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.

Acid−Base Interactions and Secondary Structures of Poly-l-Lysine Probed by 15N and 13C Solid State NMR and Ab initio Model Calculations

“…This difference is much larger than expected for a simple protonation-deprotonation in water . In other words, when alkylammonium or ammonium is transferred from a solid halide salt environment to aqueous solution, the nitrogen is shielded strongly, although the protonation state remains the same. For example, a hydration shift of about −12 ppm has been observed for ammonium chloride .…”

Nuclear Quadrupole Resonance Spectroscopy: Tautomerism and Structure of Functional Azoles

“…Thus, on the basis of multiple signals in the 35 Cl NQR spectrum, slightly different from the expected from X-ray data, it has been found that 3,5-dichloro-1,2,4-triazole (1) is present as 1Htautomer with an uncommon phase composition. Later Elguero with co-authors [72] has shown that in crystal, this triazole is present as trimer with intermolecular hydrogen bonds N(1)-H…N(2).…”

“…Structure ν, MHz s/n 1 Crystals 2019, 9, x FOR PEER REVIEW 3 of 12 The C(3)-C1(3) and C(5)-C1(5) bond lengths are slightly different [72]. So, it can be anticipated that the spectrum would contain a doublet or two sets of signals.…”

“…The value of the frequency change in going from the anion to the cation is greater for the triazole cycle than for the imidazole cycle. 36 The C(3)-C1(3) and C(5)-C1(5) bond lengths are slightly different [72]. So, it can be anticipated that the spectrum would contain a doublet or two sets of signals.…”