American Chemical Society
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Electron and Electrostatic Properties of Three Crystal Forms of Piracetam

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posted on 2011-06-01, 00:00 authored by Marie-Hélène Chambrier, Nouzha Bouhmaida, François Bonhomme, Sébastien Lebègue, Jean-Michel Gillet, Christian Jelsch, Nour Eddine Ghermani
The electron densities in three crystal forms of piracetam (triclinic form II, monoclinic form III, and triclinic piracetam monohydrate) were derived from high resolution X-ray diffraction experiments carried out at 100 K. For comparison, quantum mechanics calculations were also performed for the three compounds. The experimental electron density was refined using the Hansen–Coppens multipole model. The electron deformation density and the electrostatic potential were carefully analyzed in the three forms of piracetam. The atomic charges obtained by the flux of the electric field were found in excellent agreement with those obtained by the conventional volume integration method. The charge values compare qualitatively well with those obtained by theoretical calculations. The total electrostatic force exerted on each atom of the molecule has been investigated by using the Maxwell stress tensor as well as the interatomic forces. The experimental electrostatic interaction energies between dimers in the three crystal forms of piracetam were also estimated. Monoclinic form III exhibits the strongest piracetam–piracetam dimer electrostatic interaction energy (−131.9 kJ/mol) compared to −116.0 and −103.3 kJ/mol found in triclinic form II and the monohydrate form, respectively. The experimental interaction electrostatic energy of the piracetam–water contact was estimated as −175.0 kJ/mol in the monohydrate form.