Abstract
The structure of cholesteryl isobutyrate, $(CH_3)_2CHCOOC_{27}H_{45}$, was determined by single crystal X-ray diffraction methods. Cholesteryl isobutyrate crystallized monoclinic space group $P2_1$, with a = 15.115 (8)${\AA}$, b = 9.636 (5)${\AA}$, c = 20.224 (9)${\AA}$, ${\beta}$ = 93.15 (5)$^{\circ}$, z = 4, $D_c = 1.03 g/cm^3 $and Dm= 1.04 g/$cm^3$. The intensity data were measured for the 3417 reflections, within $sin{\theta}/{\lambda} = 0.59{\AA}^{-1}$, using an automatic four-circle diffractometer and graphite monochromated Mo-$K_{\alpha}$ radiation. The structure was solved by fragment search Patterson methods and direct methods and refined by full-matrix least-squares methods. The final R factor was 0.129 for 2984 observed reflections. The two symmetry-independent molecules (A) and (B) are almost fully extended. The molecules are in antiparallel array forming monolayers with thickness $d_{100}$ = 15.2${\AA}$, and molecular long axes are nearly parallel to the [$\bar{1}$01] directions. The two distinct molecules form separate stacks with almost the same orientations, but with differing degrees of steroid overlap. Thers is a close packing of cholesteryl groups within the monolayers. The packing type is similar to those of cholesteryl hexanoate and cholesteryl oleate.