Fig. 3. Optimized gas-phase geometry of the global minimum conformers of the amine-one (a) and imine-ol (b) tautomeric forms of 4-acylpyrazolone Schiff bases 1–9 at the B3LYP/6-311++G(d,p) level of theory. Atom color code: H (white), C (gray), N (blue), and O (red). Dotted lines designate intramolecular σ-type H-bonds and numbers indicate H-bond distances. Note that right and lift-side pyrazolone fragments of compounds 8 and 9 are characterized by the same H-bond distances and thus only those in the lift-side are presented. The atomic coordinates are provided in the ESI.†.
