Hydrogen bonds between zwitterions: intermediate between classical and charge-assisted ones. A case study.

Detailed investigation of the electron density distribution function of N-succinopyridine in the crystal and in isolated state within Bader's Atoms in Molecules theory has been carried out to analyze the charge density and energetical aspects of zwitterionic H-bonding. By means of comparative analysis of H-bonds formed between molecular, zwitterionic, and likely charged species, it was shown that, according to the criteria of H-bonding, all these interactions do not differ from each other.