Integrated intensity measurements in crystal structure analysis

Abstract

Some fundamental aspects of the photographic process are described and the relationship between exposure time and density is discussed. A review of some of the techniques employed in the measurement of the intensities of X-ray reflections is given. Among the techniques reviewed are; the visual method of estimation, the application of photometric techniques In the measurement of peak and integrated intensities and the application of counters in the direct monitoring of the X-ray beam. The Joyce-Loebl integrating microdensitometer is described, its function and operation discussed. The Instrument has a linear response up to about 1.5 D and the linear response of ILFORD INDUSTRIAL G, a double coated X-ray film, extends to 0.7 D. Ideally the I.D.V. corresponding to the limiting density of 0.7 D should be about 250 units, corresponding to a circular spot of about 0.3 mm diameter. Such a spot would be accommodated with X and Y sweeps of 3. Very large spots should be avoided because of the loss of recording sensitivity at high X sweep values and very small spots should be avoided because of their very narrow I.D.V. range. A limitation on the use of the instrument is caused by the disparity in linear response between the instrument itself and the double coated film normally used. It is recommended that double coated film be replaced with single coated film for the purposes of the intensity record. The advantage of such a move would be a wider and more accurately measured recording range. The intensity record used in the crystal structure analysis of 2-thioamidopyridine has been obtained by means of the Joyce-Loebl integrating microdensitometer. The bond lengths and angles obtained from the analysis show good agreement with similar bond lengths and angles in related molecules. It is concluded that the integrating microdensitometer provides a reliable intensity measurement and suggestions are made which should enable maximum accuracy to be obtained from the instrument in the future.