Differentiation of ferrocene D5d and D5h conformers using IR spectroscopy

Fingerprinting in the infrared (IR) spectral region of 450–500 cm−1 of ferrocene is discovered to be a sensitive way to differentiate the eclipsed (D5h) and staggered (D5d) conformers of ferrocene. The present study simulates accurate IR spectra for ferrocene using density functional theory (DFT) based on the B3LYP/m6-31G(d) model without any scaling and manipulation. The agreement with an early experiment and other theory is excellent. It is found that in the vacuum, the eclipsed conformer represents the true minimum structure of ferrocene, whereas the staggered conformer represents the saddle point structure, in agreement with a number of other theoretical and experimental studies. The study further reveals that the sandwich complexes are formed by stacking two Cp rings with an Fe atom in the middle and there are no conventionally localised Fe–C bonds in ferrocene. The vibrational frequency splitting of Δυ = 17 cm−1 in the IR region of 450–500 cm−1 (expt. Δυ = 16 cm−1) becomes the fingerprint of the eclipsed conformer in contrast with the staggered conformer of ferrocene. In addition, the present study suggests that the earlier IR spectral measurement of Lippincott and Nelson (1958) of ferrocene was not D5d alone but was likely a mixture of both eclipsed and staggered ferrocene. Finally, the study highlights that Fe-related properties of ferrocene hold the key to reveal one ferrocene conformer from the other.