The Diffraction of X-Rays by Gas Molecules A Comparison with Electron Diffraction and a Discussion of the Results for SiHCl3

Abstract

The principles of x‐ray and electron diffraction by free molecules are discussed in view of the application of these methods to the determination of molecular structures. The influence of the chemical bond is not more disturbing for x‐ray measurements than for electron measurements. The hydrogen atoms are practically nonexistent for x‐ray diffraction while it is not so for electron diffraction. This is an advantage which has not yet been fully used. The following procedure is suggested: first an x‐ray investigation in which the number of parameters to be determined is much reduced, and subsequently an electron investigation which will determine the remaining parameters, i.e., the position of the hydrogen atoms. Thus in some cases at least (CH₃Cl e.g.), the use of both diffraction methods is necessary and is sufficient for determining completely the molecular structure, including the position of the hydrogen atoms. A discussion of x‐ray measurements on SiHCl₃ shows the precision and the certainty of the method. As for the disadvantages of the long times of exposure, which have been more than 100 hours, they have been reduced to, say, 8 hours for crystal‐monochromatized radiation and they will probably be reduced by a new factor 10 in the near future. Although being less numerous than in the case of electron diffraction, the maxima given by x‐ray diffraction present two fundamental advantages. In general they are actual, well‐defined maxima. The effect of molecular vibrations on them, which is difficult to calculate, is the smallest just at the angles at which they are the most prominent.