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A Racemic Conglomerate Nipped in the Bud: A Molecular View of Enantiomer Cross-Inhibition of Conglomerate Nucleation at a Surface

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posted on 2010-10-06, 00:00 authored by Abel Robin, Patrizia Iavicoli, Klaus Wurst, Matthew S. Dyer, Sam Haq, David B. Amabilino, Rasmita Raval
The inhibition of conglomerate formation in a racemic monolayer is shown to arise from one enantiomer impeding the growth of homochiral nuclei of the opposite enantiomer. Submolecular level resolution scanning tunneling microscopy (STM) data show that nanometer scale homochiral conglomerate nuclei, containing several tens of molecules, are obstructed from developing further by enantiomeric blockers in their surroundings. We have observed this in the racemic and enantiopure samples of a bis-lactate derivative of resorcinol which was prepared because its size and symmetry made it interesting for the study of chirality in chemisorbed monolayer systems. The solid state racemate forms a racemic compound containing hydrogen bonded chains. The enantiomers and the racemate were deposited onto a Cu(110) surface under ultrahigh vacuum conditions and the monolayers formed were studied by STM, low energy electron diffraction (LEED), and reflection absorption infrared spectroscopy (RAIRS). In particular, STM observations of the racemic monolayer reveal that the individual enantiomers form small organized aggregates which are interrupted by defects of the opposite enantiomer which acts as a growth inhibitor. This situation contrasts sharply with the monolayers formed by the enantiopure compounds, which form large homochiral domains. We believe that this unique nanometer scale view of conglomerate crystallization and its growth inhibition, with each enantiomer acting as a mutual inhibitor of its mirror counterpart in a situation where diffusion is not fast, provides important insights for understanding the formation of conglomerates and racemic compounds in bulk crystals.