Theoretical Study of the Structures and Hydrogen-Bond Properties of New Alternated Heterocyclic Compounds

The conformational preferences of a new his-pyrrole derivative and its his-pyridazine precursor have been investigated through quantum chemistry calculations (HF, DFT(MPWB1K), LMP2) and observations in the solid state. The global energetic minima are planar for both structures, with the conformational preferences being explained by pi-electronic conjugation between the aromatic systems and the occurrence of intramolecular hydrogen bonds (HB). For the bis-pyridazine derivative, the all-anti preferred conformation results front CH center dot center dot Nsp(2) HB whereas the all-syn conformation of the bis-pyrrole is partly due to NH center dot center dot center dot Nsp(2) HB. For both systems, the validity of the theoretical conformational features is confirmed through the excellent agreement with the experimental data available. Calculations of electrostatic potential computed on the molecular surface of the various structures and their building blocks allow the variations to be rationalized in terms of molecular structure and are used to analyze the HB donor and acceptor sites of the compounds. The HB interaction sites predicted from the quantum chemical calculations are confirmed through the FIB interactions observed in relevant crystal structures.