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

Abstract

Staggered decamethyl-ferrocence (*Fc) becomes the lower energy conformer at low temperature, whereas the eclipsed conformer of ferrocence (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 ~800 cm-1 in Fc to ~1500 cm-1 in *Fc, and the band enhancement of the C-H stretch band ~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). Such the Fe core electron energy changes can be as large as ~10 kcal⋅mol −1 and are directional – the Fe 2pz and 3pz orbitals along the *Cp-Fe-*Cp axis (vertical) change more strongly than other Fe core electrons in px and py orbitals. Such 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 is evidence that the closest hydrogens on adjacent methyl groups of the same *Cp ring in crystal structure are 0.2-0.4 Å 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. *This article is in honour of Professor Pratim Kumar Chattaraj on the occasion of his 65th birthday, for his contributions to Theoretical and Computational Chemistry.

Future of computational molecular spectroscopy—from supporting interpretation to leading the innovation

“…10 Another CMS leading innovation case is the decade long theory guided (synchrotron sourced) spectroscopic study of ferrocene (Fc). [27][28][29][30][31][32][33][34][35][36][37] The theoretical IR spectral discovery of the more stable Fc eclipsed conformer (D 5h ) 27 shed a light on Fc studies, led and suggested several new measurements of Fc and its derivatives. 38 In the seminal theoretical IR study of Fc, it conclusively revealed that Fc eclipsed (D 5h ) and staggered (D 5d ) conformers exhibit their unique IR fingerprints in the region of 400-500 cm À1 .…”

“…[27][28][29][30][31][32][33][34][35][36][37] The theoretical IR spectral discovery of the more stable Fc eclipsed conformer (D 5h ) 27 shed a light on Fc studies, led and suggested several new measurements of Fc and its derivatives. 38 In the seminal theoretical IR study of Fc, it conclusively revealed that Fc eclipsed (D 5h ) and staggered (D 5d ) conformers exhibit their unique IR fingerprints in the region of 400-500 cm À1 . 27 That is, the IR band of Fc staggered (D 5d ) conformer does not split (the splitting of d = 2 cm À1 is sufficiently small not to be measured), whereas the band splits (d = 17 cm À1 ) in the same region for Fc eclipsed (D 5h ) conformer is measurable in almost all IR spectral measurements available.…”

“…This discovery of Fc has led to a decade long of study of Fc and derivatives. [28][29][30][31][32][33][34][35][36][37] IR spectroscopy when integrating CMS with measurements can also be used to reveal inter and intra molecular hydrogen bonding through IR frequency red-shifts 40 and blue shifts. 41 It was found that the A-H stretch in an A-HÁ Á ÁB complex redshifts (Dn) between 20 to 2500 cm À1 relative to the free molecule without such hydrogen bonding.…”

“…It was further discovered that any modifications to the unsubstituted staggered Fc ((C 5 H 5 ) 2 Fe) result in such IR band splitting. [32][33][34][35][36][37] The Fc IR spectral fingerprint is so far the unique technique which conclusively differentiates Fc eclipsed and staggered conformers. 27 The Fc IR fingerprint band 27 in Fig.…”