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

Abstract

Abstract For floppy molecules, changes in the bond distances and bond angles as a function of the large amplitude motion have to be carefully accounted for to obtain the equilibrium structure. This effect is discussed in connection with transformation of the operational bond distances r a to the equilibrium distances r e for comparison with ab initio computed values, where CaX 2 (XCl, Br and I) molecules are used as examples. The root-mean-square amplitudes of vibrational for the bond distances are affected such that the experimentally determined values from electron diffraction should always be larger than the values calculated from the vibrational force field if the bending potential is not explicitly included in the analysis. The shrinkage and the asymmetry constants are also affected by the large amplitude motion and can be either larger or smaller in magnitude compared to values calculated from the vibrational force field. The effective symmetry of the molecules and the symmetry at the equilibrium configuration is discussed when structural constraints have to be made. The use of rotational constants from microwave spectroscopy in a joint analysis is also discussed.