The geometries of eighteen 1,5-benzodiazepines including their oxo and thioxo derivatives have been calculated and compared with experimental geometries determined by X-ray crystallography using as property the methylene flip angle. Protonation of 3. H-1,5-benzodiazepine lead to a planar quasi-aromatic cation. For the non-planar structures we have calculated ring inversion barriers that are well correlated with ten experimental values determined by Dynamic NMR allowing the establishment of an empirical equation for predicting new values. © 2013 Elsevier B.V.

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