Effects of anharmonicity of molecular vibrations on the diffraction of electrons : Part IV. "Anharmonic shrinkage"

Abstract

Recent observations of anharmonic pseudo-shrinkages in polyatomic molecules greatly exceeding the genuine shrinkage due to vibrational foreshortening have forced a re-examination of anharmonically induced skewing of internuclear distribution peaks for non-bonded distances. It is found that the so called "predictive model" introduced to treat skewing in three-atom fragments X--A--X, is entirely-inadequate for multiligated cases such as AX4 and AX6. The physical basis of the observed "anharmonic shrinkage" is discussed and a theoretical treatment relating observed peak skewing to anharmonic potential terms is developed. Because practically no spectroscopic or quantum theoretical information exists to test the expressions derived, a simple mechanical model, found in previous studies to have considerable predictive power for quadratic force fields, has been extended to cubic terms. It accounts reasonably well for the observations of anomalous "shrinkages" in hot CF4 , SiF6 and SF6. Accordingly, it appears that diffraction studies may be able to provide new insights into the physical character of molecular force fields.