From Proton Disorder to Proton Migration: A Continuum in the Hydrogen Bond of a Proton Sponge in the Solid State

Abstract

The crystal structure of the ionic complex of 1,8-bis(dimethylamino)naphthalene (DMAN) and 4,5-dichlorophthalic acid has been investigated using a multi-temperature X-ray diffraction approach. The two short intramolecular hydrogen bonds exhibit different behaviors, with the [O···H···O]- hydrogen bond appearing approximately centered throughout the experiment. The [N···H···N]+ hydrogen bond, on the other hand, exhibits a complicated temperature-dependent behavior, with disordered electron density being observed at lower temperatures, with apparent migration at higher temperatures. This continuum can be explained simply in terms of the changing local environment of the [N···H···N]+ hydrogen bond induced by lattice expansion. The related complex of DMAN with 3,4-furandicarboxylic acid shows no equivalent effects; this is consistent with the explanation proposed.