CRYSTAL-STRUCTURE AND THERMAL VIBRATIONS OF CHOLESTERYL ACETATE FROM NEUTRON-DIFFRACTION AT 123-K AND 20-K

Cholesteryl acetate (C29H48O2) at 20 K is monoclinic, space group P2(1) with a = 16.521 (4), b = 9.220 (2), c = 17.620 (5) angstrom, beta = 107.18 (2)-degrees and Z = 4 (two molecules in the asymmetric unit). The crystal structure, earlier determined by X-ray diffraction at 123 K, has been redetermined at 123 and 20 K by neutron diffraction (7447 reflections at 123 K, 7281 at 20 K; sin theta/lambda < 0.69 angstrom-1) and refined by full-matrix least squares with 1423 variables to give R(F2) = 0.10 at 123 K, 0.06 at 20 K. Results at 20 K are more accurate not only because nuclear thermal vibrations have reduced amplitudes but also because a larger crystal was used for data collection. At 20 K, the average of 44 methylene C-H bond lengths is 1.102(8) angstrom (sigma from the observed distribution) and 1.103 (6) angstrom for 16 methine groups. The 22 methylene H-C-H angles [106.2 (9)-degrees] all fall within a narrow range. Analysis of nuclear anisotropic thermal parameters shows that intramolecular vibrations of the H nuclei are highly significant with similar meansquare amplitudes at 123 and 20 K. Torsional vibrations around the C-CH3 bonds of the C18 and C19 methyl groups have r.m.s. amplitudes of about 7-degrees. For methylene groups, mean-square amplitudes are a minimum for C-H bond stretching (0.006 angstrom 2) and a maximum for CH2 in-plane vibrations (0.024 angstrom 2). Values of B(eq) for the H-atom intramolecular vibrations are 0.8, 1.2 and 1.6 angstrom 2 for methine, methylene and methyl H atoms respectively. These results can be used to estimate B(eq) for H atoms in crystal structure determinations carried out by X-ray diffraction. The anisotropic intramolecular vibrations of the H nuclei contribute to the effective shape of the H atoms which, from the observed anisotropy in the distribution of intermolecular H...H distances, appears to be flattened along the direction of the C-H bond.