Space group symmetry and the location of forbidden reflections due to incoherent multiple scattering from epitaxially oriented paraffins

Abstract

We have utilized three-dimensional electron diffraction data (0 kl) and hhl) from n-tritriacontane (n-C 33H 68) epitaxially crystallized on benzoic acid to elucidate its crystal structure. Measurement of electron diffraction hhl reflection positions shows that there is no twinning in epitaxially grown crystals and that the β angle is actually 90°. The space group therefore is A2 1am rather than the monoclinic Aa proposed before. During the course of this analysis it was found that ( hhl) intensities are affected by incoherent multiple scattering, e.g. the intensity distribution on (00 l) reciprocal axis. This strong intensity change is dependent upon the subcell symmetry, i.e. when the subcell is primitive and its space group has glide plane and/or screw axes, then the incoherent scattering will strongly affect “lamellar” intensities in the ( hhl) pattern, which are independent of the methylene subcell transform. For even chain paraffin such as n-C 36H 74 (space group Pca2 1), the same symmetry rules also predict that the incoherent scattering is the major cause for extra reflections appearing in the ( hhl) pattern.