Molecular ring rotation in solid ferrocene revisited

Abstract

We report on quasielastic neutron spectroscopy experiments on ferrocene (bis(η(5)-cyclopentadienyl)iron) in its three different crystalline phases: the disordered monoclinic crystalline phase (T > 164 K), the metastable triclinic phase (T < 164 K), and the stable orthorhombic phase (T < 250 K). The cyclopentadienyl rings in ferrocene are known to undergo rotational reorientations for which the analysis of our large data set suggests partially a revision of the known picture of the dynamics and allows for an extension and completion of previous studies. In the monoclinic phase, guided by structural information, we propose a model for rotational jumps among non-equivalent sites in contrast to the established 5-fold jump rotation model. The new model takes the dynamical disorder into account and allows the cyclopentadienyl rings to reside in two different configurations which are found to be twisted by an angle of approximately 30°. In the triclinic phase, our analysis demands the use of a 2-ring model accounting for crystallographically independent sites with different barriers to rotation. For the orthorhombic phase of ferrocene, we confirm a significantly increased barrier of rotation using neutron backscattering spectroscopy. Our data analysis includes multiple scattering corrections and presents a novel approach of simultaneous analysis of different neutron scattering data by combining elastic and inelastic fixed window temperature scans with energy spectra, providing a very robust and reliable mean of extracting the individual activation energies of overlapping processes.

Molecular dynamics study of ferrocene topology under various temperatures

“…The δ angle defines the relative positions of the Cp pair in Fc as a pseudomolecular carousel, with δ = 0 corresponding to the eclipsed conformation, while δ = 36 corresponds to the staggered one. Here, the rotational dihedral angle, δ, is equivalent to the α angle in the simulation of Appel et al [34] To define the δ angle for the Fc configurations along the MD trajectories, the 5-fold symmetry of the pentagonal cyclopentadienyl rings (Cp) is considered. Similar to the studies of Appel et al [16,34] that the dynamic disorder of Fc is projected into a nonequivalent site rotational jump diffusion model and into a two 5-fold model in Scheme 1A.…”

“…Here, the rotational dihedral angle, δ, is equivalent to the α angle in the simulation of Appel et al [34] To define the δ angle for the Fc configurations along the MD trajectories, the 5-fold symmetry of the pentagonal cyclopentadienyl rings (Cp) is considered. Similar to the studies of Appel et al [16,34] that the dynamic disorder of Fc is projected into a nonequivalent site rotational jump diffusion model and into a two 5-fold model in Scheme 1A. As a result, the ferrocene structure will repeat itself 10 times if the rotational dihedral angle S C H E M E 1 A, A dihedral angle δ defines the rotation around the axis passing through the centroids (c 1 for Cp1 and c 2 for Cp2) of the cyclopentadienyl rings (c 1 and c 2 are the dummy atoms in the ring centroids).…”

“…[41] It is noted that this angle is approximately 30 as calculated by Appel et al for Fc solids. [34] As a result, the eclipsed Fc conformer dominates at very low temperatures (up to T c ) in the gas phase, while at higher temperatures, "free" rotations of the cyclopentadienyl rings lead to the population of other possible Fc configurations (Figure 1).…”

“…The present MD study shows that the mean values of angle α (Table 1) The MD simulated distance at 300 K is approximately 3.24 Å obtained in 1.5 ps. [34] which peaks at 14 at 500 K ( Figure 2). The linear correlation coefficient, r(δ, α), between the rotational dihedral angle δ and tilt angle α indicates that there is no correlation between the Cp rotation and tilt motions as |r(δ, α)| is a small value around 0.05 at all temperatures (except for at 500 K when it is equal to 0.137), and therefore, this pair of angles has no linear relationship between them.…”

“…This may explain the fact that the Fc crystalline exhibits monoclinic structures (staggered) at higher temperatures (T > 164 K), but triclinic structures (eclipsed) does so at lower temperatures (T < 164 K). [34,46] Triclinic structures are also known as the rhombohedrum system, which may be skewed or inclined to one side, making it oblique.…”