Anomalous scattering by oxygen: measurements on (+)-tartaric acid

The structure of (+)-tartaric acid has been used as a test case to study the feasibility of absolute configuration determinations of compounds in which oxygenis the heaviest atom. Based on about 1100 Ni-filtered Cu Kα reflections (−7 ≤ k ≤ 7) measured on a Picker automatic diffractometer the (2R, 3R) and (2S,3S) configurations could be refined to R = 0.0228 and R = 0.0231 respectively. 94 selected enantiomer sensitive reflections gave R(2R, 3R) = 0.0336 and R(2S, 3S) = 0.0374 (Δf′′_o = 0.032). Among the 38 most sensitive Bijvoet differences only one had the incorrect sign. A value of 0.041 (4) for Δf”_o (Cu Kα) was derived from 36 observed Bijvoet differences. Similar measurements with Cr Kα radiation also permitted the unequivocal assignment of the correct absolute configuration; however, Cr Kα radiation was not found to offer any overall advantage over Cu Kα radiation. The cell dimensions used in this study are a = 7.7291 (5), b = 6.0069 (2), c = 6.2118 (3) Å, β = 100.147 (2)° (space group P2₁; Z = 2).