The search for proton mobility in solid pyrazoles: molecular and crystal structure of 3(5)-phenyl-4-bromo-5(3)-methylpyrazole

Llamas-Saíz, Antonio L.; Foces‐Foces, Concepción; Fontenas, Christophe; Jagerovic, Nadine; Elguero, José

doi:10.1016/s0022-2860(98)00905-3

Cited by 24 publications

(12 citation statements)

References 18 publications

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“…In this regard, it should be mentioned that another possible reason for the degenerate COOH/COO − signal, that cannot be ruled out at this point, might be a fast proton exchange. Indeed, fast proton exchange in the solid state has been reported in several systems 30–32. To determine whether or not the lack of resolution between NMR peaks in the freeze‐dried amorphous samples occurs because of fast proton exchange, studies at low temperatures would have to be performed.…”

Section: Discussionmentioning

confidence: 99%

Acid–Base Characteristics of Bromophenol Blue–Citrate Buffer Systems in the Amorphous State

Chatterjee

Medek

et al. 2004

Journal of Pharmaceutical Sciences

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Section: Discussionmentioning

confidence: 99%

Acid–Base Characteristics of Bromophenol Blue–Citrate Buffer Systems in the Amorphous State

Chatterjee

Medek

et al. 2004

Journal of Pharmaceutical Sciences

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“…23 The ring in the VITRUL derivative 3a is symmetric, showing a binary axis passing through N4 and the midpoint of the N1-N2 bond, probably resulting from proton disorder (occupancy factors of the disordered atoms close to 50%), although, as reported, electron density was only found close to N1. 23 The ring in the VITRUL derivative 3a is symmetric, showing a binary axis passing through N4 and the midpoint of the N1-N2 bond, probably resulting from proton disorder (occupancy factors of the disordered atoms close to 50%), although, as reported, electron density was only found close to N1.…”

Section: Crystallographic Structures and Structures Calculated Using mentioning

confidence: 61%

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

et al. 2000

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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.

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“…Some N-unsubstituted pyrazoles exist as tautomers that can interconvert in the solid state by intermolecular proton transfer. From 34 structures of NH pyrazoles reported by 1999, for instance, 8 structures displayed intermolecular solid-state proton transfer [11]. A typical example of a system that displays proton mobility in the solid state is 3(5)-phenyl-4-bromo-5(3)-methylpyrazole.…”

Section: Examples Of Thermally Induced Tautomerismmentioning

confidence: 99%

Direct Evidence of Solid‐State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics

Naumov

Sahoo

2013

Tautomerism

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Application of X-Ray Diffraction to Study TautomerismX-ray diffraction analysis can be utilized as an alternative method to characterize tautomeric equilibria in the solid state. X-ray diffraction is oftentimes utilized to study tautomerism in the solid state, either as a stand-alone technique for structure elucidation or as an important complement to various single or multidimensional NMR spectroscopic techniques, as well as to a variety of steady-state and time-resolved electron and IR spectroscopies. As the most important and general asset, X-ray diffraction is a very useful tool that provides direct evidence of the three-dimensional molecular and crystal structure with atomic-scale resolution that complements structure-or energy-related spectroscopic data. Moreover, temperature-resolved, time-resolved, and spatially resolved diffraction studies could be implemented to unravel details on the kinetics, dynamics, and spatial progress of the reaction. At a qualitative level, diffraction methods can afford conclusive results that could aid in reaching a decision on the chemical identity of the tautomers from the viable chemical structures in the solid state. In more quantitative terms, X-ray diffraction analysis is of invaluable relevance in terms of the possibilities that it provides to directly probe the shape and depth of the potential well of the proton, simultaneously acquiring information on the atomic-scale-resolution geometry of the system of interest. Its applications range from routine structure determinations where diffraction methods are capable of determining the identity of the ''pure'' tautomers that exist in the crystal (in cases where the equilibrium is shifted toward one of the tautomeric forms and the dynamic proton is located on only one atom) to temperature-resolved studies in which variants of the technique are employed to monitor dynamic processes to obtain evidence of dynamic tautomeric equilibria where the proton is distributed between two or more atoms. The diffraction methods continue to provide a significant contribution to the understanding of the underlying tautomeric processes. In addition to steady-state diffraction, the recent advent of time-resolved (pump-probe) crystallography, currently available at several third-generation (synchrotron) light sources at 100 ps time scale, provides an additional means to peek into the structural rearrangement as it happens. The Tautomerism: Methods and Theories, First Edition. Edited by Liudmil Antonov.

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The search for proton mobility in solid pyrazoles: molecular and crystal structure of 3(5)-phenyl-4-bromo-5(3)-methylpyrazole

Cited by 24 publications

References 18 publications

Acid–Base Characteristics of Bromophenol Blue–Citrate Buffer Systems in the Amorphous State

Acid–Base Characteristics of Bromophenol Blue–Citrate Buffer Systems in the Amorphous State

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

Direct Evidence of Solid‐State Tautomerism by Diffraction Methods: Isomers, Equilibria, and Kinetics

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