Molecular dynamics study of ferrocene topology under various temperatures

Abstract

We present the first quantum mechanical Atom-Centered Density-Matrix Propagation molecular dynamic (MD) study to investigate ferrocene (Fc) conformation in gas phase. The MD simulations were performed at several temperatures (7, 18, 80, 120, 180, 293, and 500 K) for a period of 10 ps. It is found that, at very low temperatures (≤18 K), ferrocene prefers eclipsed-like conformation. At higher temperatures (>18 K), the cyclopentadienyl rings (Cp) of ferrocene exhibit apparent fluxional rotations, leading to configurations with the rotational angle δ distributing within a range of 0 (eclipsed) to 18 (approximately half of 36 for the staggered conformation), accompanied by the cyclopentadienyl ring tilt up to approximately 12 at 500 K. The simulated mean inferred (IR) spectrum of ferrocene at 7 K is clearly dominant by a doublet-splitting band of eclipsed-like Fc features in the region of 400 to 600 cm −1 , in agreement with previous IR studies. The animation obtained from the MD simulations indicates that, at room temperature, the Fe-C distances in ferrocene are in fact not strictly congruent but 2:2:1-fold.

Dominant changes in centre Fe atom of decamethyl-ferrocene from ferrocene in methylation

“…Kaspi-Kaneti and Tuvi-Arad further indicated that the symmetry transition point cusp at 18° represents the point at which the molecular geometry has an equal distance from an eclipsed and staggered Fc [11]. These conclusions are confirmed by our most recent first principle molecular dynamics study [12] which also revealed that conformational changes of ferrocene with temperature and confirmed by the most recent study when Fc crystallised with hexafluorobenzene of Bear et al [13].…”

“…1) exhibit eclipsed and staggered conformers with small energy differences [8,10,24]. The stability of staggered and eclipsed Fc conformers has been subjected to debate since the discovery some seventy years ago until very recently [6-8, 10, 12], when a series of combined experiment and theoretical research has provided compelling evidences that the eclipsed Fc in very low temperatures (under 18 K) is the more stable conformer [7,10,12]. Fc does not become a mixture of both eclipsed and staggered conformers [11], nor does it become an orientation with a mean rotation angle along the reaction coordinate.…”

“…The debate on the most stable conformer of Fc, whether it is the eclipsed (D 5h ) or the staggered (D 5d ), has been raised by a number of studies using combined DFT calculations and experimental measurements [6][7][8][9][10]. A new careful analysis of IR spectra has provided the strongest evidence so far of the stability of the eclipsed conformation at lower temperatures for Fc [6,12]. The studies reveal that the conformational interchange process from eclipsed Fc to staggered Fc involves a twist around the C 5 rotation axis that changes the torsion angle connecting any carbon atom of one ring to the corresponding carbon atom of the second ring through the two ring centroids [11].…”

Dominant changes in centre Fe atom of decamethyl-ferrocene from ferrocene in methylation

“…(2003), the measurement at 150 K does not show apparent differences from the IR spectrum measured in very low temperature of Duggan et al(49). Figures2(a) and (b) compare the calculated IR spectra of Fc and *Fc in the eclipsed and staggered conformer forms, respectively using the same DFT methods.…”

“…The IR of *Fc but splits into two well-separated peaks at ca 1035 cm -1 and 1141 cm -1 , respectively. The former (1035 cm -1 ) for the Cp in-plane stretching turtle swimming (49)) and the latter (1141 cm -1 ) for the opposite Cp ring breathing (small-large). The IR spectral band with largest frequency at around 3100-3200 cm -1 , refers to vibrations of the C-H stretches of the molecules.…”