton transfer increase monotonically with the distance between the two nitrogen atoms involved in the proton transfer. It is shown that the intrinsic nitrogen chemical shifts are related to the N...H distances. Copyright © 2000 John Wiley & Sons, Ltd.