Measurement of Molecules in the Gaseous State

The interference pattern of a crystal arises from myriad (about 10 ) atoms or ions arranged regularly in space, and in it are mirrored the numerous distances which separate the crystallographically equivalent points in a crystal from one another. In gases, on the other hand, according to Debye and Ehrenfest, only the few intra-molecular distances are responsible in the interference pattern, because the distances between separate molecules have no fixed values. This gain in simplicity is of 

X-rays and Crystal Structure London 1924 and 1934 R. W. G. Wyckoff, The Structure of Crystals 2nd ed.. New York 1931, SuppL 1936 G. L. Clark, Applied X-rays New York 1940. 

Debye has developed a theory for evaluating gas exposures which, for given atomic coordinates, represents the intensity of the scattered beam as a function of the angle of diffraction. It is obviously not possible in general to deduce a definite molecular model from one experimental diagram without further assumptions (harmonic analysis). Frequently the evaluation of the diffraction pattern must be based on a tentatively assumed plausible model, the interferences given by it calculated and th( model compared with the diagram actually obtained. 

The evaluation is particularly simple and clear if in the conjectural molecular model only a single effective (independent) distance connects the different atoms. This is the case, for example, in CCI4 where, according to well founded assumptions of structural organic chemistry, the C-atom lies at the central point and the Cl-atoms at the apices of a regular tetrahedron. Here and in other similar instances the intensity distribution admits of a definite conclusion regarding the atomic distances and the angle between the individual valences. 

for example J. T. Randall, The diffraction of x-rays and electronSy London 1934, p. 62 G. P. Thomson, Electronic DiffractioUy London 1939. 

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