Glass-forming Schiff bases: Peculiar self-organizing systems with bifurcated hydrogen bonds

A combination of quantum computations with calorimetric, dielectric, infrared, and diffraction studies was used to dissect the nature of atypical bifurcated (three-centered) hydrogen bonds (BHBs). Based on the analysis of four glass-forming Schiff bases, this article experimentally proves that self-organization via BHBs may be prevalent in H-bonding systems. Moreover, it explicitly shows that even high screening of the proton-donor and proton-acceptor groups is not a sufficient obstacle to form the BHBs. Even highly sterically hindered Schiff bases form lasting centrosymmetric dimers via a pair of BHBs in the liquid and glass phases. The driving force for their medium-range-scale ordering tendency is a disorder in the molecular scaffold, strictly bound to the N center dot center dot center dot H center dot center dot center dot O bond character. As a result, the protons of the BHBs center gain translational freedom. Finally, it is shown that, contrary to the prevailing opinion, Schiff bases can be good glass-forming systems with high glass transition temperatures. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

This article uses a combination of quantum computations and various studies to analyze the nature of atypical bifurcated hydrogen bonds (BHBs). The experiments prove that self-organization through BHBs is prevalent in H-bonding systems, even with high screening of proton-donor and proton-acceptor groups. It is also shown that even highly sterically hindered Schiff bases can form lasting centrosymmetric dimers through BHBs. The driving force for their ordering tendency is the disorder in the molecular scaffold.