The crystal and molecular structure of 1,4:3,6-bis (thioanhydro)-2,5-O-acetyl-D-iditol (R,R)-disulphoxide. Analysis of the rotational freedom of the ester group by means of potential-energy calculations

Abstract

The crystal structure of the title compound was determined from three-dimensional X-ray diffractometer data. It was solved by the heavy-atom method and refined to R = 0.054. Crystals of CIoH~406S 2 are monoclinic, space group C2, with a = 18.714 (3), b = 4.9317 (10), c = 15.101 (3) A, fl = 109.72 (3) °, and Z = 4. There are two crystallographically independent sets of molecules, A and B, in the structure; angles, interatomic distances and ring conformations are almost identical for the two sets, however. The molecules have twofold symmetry, a crystallographic axis passing through the centre of each, and the rings possess a 'near envelope' conformation. The 'semi-torsion angles' H(2)-C(2). . C(4)--O(3) between the carbonyl bond in the ester group and the nearest C H bond in the ring are "-~40 ° instead of 0 °, i.e. the ideal eclipsed conformation value. Potential-energy calculations show that the deviation is caused mainly by intramolecular interactions; therefore the molecules probably retain this conformation also in solution.