X-RAY-POWDER DIFFRACTION STUDIES OF ALKANES - UNIT-CELL PARAMETERS OF THE HOMOLOGOUS SERIES C18H38 TO C28H58

Synchrotron radiation high-resolution powder diffraction has been applied to determination of the unit-cell parameters of n-alkanes in the range C18H38 to C28H58. The data confirm the previously reported results for C18H38, C20H42, C23H48 and C25H52. With the exception of C24H50 and C26H54, these data support the predictions made for the other homologues in this series by Nyburg & Potworowski [Acta Cryst. (1973), B29, 349-352] i.e. C18H38, C20H42 and C22H46 - triclinic unit cells (Z = 1), C28H58 - monoclinic unit cell (Z = 2), and C19H40, C21H44, C23H48, C25H52 and C27H56 - orthorhombic unit cells (Z = 4). In contrast, C24H50 and C26H54 are found to have lattice parameters consistent with a triclinic (pseudomonoclinic) unit cell containing two molecules possibly arranged in a polytypic conformation of two triclinic (Z = 1) unit cells related by a pseudo-twofold axis.