THE EFFECT OF LARGE-AMPLITUDE MOTION ON THE COMPARISON OF BOND DISTANCES FROM AB-INITIO CALCULATIONS AND EXPERIMENTALLY DETERMINED BOND DISTANCES, AND ON ROOT-MEAN-SQUARE AMPLITUDES OF VIBRATION, SHRINKAGE, ASYMMETRY CONSTANTS, SYMMETRY CONSTRAINTS, AND INCLUSION OF ROTATIONAL-CONSTANTS USING THE ELECTRON-DIFFRACTION METHOD

For floppy molecules, changes in the bond distances and bond angles as a function of the large amplitude motion have to be carefully account ed for to obtain the equilibrium structure. This effect is discussed i n connection with transformation of the operational bond distances r(a ) to the equilibrium distances r(e) for comparison with ab initio comp uted values, where CaX2 (X=Cl, Br and I) molecules are used as example s. The root-mean-square amplitudes of vibration for the bond distances are affected such that the experimentally determined values from elec tron diffraction should always be larger than the values calculated fr om the vibrational force field if the bending potential is not explici tly included in the analysis. The shrinkage and the asymmetry constant s are also affected by the large amplitude motion and can be either la rger or smaller in magnitude compared to values calculated from the vi brational force field. The effective symmetry of the molecules and the symmetry at the equilibrium configuration is discussed when structura l constraints have to be made. The use of rotational constants from mi crowave spectroscopy in a joint analysis is also discussed.