Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Refinement. All H atoms were positioned geometrically and refined using a riding model, with C—H = 0.98 (methyl groups), 0.99Å (methylene groups), 1.00Å (methine groups) or 0.95 Å (aryl CH) and with Uiso(H) = 1.5 times Ueq(C) (methyl groups) or with Uiso(H) = 1.2 times Ueq(C) (methylene groups, aryl CH, methine groups). Torsion angles of all methyl groups were allowed to refine. Refinement of F2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F2, conventional R-factors R are based on F, with F set to zero for negative F2. The threshold expression of F2 > 2σ(F2) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F2 are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger. Refined as a two-component inversion twin.