Dominance of eclipsed ferrocene conformer in solutions revealed by the IR spectra between 400 and 500 cm-1

A combined Fourier transform infrared (FTIR) spectra of ferrocene (Fc) and density functional theory (DFT) based quantum mechanical calculations confirmed the dominance of the eclipsed Fc conformer in the fingerprint region of 400–500 cm⁻¹ in solutions. Solution IR spectra of Fc measured in acetonitrile (ACN, ε = 35.69), dichloromethane (DCM, ε = 8.93), tetrahydrofuran (THF, ε = 7.43) and dioxane (DOX, ε = 2.21) show two well-defined bands in the 480-500 cm⁻¹ region with the higher-wavenumber band higher in intensity. The band profile agrees well with the earlier IR spectra of Lippincott and Nelson (1958) in tetrachloromethane solution as well as more recent measurement in dichloromethane solution of Duhović and Diaconescu (2013). DFT based quantum mechanical calculations predict that the eclipsed (D_5h) Fc conformer is lower in energy than the staggered (D_5d) conformer and that the room-temperature solution spectrum of Fc is dominated by that of the D_5h form (Best et al., 2016). The present study confirms that solvent effects enhanced the dominance of the D_5h Fc conformer which resulted in switch of the IR profile patterns in 400–500 cm⁻¹, from low-wavenumber band higher in intensity pattern in gas phase to higher-wavenumber band higher in intensity in solutions. It further suggests that the effects of solvents on the IR spectra of Fc in this region are small and the solvent model effects are also small for the IR spectrum in the region of 400–500 cm⁻¹ of Fc.