Hyperfine Interactions and Metal Atom Dynamic Effects of Pentafluorophenyl Substituents on Ferrocene Complexes

clarity, the structures of these compounds are collected in scheme 1. In the present study, these compounds have been investigated by temperature-dependent MES and the dynamical data concerning the iron atom(s) have been compared to the temperature factor Ui,j values determined by single crystal X-ray diffraction. A comparison between the MES and X-ray based data relating to the vibrational amplitudes of the metal atom in organometallics should consider the effect of lattice imperfections which play a more significant role in the magnitude of the Ui,j values extracted from X-ray diffraction data than in the mean-square-amplitude-of-vibration data contributing the the recoil-free fraction (MES) values. Inter-molecular motions will contribute more to the former than the latter, since the X-ray measurements sample all atomic positions over a lengthy period of time while the MES measurements involve only a single atom during the scattering event. It should also be noted that the presence of solvate molecules in the unit cell will influence the atomic positions sensed by the X-ray technique relative to the MES measurements. This point will be addressed again in the discussion to follow. Hence, in general the vibrational amplitudes calculated from the X-ray data are larger than those derived from the MES data at the same temperature, as will be discussed in greater detail, below. This observation has also been examined in detail in relation to protein dynamics by Parak et al. 8 The second point relates to the temperature dependence of the vibrational amplitudes. The frequency of such vibrations can be expressed as ω ∝ [k/M] 1/2 where k is the appropriate force constant and M is the vibrating mass. For intra-molecular motions, k is large and M is small (relatively), whereas for inter-molecular motions, k is small and M is large. Thus, at low temperatures where the phonon frequencies are low, the major contribution to the vibrational amplitude will be due to the inter-molecular motions, while with increasing temperature the intra-molecular motions become more appreciable. These considerations lead to the expectation that the tempera