Elucidation of Crystallization Process and Crystal Engineering of Conglomerates

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It is known that a few salts consisting of a chiral amine and an achiral carboxylic acid crystallize into conglomerates. This project aimed to elucidate the factor and to establish a guideline for converting racemates into conglomerate crystals. By means of crystal structural analysis of a large number of primary ammonium carboxylates, we tried to extract the characteristic structural feature of conglomerate crystals. As a result, for the most part of the salt crystals, acid and amine components make a column constructed by hydrogen bonds, and the crystals were made up from these columns. The column could be classified into two categories ; one is characterized by a two-fold screw axis and others by centrosymmetry. In the case of conglomerate salts, only the former is possible, while the two types of columns exist in a competitive probability in achiral salts or racemic compound salts. When a racemate formed centrosymmetric columns, the salt could be a racemic compound. In contrast, when a racemate formed helical columns, a conglomerate can be formed by gathering the columns having the same chirality. However, a racemic compound is also possible even in this case by gathering the enantiomeric columns in 1 : 1 ratio. These two types of structures can be seen in racemic compound salts in a competitive probability. The factors influencing the potential energies between these two types of packing are the shape of column and van der Waals potential, and would be estimated by computational methodology such as molecular mechanics.The primary factor for the formation of a conglomerate crystal by derivatization of a chiral amine to a salt with an achiral carboxylic acid could be attributable to the formation of optically active, helical column. The estimation for the formation of a conglomerate crystal with computational methodology is a prospective subject.