Intramolecular electronic redistribution coupled to hydrogen bonding: An important mechanism for the "neutral-to-ionic" transition

The part played by the electronic interactions in the uncommon "neutral-to- ionic" (N-1) structural transition occurring in some mixed stack organic co mpounds, such as tetrathiafulvalene-chloranil (TTF-CA), is analyzed by ab i nitio electronic structure calculations. It is shown that the charge-transf er variation occurring in the valence band along the mixed stacks of donor and acceptor molecules is coupled via an intramolecular mechanism to a defo rmation of the electronic cloud in lower bands. The lower bands affected by this redistribution are also those which are implicated in the interstack hydrogen bonds, and a charge-transfer increase produces a strengthening of these hydrogen bonds. This important mechanism, which associates the charge -transfer variation to the anisotropic lattice contractions in the crystal, has never been considered up to now.