The Potential Functions of the Methyl Halides

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Abstract
The problem of the potential functions of the methyl halides is re‐examined in an attempt to find a function which is both adequate and simple. A valence form of potential was tried which contained four constants: k1, the C–H elongation; c, the C—X elongation; k2, the deformation of the H–C–H angle; and k3, the deformation of the X–C–H angle. It was found that this simple valence potential must be modified by the inclusion of a cross product term between the X—C distance and the X–C–H angle, thus introducing a fifth constant, k4. The constants k1 and k2 were determined from the methane frequencies (k1=4.88×105 and k2=0.443×105) and were taken to be the same for all the methyl halides. By adjusting the three remaining constants it was possible to predict eight quantities, the six fundamental frequencies and two of the fine structure spacings. The agreement with the observed values was satisfactory, the average deviation being less than 1 percent.