The microwave rotational spectra of malonaldehyde and a number of its isotopic forms have been investigated. In the vapor phase the molecule is found to exist in a planar, intramolecularly hydrogen-bonded form with two equivalent, individually asymmetric equilibrium configurations between which tunneling occurs. The data indicate that the hydrogen bond may be described by a double-minimum potential function. The large amplitude tunneling motion complicates structure determination,(1) (a)

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Caffeine, a pharmaceutical model compound, generally forms cocrystals with carboxylic acids that involve an O-H • • • N hydrogen bond between the carboxylic acid group of the acid and the caffeine imidazole moiety. Such solids are typically based on a single heterosynthon. To examine the facility of introducing additional supramolecular heterosynthons to the caffeine-carboxylicacid cocrystal system, structural isomers of aromatic carboxylic acids (i.e., hydroxy-and dihydroxybenzoic acids) with additional hydrogen-bonding functional groups (i.e., hydroxy group) were screened for cocrystal formation. For this purpose, a highly efficient screening method based on thermodynamically driven solution-mediated phase transformation was utilized. This method enabled the rapid discovery of nine new solid phases involving caffeine. These caffeine phases were identified using single-crystal X-ray diffraction and FT-IR spectroscopy as cocrystals based on 2-, 3-, 4-hydroxybenzoic-and 2,3-, 2,4-, 2,5-, 3,4-, and 3,5-dihydroxybenzoic acids. Structural analyses of the cocrystals revealed solids composed of up to three caffeine-carboxylic acid heterosynthons.

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