Research paper
Resolution of ferrocene and deuterated ferrocene conformations using dynamic vibrational spectroscopy: Experiment and theory

https://doi.org/10.1016/j.ica.2020.119491Get rights and content

Highlights

  • High-resolution measurements display a complex dependence on temperature.

  • We explain them quantitatively.

  • Compelling evidence that the single conformer model is invalid.

  • Compelling evidence that the mixed conformer models are invalid.

  • New methodology applying advanced modelling with the latest theory.

Abstract

The signature of molecular vibrations and distortions in dynamic molecules gives a complex fingerprint which is insightful and can substantiate (or otherwise) chemical hypotheses regarding molecular and conformer stability. Using high-accuracy experimental data of ferrocene (Fc) and deuterated ferrocene (dFc, Fc − d10) at temperatures from 7 K through to 388 K, we obtain complex spectral profiles which require an advanced reaction coordinate model to explain. We obtain compelling evidence that the single conformer model (staggered D5d or eclipsed D5h) used to interpret and explain many experimental results on ferrocene is invalid. However we also present compelling evidence that mixed conformer models are invalid, where ferrocene is represented by an effective dihedral angle between the cyclopentadienyl (Cp) rings; or by a mixture of Boltzmann populations of the two conformers. We find no evidence for single or mixed conformer models despite covering almost all conclusions from past literature for gas, solution or solid phase Fc. Some molecular dynamics computations have imputed free rotation at liquid helium temperatures or at room temperature – we find no evidence for either of these hypotheses.

We measure and derive point-wise experimental uncertainty of the spectra, enabling quantitative assessment of specific chemical and physical models about the origin of the spectral line-shapes. A new principle based on the reaction coordinate is introduced. Advanced spectroscopy and modelling is introduced for hypothesis testing, to articulate the nature of the potential surface, the reaction coordinate and subtle conformational changes in dilute systems. Two expected spectral peaks appear inverted in the gas phase, but are explained by our Reaction Coordinate Method (RCM) model. The non-uniform broadening of the singlet and doublet peaks with increasing temperature is explained. Our experimental analysis shows that the lowest energy conformer is D5h for both Fc and dFc. We are able to represent the reduced mass ratios of the lowest vibrational modes for Fc and dFc of 1.11 for ν1 for Fc to Fc-d9 and of 1.10 for Fc to Fc-d10. The measured barrier height for rotation is 7.4 kJ mol−1 and 6.3 kJ mol−1 for Fc and dFc respectively, in comparison to numerous theoretical treatments and past experimental studies. For the first time, we obtain agreement of the model with the complex spectral evolution of profiles. These new techniques are sensitive discriminants of alternate models and chemical systems, which argues for wider application to other complex or impenetrable problems across fields arising for numerous other solutions, frozen or at room temperature.

Section snippets

Motivation

Ferrocene (Fc, [Fe(C5H5)2]) is the iconic molecule of organometallic chemistry, where the ferrous sandwich structure exemplifies metal-aryl bonding, yet is also one of the most subtle. A serendipitous discovery of Fc more than sixty years ago, reported by two groups in late 1951 [4] and early 1952 [5], ultimately proved to be a breakthrough introducing a new era of organometallic chemistry. Wilkinson and coworkers proposed the structure of Fc to be a sandwich compound (a metal atom sandwiched

Profile variation from theory between the conformers

The conformers are remarkably similar and IR, XAFS and XRD experimental data are almost identical, with little ability to discriminate between the D5h and D5d conformations. Fortunately, recent advanced DFT computation [2] has predicted a significant splitting of the IR spectrum according to conformation and thereby enabled a critical probe of the conformation. IR spectra of Fc were calculated by our group for the D5h and D5d conformations in gas-phase in a broad spectral region (400–3500 cm-1

Experimental data

The experimental gas-phase deuterated ferrocene spectra (Fig. 2) show two clearly separated peaks in the band which roughly line up with the D5h conformation, but with quite different separation of components and quite different broadening. Whilst the frequency splitting of two broad bands in the 450–500 cm-1 region is clear for Fc, the more intense band lies at the low wavenumber, which is predicted for the D5d rotamer but with very narrow separation of components. Neither D5h nor D5d

The ability to resolve hypotheses

This high quality data demonstrates the power of advanced infrared spectroscopy to probe the structure of disordered systems. Using goodness-of-fit (χr2 analysis) with established uncertainty estimates and advanced hypotheses, we can investigate and explicitly discriminate between hypotheses:

  • A Single Conformer Model (Hypothesis A) assumes that the sample can be well-modelled either as a purely eclipsed or as a purely staggered conformer. This has been the conclusion from many sources in the

