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

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.

本研究采用多温度X射线衍射（multi-temperature X-ray diffraction）方法，对1,8-双(二甲氨基)萘（1,8-bis(dimethylamino)naphthalene，DMAN）与4,5-二氯邻苯二甲酸形成的离子配合物的晶体结构进行了研究。该配合物中存在两条短分子内氢键（intramolecular hydrogen bonds），二者展现出迥异的行为特征：其中[O···H···O]-型氢键在整个实验过程中大致处于中心对称位置。而[N···H···N]+型氢键则表现出复杂的温度依赖特性：在较低温度下可观测到无序电子密度（electron density），在较高温度下则出现明显的质子迁移现象。这种连续变化的行为可通过晶格膨胀（lattice expansion）诱导的[N···H···N]+型氢键局部环境改变得到简洁解释。DMAN与3,4-呋喃二甲酸形成的相关配合物未展现出同等效应，这与所提出的解释相一致。