NaF gives rise to very weak odd-index reflexions in X-ray diffraction. These reflexions have been studied to determine the best models for electron distribution in the NaF crystal. Data were obtained from a single crystal at room temperature for Ag K~, Mo K~, Cu Kct and Co K~ radiations. Least-squares analyses were made on the Ag K0c and Mo K~ data. Scattering factors for Hartree-Fock singly charged free ions allowed a good fit to the data, and there was a marginal improvement of the fit with similar scattering factors modified for effects of crystalline environment. The mean thermal parameter for the ions was B= 0-905+0-025 A 2. An attempt to account for the wavelength dependence of intensities of the strong reflexions, with current theories of extinction, was unsuccessful.

The crystal structure of the enol tautomer of 1,3-diphenyl-l,3-propanedione, C15H1202, has been determined from three-dimensional, neutron-diffraction data. The structure was refined by full-matrix, least-squares methods to an R value of 0.090 (0.042 for reflexions with I_>2.3a~) and wR of 0"045 (0.038) using 1660 reflexions. The short, intramolecular O... O contact is 2.463 (4) .~. The enol H atom is asymmetrically placed with respect to the O atoms, the difference in O-H bond lengths being 0.199 (17) ]~. Reasons for the asymmetry are discussed in terms of a unique enol tautomer and the disposition of near-neighbour atoms. The results are compared with two previous X-ray diffraction studies.

An Optronics P‐1700 film‐writing device is used to produce simulations of disordered organic molecular crystals. The simulations may be used directly to obtain optical diffraction patterns for comparison with X‐ray diffuse scattering photographs of real crystals. Examples of some substituted anthracenes are given to demonstrate the diversity of disorder effects which may be included and the level of precision achieved.

The crystal structure of C ~sHI1Br has been determined from X-ray (Mo Ka radiation) and neutron (2 ---0.988 /k) diffraction data. The compound crystallizes in the monoclinic space group P2~./c with a = 8.307 (1), b = 9.844 (1), c = 13.815 (2) A, fl = 95.96 (1) °, Z = 4. Refinement led to R and wR values of0-101 and 0.045 for 2197 X-ray reflexions (0-047 and 0-042 for reflexions with I > 2-3oi) and 0.167 and 0.056 for 2299 neutron reflexions (0.059 and 0.046). The 9,10-substituents show disorder of Br and CH 3, the 9-substituent occupied on average by 0.643 Br and 0.357 C and vice versa for the 10-substituent. Additionally, the neutron data show two orientations for the H atoms ofthe CH a.

The crystal structure of the enol form of 1-phenyl-l,3-butanedione, Cl0H10Oz, has been determined from three-dimensional, neutron-diffraction data. The structure was refined by full-matrix least-squares methods to an R of 0.098 (0.046 for reflexions with I>_ 2"3o-i) and wR of 0.049 (0.042) for 1184 reflexions.The short, intramolecular O. • • O contact is 2.489 (5) A,. The hydrogen bond is slightly asymmetric but, because of large thermal motion, the asymmetry is of doubtful significance. From the bond lengths in the enol ring, it would appear that the structure is either an average of the two possible cis enol tautomers or a resonance hybrid. The structure is compared with a previous X-ray analysis.