Rotation barriers in crystals from atomic displacement parameters

Atomic displacement parameters (ADPs) from X-ray diffraction studies are analysed to yield mean-square liberation amplitudes and rotation barriers for cyclopentadienyl rings in 22 different environments in crystalline ferrocene, nickelocene, and ruthenocene. The results agree well with estimates obtained by various spectroscopic methods. In principle, the ADP method may be applied to experimental data at a single temperature; thus the temperature-dependence of rotation barriers in crystals can be followed. Among the other advantages of the ADP method is the resolution of different motions for symmetry-independent rotors. Analysis of ADPs seems to be a valuable complement to other methods of studying molecular motion in the solid state.