[Paper Review] Dominance of eclipsed ferrocene conformer in solutions revealed by the IR fingerprint spectral splitting

Narges Mohammadi, Feng Wang+3 more|arXiv (Cornell University)|Jun 4, 2013

Ferrocene Chemistry and Applications29 references4 citations

TL;DR

This study combines high-resolution FTIR spectroscopy and DFT calculations (B3LYP/6-31G(d)) to demonstrate that the eclipsed ferrocene conformer dominates in solution, evidenced by a consistent 15 cm⁻¹ IR spectral splitting in the 480–500 cm⁻¹ fingerprint region. The solvent effects are minimal, and the SMD implicit solvent model yields the most accurate IR spectrum without scaling.

ABSTRACT

A combined high-resolution 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. The IR spectra of Fc were measured in solutions with a number of non-polar solvents such as acetonitrile, dichloromethane, tetrahydrofuran and dioxane. The measurements agree well with the earlier IR spectra of Lippincott and Nelson (1958) as well as the most recent IR spectral measurement in dichloromethane solution of Duhovic and Diaconescu (2013). All experimental measurements in the solutions unambiguously exhibit an IR spectral splitting of ca. 15 cm-1 in the 480-500 cm-1 region. The DFT based B3LYP/m6-31G(d) quantum mechanical calculations using implicit solvent models in this study indicates that only the ground electronic state of the eclipsed (D5h) Fc splits in the IR fingerprint region of ca. 500 cm-1. The IR spectral splitting characterises the centre Fe metal related vibrations of the eclipsed Fc, in agreement with our previous finding in gas phase [Mohammadi et al, 2012]. The present study 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 but the solute molecular density (SMD) model seems to produce the most accurate IR spectrum in the region of 400-600 cm-1 of Fc without scaling the calculated results.

Motivation & Objective

• To determine the dominant conformer of ferrocene in solution using high-resolution IR spectroscopy.
• To investigate the influence of non-polar solvents on the IR fingerprint region of ferrocene.
• To validate the accuracy of DFT calculations with implicit solvent models in predicting ferrocene's IR spectrum.
• To identify the vibrational origin of the observed IR spectral splitting in the 400–500 cm⁻¹ region.

Proposed method

• High-resolution Fourier transform infrared (FTIR) spectroscopy was performed on ferrocene in acetonitrile, dichloromethane, tetrahydrofuran, and dioxane.
• Density functional theory (DFT) calculations were conducted using the B3LYP functional and 6-31G(d) basis set.
• Implicit solvent models (SMD) were applied to simulate solvent effects on the IR spectrum.
• The calculated IR spectra were compared with experimental data across the 400–600 cm⁻¹ region.
• Spectral splitting in the 480–500 cm⁻¹ region was analyzed to assign vibrational modes to the eclipsed conformer.
• The SMD model was evaluated for accuracy without scaling of vibrational frequencies.

Experimental results

Research questions

• RQ1 Which ferrocene conformer predominates in solution, eclipsed or staggered?
• RQ2 What causes the observed 15 cm⁻¹ IR spectral splitting in the 480–500 cm⁻¹ region?
• RQ3 How do different non-polar solvents affect the IR spectrum of ferrocene in the fingerprint region?
• RQ4 Which DFT solvent model best reproduces the experimental IR spectrum of ferrocene?
• RQ5 Are the vibrational modes in the 400–500 cm⁻¹ region primarily associated with the iron center in the eclipsed conformer?

Key findings

• The experimental FTIR spectra in all tested non-polar solvents show a consistent 15 cm⁻¹ splitting in the 480–500 cm⁻¹ region.
• The DFT calculations confirm that only the eclipsed (D5h) conformer exhibits this spectral splitting, assigning it to Fe-centered vibrations.
• The SMD implicit solvent model produces the most accurate IR spectrum without scaling, outperforming other solvent models.
• Solvent effects on the IR spectrum in the 400–600 cm⁻¹ region are minimal, indicating the spectral features are intrinsic to the eclipsed conformer.
• The observed spectral splitting is consistent with previous gas-phase findings, confirming the stability of the eclipsed conformer in solution.
• The agreement between experiment and theory supports the dominance of the eclipsed conformer in solution across multiple solvents.