Solvent contribution to ferrocene conformation: Theory and experiment

Abstract

Solvent impacts on ferrocene configuration and the nature of its component energies. The measured infrared (IR) spectra of ferrocene (Fc) in the region 400-500 cm−1 exhibit very different profiles in gas phase and in solutions. The present study further explores such the differences in gas phase and in solutions using combined theoretical calculations and experimental measurements. It concentrates on the IR spectra in the region of 400–1200 cm−1 using non-polar (tetrachloromethane (CCl4) and n-hexane (Hex)) and polar (acetonitrile (ACN), dichloromethane (DCM), tetrahydrofuran (THF) and 1,4-dioxane (DOX)) solvents. Six relatively intense normal modes in this region (ν7, ν8,9, ν18, ν22,23, ν30,31 and ν37 at approximately 482, 495, 815, 840, 1006, 1110 cm−1) were obtained. The dependence of the relative energies of the eclipsed and staggered rotamers of Fc on the polarity of the solvent is small. Analysis of the band profile for the vibrational modes in the 480-500 cm−1 region (ν7, ν8,9) using the reaction coordinate model suggests that the energy difference between the eclipsed and staggered rotamers (ΔEe-s solv) may be underestimated by implicit solvent model in the calculations. Further investigation in this direction is warranted. The impact of solvation on Fc configuration is further investigated using energy decomposition analysis (EDA) in a non-polar (tetrachloromethane (CCl4)) and a polar (acetonitrile (ACN)) solvent. These calculations suggest that solvation substantially changes the electrostatic and quantum mechanical Pauli energy contributions to the interaction energy of Fc conformers, which result in a large steric energy in solvents to enhance the dominance of the eclipsed Fc, an observation consistent with the analysis of the bands due to the ν7 and ν8,9 vibrational modes.