Metamorphosis of caffeine hydrate and anhydrous caffeine

Abstract

The phase stability, interconversion and physicochemical characterisation of caffeine (1,3,7-trimethylpurine-2,6-dione) hydrate and anhydrous caffeine relate to the strength of the available hydrogen-bonds. A hydrogen-bonded T-branched spine of hydrate molecules establishes a weak lattice bond for the caffeine hydrate which, under ambient conditions, transforms to a β-anhydrous phase which in turn, at 155 °C and with an enthalpy of 3.6 kJ mol^–1, converts to a trigonal phase α-anhydrous caffeine. The anhydrous phases are stabilised by weak CH₃ to CO hydrogen bonds.