The Brønsted-Lowry Reaction Revisited: Glass-Forming Properties of Salts of 1,5-Dimexylbiguanide

Ionic molecular glasses can be conveniently prepared by reacting 1,5-dimexylbiguanide with a wide variety of carboxylic and sulfonic acids. A library of 51 salts has been prepared and studied by differential scanning calorimetry, 25 of which readily form glassy phases and do not recrystallize upon heating or standing. The crystal structures of 1,5-dimexylbiguanide and six of its salts have been solved and studied, and show extended hydrogen-bonded networks. Curiously, 1,5-dimexylbiguanide, which is extremely stable toward crystallization, crystallized in a close-packed, regular structure, unlike most related glass-forming compounds. This apparent paradox was probed with density functional theory calculations which revealed the presence of two conformers of similar energy, thus contributing to frustrate crystallization.