Structure analysis of polymer single crystals by electron diffraction

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Abstract
The crystal structure of alpha-poly(3,3-bis(chloromethyl) oxacyclobutane) has been solved by electron diffraction, on the basis of data obtained from thin single crystals grown from solution. The structure analysis required the assumption that the electron scattering was at least approximately kinematical. This assumption was justified in two ways. Firstly, the refined model was found to have acceptable stereochemistry. Secondly, dynamical scattering amplitudes calculated from the final structure reduced the R-factor by an insignificant amount. The structure was refined by rigid body least-squares methods. The two unit cell dimensions which could be measured were a = 17·85 Å, b = 8·15 Å, and the space group was found to be Pna21. Four molecular chains pass through the unit cell in a planar zigzag conformation, with the-CH2Cl side-groups in an approximate trans-trans conformation such that close packing of chlorine atoms on adjacent groups is achieved.