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-temperature N-15 NMR and ab initio calculations

Results are reported of variable-temperature N-15 cross-polarization magic angle spinning (CP/MAS) NMR experiments performed on [N-15(2)]-labelled pol ycrystalline 3,5-dibromo-1H-1,2,4-triazole (2b) and 3,5-dichloro-1H-1,2,4-t riazole (3b), the synthesis of which is also described. According to the cr ystal structures these compounds form cyclic trimers in the solid stale. Th e molecular and hydrogen bond structures were compared with those derived f rom ab initio calculations. The N-15 CP/MAS NMR spectra show temperature-de pendent lineshapes which were analysed in terms of near-degenerate triple p roton 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 an gles at N1 and N2 and compared with those obtained from solid-state NMR; bo th methods agree fairly well. By lineshape analysis rate constants of the t riple proton transfer processes were obtained at different temperatures. Th e proton transfer kinetics of 2b and 3b were compared with those of 3,5-dim ethylpyrazole, 4-nitropyrazole and 4-bromopyrazole, studied previously, whi ch also form cyclic trimers in the solid state, exhibiting more or less con certed, degenerate triple proton transfer processes proceeding by tunnellin g at low temperatures. It is shown that the triazoles behave in a similar w ay to the pyrazoles. The barrier heights of the triple proton transfer incr ease monotonically with the distance between the two nitrogen atoms involve d in the proton transfer. It is shown that the intrinsic nitrogen chemical shifts are related to the N...H distances. Copyright (C) 2000 John Wiley & Sons, Ltd.