Electron diffraction studies of hot molecules. II. ‘‘Anharmonic shrinkage effects’’ in SF6, CF4, and SiF4

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Abstract
Least squares refinements of diffraction intensities for SF6, CF4, and SiF4 yielded nonbonded internuclear distances that lagged increasingly behind values expected from the bond lengths, the higher the temperature. The observation that observed nonbonded shrinkages greatly exceed calculated shrinkages cannot be attributed to a deficiency in the theoretical expressions customarily invoked. It is explained, instead, by anharmonic effects in bending modes that give the illusion of anomalous shrinkage where it does not, in fact, exist. These effects, which skew the nonbonded distribution functions and displace the true mean distances from the peak maxima, contain heretofore unexploited information about anharmonic potential constants.