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Single crystals of DL-6-thioctic acid, (~-OL-lipoic acid), {5-[3(1,2-dithiolanyl)] pentanoic acid}, were obtained by recrystallization of a racemic mixture of DL-6-thioctic acid from cyclohexane. The crystals are predominantly twinned, and show a high degree of disordering. The space group is P21/c with a = 11.744, b = 9.895, c = 9"246 A, and fl= 109 ° 35. There are four molecules per unit cell, and the structure was solved by the heavy-atom technique. Hydrogen atoms were located using difference Fourier syntheses. The molecules are held together by hydrogen bonds between pairs of carboxyl groups, and by van der Waals contacts between sulphur atoms. Karle, Estlin & Britts (Acta Cryst. (1967) 22, 567) previously solved the crystal structure of the twinned crystal of D L-6-thioctic acid, using a restricted data set. This single-crystal study reports more accurate bond distances and angles for the same molecule.

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