Open AccessOn the distribution of intensity in the X-ray spectra of certain long-chain organic compounds
Publication year - 1925
Publication title -
proceedings of the royal society of london. series a, containing papers of a mathematical and physical character
Language(s) - English
Resource type - Journals
eISSN - 2053-9150
pISSN - 0950-1207
DOI - 10.1098/rspa.1925.0102
Subject(s) - perpendicular , molecule , series (stratigraphy) , crystal (programming language) , plane (geometry) , spectral line , intensity (physics) , chain (unit) , crystallography , glide plane , carbon chain , distribution (mathematics) , chemistry , molecular physics , geometry , physics , optics , mathematics , quantum mechanics , mathematical analysis , organic chemistry , paleontology , dislocation , computer science , biology , programming language
Recently, many compounds, having as the basis of their structure a long chain of carbon atoms, have been investigated by means of X-rays. Although the complete solution of the crystal structure of these compounds presents great difficulties, certain measurements, which can readily be made, appear capable of a simple interpretation in terms of the molecular structure. It is found, for example, that there is one plane in the crystal which has a very much larger spacing than any of the others, and the magnitude of this spacing is very simply related to the number of carbon atoms in the molecule. In any series, as the molecular chain is increased by the continued addition of CH2 groups, this spacing increases uniformly. This must mean that the molecules are oriented lengthwise between such planes, so that an increase in the length of the molecule results in a corresponding increase in the spacing. It does not follow that the molecules are so arranged that the direction of their length is perpendicular to these planes ; but, for any one series, the angle which this direction makes with the normal to the plane must remain very nearly constant; in certain series the molecules appear to be very nearly at right angles to the planes. A consideration of the results of many such measurements led to the conclusion that, from an X-ray point of view, the various series may be divided into two groups, (a ) those in which the planes are separated by the length of a single molecule, (b ) those in which the spacing corresponds to the length of two molecules. It is possible to predict with considerable certainty to which of these two classes a series will belong. If, at the end of the molecule, there is a group which is chemically active as in the cases of the fatty acids, alcohols, etc., series, it seems reasonable to conclude that, where the double length is found, the two molecules will be arranged end-to-end in opposite directions, the active group of the one molecule being oriented towards the active group of the other.