The single-crystal X-ray diffractometer installed at the PWA Laboratory in Frascati is described. The Huber four-circle goniometer is controlled by an IBM-compatible personal computer by means of a plug-in board interface based on the LSI chip Am9513. The program CS running under MS-DOS performs a wide variety of operations from random search to data collection. The angular resolution of the diffractometer is 0.0025 °, while the instrumental resolution at 8 keV, with an Si(lll) monochromator, is 0.0026 °. After preliminary tests, 2Zn insulin crystals were mounted on the diffractometer and reflections were measured using synchrotron radiation. The rocking curves show full widths at half-maxima (FWHM) in the range 0.013-0.031 ° mainly due to the mosaicity.

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Abstract A new X-ray diffraction study of p-nitroaniline shows that through-conjugation has highly significant effects on the geometry of the molecule, as compared to aniline and p-nitrobenzoic acid. The effects include a moderate decrease of the internal angles of the ring at substituted carbons, with respect to values derived by superimposing independent angular distortions from each substituent.

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The crystal structure of p-hydroxybenzoic acid monohydrate, CTH6Oa.H20, determined from photographic data [Fukuyama, Ohkura, Kashino & Haisa (1973). Bull. Chem. Soc. Jpn, 46,[804][805][806][807][808], has been refined to R = 0.0414 on 1454 counter reflexions. Crystals are monoclinic, space group P2~/a, with a = 17.752 (9), b = 6.442 (2), c = 6.731 (3) A, fl= 105.48 (6)°, Z=4. The benzene ring has approximate C2, (mm) symmetry and shows a slight quinoidal shortening of the central C-C bonds. The internal angles at the ipso positions of the ring are aon = 120.5 (1) °, acoon = 119.4 (1) °.

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The molecular structures of p-methylsulphonylbenzoic acid and methylphenylsulphone have been accurately determined by X-ray crystallography and gas electron diffraction, respectively. After correction for systematic effects, the geometry of the crystal molecule is seen to agree with that of the free molecule within a few thousandths of an Å unit for bond distances and a few tenths of a degree for bond angles. An exception is the S-Me bond distance, which is ca. 0.02 Å shorter in the crystal. The distortion of the benzene ring angles from 120°, an effect of the - SO2Me substituent, is virtually the same from both experiments