Eclipsed or staggered? A Single Conformer Model

The simplest model of Fc would be: (i) that the Fc or dFc system is either purely eclipsed (Ec) D5h or purely staggered (St) D5d; and (ii) that the theoretical models exactly represent this ideal. In other words, the first model (Hypothesis A) represents the spectrum of Absorbance A versus ν (wavenumber in cm-1), as Eqs. (1) or (2) (see Supp. material). The locations and offsets of the IR spectra are approximately aligned with the experimental data across the IR spectral range (400–1200 cm−1)

Mixed conformer model? Hypothesis B

Fitting a mixture of the D5h and D5d theoretical gas phase DFT computations at a given temperature would reveal the relative contributions of the conformations in this experimental observation. This allows an investigation beyond the facile Single Conformer Model and permits the investigation of the dependence of changes of conformation. This might give information on the energy difference between two stable minima on a potential surface. Hence the simplest Hypothesis B is (i) that the system

RCM model: Hypothesis C

If we consider the potential surface of the rotation around the Cp ring between conformations, we should expect a periodic potential with either two stable conformations or a stable conformation and a transition state conformation (Fig. 4). A key parameter from literature – the barrier height ΔE to the internal rotation of the Cp rings – has been investigated using different techniques and media.

The electron scattering technique was applied to gaseous Fc at high temperatures 673 K [46], and

Advanced RCM Model in solution: Hypothesis D

This model hypothesis allows parameters to vary from the DFT predictions, especially because of the theoretical uncertainty and convergence limitations. For the very lowest temperatures (7 K), only the lowest occupied mode, the ground state D5h mode, will contribute. Hence several critical parameters are found or fitted best in the lowest temperature data, yet should be consistent and stable for all temperatures, viz. ν7;ν8;ν9(D5h),I7;I8;I9(D5h). Conversely, parameters ν7;ν8;ν9(D5d),I7;I8;I9(D5d

Advanced RCM IR gas-phase spectra: Hypothesis D

Whilst this agreement in solution is compelling for the Advanced RCM Model, the gas-phase spectra represent an even more difficult challenge. This is a different quantum chemistry system, where the molecule is not relatively isolated in a frozen solvent but is isolated in a gas-phase system at high temperatures T=388K. Statistics, adsorption and background subtraction can be more challenging. The model for solution may not agree for high temperature gas phase for a range of reasons –

Results and discussion

The first question of our inquiry was whether we could tell between the eclipsed and staggered conformers. We conclude that the lowest-temperature IR spectra of dilute Fc is dominated by the D5h eclipsed conformation. As the temperature increases, we predict and observe an increasing population of occupied modes along the reaction coordinate towards the D5d frequencies. In order to fit the Fc and dFc spectra across the temperature series we must allow:

  • (i) An energy offset (10–30 cm-1) from DFT

Conclusions

Our high quality FTIR experimental data with uncertainty across a wide temperature series allows quantitative investigation of the quantum chemistry, reaction surface and dynamics of Fc and dFc. The Reaction Coordinate Method and the developed model, presented herein, predicts the detailed spectrum and spectral changes across a wide range of energy including the highest temperatures, and even for a different phase, consistently. Compared with qualitative fits or direct DFT fits the model is

CRediT authorship contribution statement

R.M. Trevorah: Formal analysis, Writing - original draft, Writing - review & editing. N.T.T. Tran: Formal analysis, Writing - original draft, Writing - review & editing. D.R.T. Appadoo: Investigation, Methodology, Writing - review & editing. F. Wang: Investigation, Methodology, Writing - review & editing. C.T. Chantler: Supervision, Conceptualization, Formal analysis, Writing - original draft, Writing - review & editing.

Acknowledgement

This research was undertaken on the THz/Far-IR beamline at the Australian Synchrotron, Victoria, Australia. We acknowledge the Australian Research Council (ARC) and the science faculty of the University of Melbourne for funding this work. This research is supported by the AINSE Honours Scholarship Program. We acknowledge M. T. Islam, S. Islam and S. P. Best for their experimental and conceptual contributions to this work. In particular, this work and development would not have been possible

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      Both the calculated relative energies of the D5h and D5d forms and the IR band profile in the 400-500 cm−1 region are consistent with the D5h (eclipsed) conformer of Fc being lowest in energy in all solvents. While it is difficult to estimate the relative energies of the two conformers from these measurements the band profile suggests an energy difference similar to that obtained for solid solutions of Fc in paraffin (ca. 10 kJ mol−1) (Trevorah et al., 2020). These observations support our previous assertion that the IR spectra can be combined with good quality DFT calculations to explore the geometry of Fc (Mohammadi et al., 2012).

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