Study of in situ crystallization method, of liquid compounds and their crystal structures

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Low melting point acetic esters are important solvent and also used as industrial materials. Therefore, understanding their physical and chemical properties is essential for utilizing these compounds further. As melting point (m.p.) is one of the fundamental property, explaining m.p.behavior of "m.p.alternation" by structural feature is attracting the interests of crystallographers.Different from n-alkane with two functional group (e.g.alkanedicarboxylic acid and alkanediol), acetic esters (AcO(CH_2)n-1 CH_3) show no m.p.alternation behavior in longer alkyl chain region, however, smaller acetic esters show prominent m.p.alternation (-96,-81,-91,-75,-68,-60℃ for n=1 to 6). In order to investigate this behavior of small acetic esters, crystal structures of methyl acetate (n=1) to amyl acetate (n=5) were investigated by single crystal X-ray structure analysis.As they are liquid in room temperature, single crystals were grown by "in situ crystallization" method on the R-AXIS RAPID diffractometer. The diffraction data were collected at 93K within short measurement time just after a single crystal was obtained. Their crystal structures were determined in good R values (R1=0.039-0.044).Investigation of the crystal structures revealed that both C-H...O type hydrogen bonds between acetic groups and van der Waals interactions between alkyl chains play important role to stabilize the structure. Iwo distinctive H-bond motif recognized were "dimer" (n=2,4,5) and "tetramer" (n=1,3) structures around inversion center. Because of this packing differences, methyl (n=1) and propyl (n=3) acetates have fewer number of H-bond comparing to other acetates. Therefore, lower m.p.of them is caused by this kind of inefficiency of crystal packing