The valence electron density distribution of hydrogen bonded systems in the iterative extended Hückel approach. III. The pyrrole-pyridine system

The valence electron distributions of the hydrogen‐bonded systems C4H4NH … NC5H5 formed from pyrrole and pyridine and C4H4N− … HNC5H+ from pyrrole anion and pyridinium ion have both been investigated using the charge iterative extended Hückel method. The results are presented in the form of contour diagrams showing the charge densities in pertinent sections of the hydrogen‐bonded systems, and also in the corresponding parts of the non‐interacting constituent molecules. The density differences between interacting and non‐interacting systems brought about by hydrogen bonding are also presented. Like earlier investigated hydrogen bond types, an NH … N bond is characterized by an increase of charge density in the covalent NH bond, and a decrease at the acceptor atom. The formation of a rather low maximum in the centre of the H … N bond is also observed. The hydrogen bonding interaction causes a transfer of charge to the donor molecule of the same order of magnitude in both systems considered.