Absolute optical chirality of dirubidium (+)-tartrate and dicaesium (+)-tartrate crystals

The crystal structures and absolute optical chiralities of Rb2[(2R,3R)-C4H406], RBT (Mr=319"01) and Cs2[(ER,3R)-C4H406], CST (Mr = 413.88) have been determined. For crystals grown from aqueous solutions containing (+)-tartrate ions the specific rotation was observed to be laevo over the visiblewavelength range along the optic axis and the space group was found to be P3221 (D 6) in both cases. Crystal data at 298K, CuKa, A=l.54178/k; for RBT: a=7.168(1), c=13.097(1)A, V= 582-8 (2) A 3, Z = 3, Dx = 2.726, D,,, = 2.727 (3) Mg m -3, ~ = 16.61 mm-I, F(000) = 450, final R--0.0281 and wR = 0.0572 for 804 unique observed reflections; for CST: a= 7.432 (2), c = 13.526 (3) A, V = 647.0 (5) ,~3, Z = 3, Dx = 3.187 Mg m -3, /x --- 67.56 mm- 1, F(000) = 558, final R = 0.0348 and wR = 0.0521 for 837 unique observed reflections. The relationship between optical rotation and structural chirality has been traced by following the rules established earlier for inorganic ionic crystals. It is shown that in the reported structures the intermolecular helical atomic arrangement of highly polarizable atoms (mainly oxygens and cations) rather than the contribution from the individual optically active organic molecules is responsible for optical rotation of the crystals. Special attention is paid to possible hydrogen bonds, both inter- and intramolecular, as they significantly influence the shape of the helices. The refractive indices and rotatory power, calculated from the structural data using a point-dipole polarizability theory, agree with the experimental results and support this point of view. Evidence is found that in concentrated solution RBT molecules also form helical arrangements.