The crystal structures of two organic molecules have been determined by the methods of X-ray diffraction. Epi-limonol iodoacetate, C28H33O9I, belongs to the monoclinic space group P2 1 with four molecules per unit cell. The crystal structure analysis has been based on the heavy-atom technique. Three-dimensional data have been used for the computations, the chief methods used being the Patterson synthesis and corresponding minimum function combined with Fourier syntheses. This analysis has led to a complete determination of the molecular structure and stereochemistry of epi-limonol iodoacetate and hence of the parent compound, limonin, C26H30O8. The latter is of great interest to organic chemists and the determination of its structure may make a significant contribution to the elucidation of other closely-related structures. Azulene, C10H8, is monoclinic, space group P2 1/a, with two molecules per unit cell. The partially refined structure, postulated as belonging to the space group Pa, has been analysed by the method of least-squares and found to be incapable of refinement. Re-examination of the space group requirements has led to a statistically disordered structure in the space group P2 1/a. This structure has been exhaustively refined by three-dimensional least-squares analysis but the results are rather unsatisfactory. The extreme 'closeness' of pairs of atoms prohibits a very accurate determination of atomic coordinates. However, the transannular C-C bond has been shown to he considerably larger than the other bonds in the molecule in agreement with theoretical considerations. Several programmes have been developed for use with English Electric 'DEUCE'. These programmes, written with interpretive coding schemes, facilitate various computations encountered in X-ray crystallographic analysis
