Molecular and crystal structures of novel second-order nonlinear optical crystals:: α,α′-dibenzylidenecycloalkanones

X-Ray crystallographic analyses have been performed for the crystals of alpha,alpha'-dibenzylidenecycloalkanone (DBCA) derivatives which preferentially crystallize in non-centrosymmetric space groups and become second-order nonlinear optical crystals. We examined the obtained structures while paying attention to the intermolecular short contacts common to their structure. As a result, relatively short C-H ... O contacts between the cycloalkanone rings were found for 8 out of 10 DBCA derivatives. The C-H ... O distances for all cyclopentanone derivatives are almost the same as the sum of the van der Waals radius of carbon and that of oxygen; also, the geometrical characteristics found from the C-H ... O contacts are consistent with the C-H ... O hydrogen bond. The derivatives form a non-centrosymmetric one-dimensional chain with this contact. Their non-centrosymmetric crystal structures are led not only by their molecular shape, but also by this C-H ... O interaction.