The structure of an anhydrate form of ampicillin, C16H19N3045, has been determined. \Ct --C,H c--CH3EJ 02 &-coo-It crystallizes in the monoclinic system in space group P2t with two molecules in a unit cell of dimensions a= 12.40, b=6.20, c= 12.00/~, ,8=114"5 ° (standard deviation 0.30%). The structure was solved by Patterson heavy-atom methods and Fourier refinement from X-ray intensity measurements obtained from visual comparison and the equi-inclination Weissenberg method. Full-matrix least-squares refinement yielded an R value of 0.106 for observed data. The molecular configuration is very similar to the known configuration of the trihydrate form (space group P212121). Both forms exist in the crystalline state as zwitterions with hydrogen bonding between the amino nitrogen atom and the ionized carboxyl group. In both forms the thiazolidine ring is puckered with four of the atoms forming an approximately planar arrangement with the remaining atom significantly out of this plane. The non-coplanar atoms are C(2) in the anhydrate and S(1) in the trihydrate. A comparison is made with the known thiazolidine ring configurations of other penicillins. The nitrogen atom N(4) of the,8-1actam of both forms exists in a pyramidal configuration with its neighbouring atoms. The molecular packing of the two forms is completely different. The amino group in the anhydrate is hydrogen bonded to the ionized carboxyl groups of two molecules. In the trihydrate the amino group is hydrogen bonded to a single carboxyl group and to an intricate arrangement of the three hydrogen-bonded water molecules which also form hydrogen bonds to three other atoms within the molecule. There is no further hydrogen bonding in the anhydrate structure.

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