Structural Chemistry

, Volume 28, Issue 5, pp 1343–1357 | Cite as

Tautomerism in acyl-pyrazolones and in a novel photolysis product—importance and impact of the accurate localization of hydrogen atoms in crystal structures

  • Lorraine A. Malaspina
  • Allan H. White
  • Dieter Wege
  • Michael B. Tolmie
  • Brian W. Skelton
  • Simon GrabowskyEmail author
Original Research


Acyl-pyrazolones exist in four different tautomeric forms (two keto and two enol) in crystal structures. Routine crystal structure refinements using an independent atom model and routine isolated-molecule calculations fail in locating the mobile hydrogen atoms accurately in 22 investigated acyl-pyrazolone examples. However, a combination of both within the framework of quantum crystallography represented by the method of Hirshfeld atom refinement accurately locates the mobile hydrogen atom in a resonance-assisted hydrogen bond of title compound 3, a novel photolysis product. The impact of the hydrogen atom position on the resonance system of the non-hydrogen framework of the various tautomers is discussed, and the importance of intermolecular interactions for the positioning of the hydrogen atom is highlighted.


Tautomerism Resonance-assisted hydrogen bond Hirshfeld atom refinement Hydrogen atom parameters Photolysis 



We gratefully acknowledge useful consultations with Prof. C. Pettinari of the University of Camerino in respect of accessing/acquiring data on relevant compounds. S. Grabowsky thanks the German Research Foundation (Deutsche Forschungsgemeinschaft DFG) for funding within the Emmy Noether scheme GR 4451/1-1.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no conflict of interest.

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Copyright information

© Springer Science+Business Media, LLC 2017
Corrected publication August/2017

Authors and Affiliations

  1. 1.Universität Bremen, Fachbereich 2, Biologie/ChemieInstitut für Anorganische Chemie und KristallographieBremenGermany
  2. 2.The University of Western AustraliaSchool of Molecular SciencesPerthAustralia

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