ANISOTROPIC THERMAL MOTION OF TRIGONAL BIPYRAMIDAL MOLECULES FROM SPECTROSCOPIC DATA . PENTACOORDINATED MOLECULES .13.

A systematic vibrational analysis has been carried out on trigonal bipyramidal molecules for which adequate spectroscopic data exists. From the resulting description of the normal modes based on the harmonic approximation, the mean-square amplitudes of thermal motion are computed as a function of temperature. Considerable anisotropy is evident in the displayed thermal-vibrational ellipsoids. The thermal parameters are listed in internal coordinates of interest to the electron diffractionist as well as Cartesian coordinates for comparison with crystallographic studies. In dealing with molecular crystals containing light ligands or having low-frequency internal modes, the importance of correcting bond distances for the effects of thermal motion due to internal vibrations is emphasized in connection with the problem of increasing the precision of structural parameters obtained from diffraction studies. The use of thermal ellipsoids representing internal motion in determining missing fundamentals and in aiding frequency assignments is discussed.