The Mechanism of Bending in Co-Crystals of Caffeine and 4-Chloro-3-Nitrobenzoic Acid

In a recent study, Dey et al.1 propose a mechanism of elastic bending in co-crystals of caffeine, 4-chloro-3-nitrobenzoic acid and methanol (1) in which mechanical interlocking is proposed to allow for the reversible flexibility observed. We have now determined the mechanism to atomic resolution using micro-focused synchrotron radiation,2 which is different to that previously reported. When subjected to strain the inter-molecular distances change and hydrogen-bonded dimers rotate over two orthogonal directions to allow the compression and expansion producing flexibility.