850LOCAL ANAESTHETICS ROPIVACAINE AND BUPIVACAINE prediction of the conformation from VAMP optimizations to serve as a starting model for the structure determination would be an unacceptable approximation. Concluding remarksRopivacaine and bupivacaine crystallize in the same conformation, although their crystallographic packing schemes are quite different. This conformation of the active (protonated) form of both molecules was recognized earlier (Yoo et al., 1975) in related lidocaine derivatives. Molecular-mechanics calculations as implemented in MacroModel and Biograf can be used to predict these conformations relatively accurately, in contrast to the semi-empirical molecularorbital calculations.We are grateful to Dr Jan Noordik for valuable discussions, and we thank Dr Jos Lange for preparing the crystals and Drs Jos6 van der Mijden for her assitance during the crystal structure determinations. AbstractThe title compound, 2,5-dimethyl[1,2]dithiolo[1,5-b]-iv [1,2]dithiole-7-S , C7H853, was studied by X-ray diffraction at both 110 and 300 K. Space group

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The title compound (I), C 23 H 23 NO 3 Fe, crystallized in the space group P2 1 /c with Z = 4 whereas the compound (II), C 23 H 23 NO 4 Fe, crystallized in the space group Pbca with Z = 8. In each compound of (I) and (II), in addition to a strong [N-HÁÁÁO=C] intramolecular hydrogen bond, an infinite one-dimensional chain in the [010] direction is formed by a C-HÁÁÁO=COEt hydrogen bond for (I) and by C-HÁÁÁO=COEt and C-HÁÁÁO=C hydrogen bonds for (II).