A Molecular Mechanics Study of the Cholesteryl Acetate Crystal: Evaluation of Interconversion amongrg,rz, andrαBond Lengths

Abstract

MM3 calculations on the cholesteryl acetate crystal were carried out to study the accuracy of the MM3 force field, and to evaluate previous approximations (Kuchitsu and Cyvin) in the relationships among rg, rz, and rα bond lengths. It has been found that the previous approximations are good for treating the skeletons of relatively rigid molecules, for chemical bonds not involving hydrogen atoms. However, additional corrections have to be included to interconvert rg, rz, and rα bond lengths for general purposes. New relationships among rg, rz, and rα bond lengths have been derived. Using the improved approximations, our MM3 rα structure for cholesteryl acetate in the crystal agrees well with experimental results (neutron diffraction, 20 K), including for the bonds involving hydrogen atoms.