title: Crystal structure and thermal vibrations of cholesteryl acetate from neutron diffraction at 123 and 20 K.
abstract: 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) A, 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 less than 0.69 A-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) A (sigma from the observed distribution) and 1.103 (6) A 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 mean-square 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 A2) and a maximum for CH2 in-plane vibrations (0.024 A2). Values of Beq for the H-atom intramolecular vibrations are 0.8, 1.2 and 1.6 A2 for methine, methylene and methyl H atoms respectively. These results can be used to estimate Beq 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.
authors.affiliation: Crystallography Department, University of Pittsburgh, PA 15260.
authors.forename: H P
authors.initials: HP
authors.lastname: Weber
authors.forename: B M
authors.initials: BM
authors.lastname: Craven
authors.forename: P
authors.initials: P
authors.lastname: Sawzik
authors.forename: R K
authors.initials: RK
authors.lastname: McMullan
mesh_terms: D002788:Cholesterol Esters / Q000737:chemistry*; D013058:Mass Spectrometry / Q000379:methods; D008958:Models, Molecular; D008968:Molecular Conformation; D009502:Neutrons
chemical_list: D002788:Cholesterol Esters; C060123:cholesterol acetate