Electron diffraction studies of hot molecules. IV. Asymmetries of nonbonded distribution functions of SF6, SiF4, and CF4

Abstract

Highly significant improvements in the agreement between observed and calculated intensities of electrons diffracted by hot molecules were obtained by optimizing the skew parameter â for the nonbonded distributions. Derived â values (±2σ) were 2.75(11), 0.72(13), and 2.0(4) Å−1 for SF6, SiF4, and CF4, respectively. These measured skew parameters are approximately 2 Å−1 higher in each case than values previously proposed on the basis of Morse asymmetry factors and the nonlinear transformation between curvilinear and normal coordinates of molecules. The principal factor responsible for the increase is the previously unknown intrinsic anharmonicity in bending deformations. Silicon tetrafluoride has a lower â value than the other molecules studied primarily because its bending force constant is lower, relative to stretching. Practical as well as theoretical implications of present findings are discussed.