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

Staggered decamethyl-ferrocene (*Fc) becomes the lower energy conformer at low temperature, whereas the eclipsed conformer of ferrocene (Fc) is more stable. The powerful infrared (IR) spectroscopy which has remarkably provided signatures of ferrocene (Fc) in eclipsed and staggered conformers recently is employed to investigate methylation of Fc. The most significant consequences of the full methylation of Fc in the IR spectra are the blue shift of the band at similar to 800 cm(-1) in Fc to similar to 1500 cm(-1) in *Fc, and the enhancement of the C-H stretch band at similar to 3200 cm(-1) region in *Fc. Further analysis reveals large impact of Fc methylation on core electron energies of the centre Fe atom (1s(2)2s(2)2p(6)3s(2)3p(6)). The Fe core electron energy changes can be as large as similar to 10 kcal mol(-1) and are directional-the Fe 2p(z) and 3p(z) orbitals along the *Cp-Fe-*Cp axis (Cp centroids, vertical) change more strongly than other Fe core electrons in p(x) and p(y) orbitals. The directional inner shell energy changes are evidenced by larger inner shell reorganization energy. Energy decomposition analysis (EDA) indicates that methyl groups in *Fc apparently change the physical energy components with respect to Fc. The large steric energy of *Fc evidences that the closest hydrogens on adjacent methyl groups of the same *Cp ring in crystal structure are 0.2-0.4 angstrom closer than the hydrogens on nearest-neighbour methyl groups on opposing rings in *Fc. A significant increase in Pauli repulsive energy contributes to the large repulsive steric energy in *Fc.