Geometric isotope effect of deuteration in a hydrogen-bonded host-guest crystal

Deuteration of a hydrogen bond by replacing protium (H) with deuterium (D) can cause geometric changes in the hydrogen bond, known as the geometric H/D isotope effect (GIE). Understanding the GIEs on global structures and bulk properties is of great importance to study structure-property relationships of hydrogen-bonded systems. Here, we report a hydrogen-bonded host-guest crystal, imidazolium hydrogen terephthalate, that exemplifies striking GIEs on its hydrogen bonds, phases, and bulk dielectric transition property. Upon deuteration, the donor-acceptor distance in the O-H center dot center dot center dot O hydrogen bonds in the host structure is found to increase, which results in a change in the global hydrogen-bonded supra-molecular structure and the emergence of a new phase (i.e., isotopic polymorphism). Consequently, the dynamics of the confined guest, which depend on the internal pressure exerted by the host framework, are substantially altered, showing a downward shift of the dielectric switching temperature.