Research paperResolution of ferrocene and deuterated ferrocene conformations using dynamic vibrational spectroscopy: Experiment and theory
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Motivation
Ferrocene (Fc, ) 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 and 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 and conformations in gas-phase in a broad spectral region (400–3500
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 conformation, but with quite different separation of components and quite different broadening. Whilst the frequency splitting of two broad bands in the 450–500 region is clear for Fc, the more intense band lies at the low wavenumber, which is predicted for the rotamer but with very narrow separation of components. Neither nor
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 ( analysis) with established uncertainty estimates and advanced hypotheses, we can investigate and explicitly discriminate between hypotheses:
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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) or purely staggered (St) ; 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 (wavenumber in ), 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 and 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 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 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. . Conversely, parameters
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 . 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 eclipsed conformation. As the temperature increases, we predict and observe an increasing population of occupied modes along the reaction coordinate towards the frequencies. In order to fit the Fc and dFc spectra across the temperature series we must allow:
(i) An energy offset (10–30 ) 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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Solvent contribution to ferrocene conformation: Theory and experiment
2021, Radiation Physics and ChemistryCitation Excerpt :In particular, different independent methods have confirmed that the D5h conformation is lowest in energy (Bohn and Haaland, 1966; Seiler and Dunitz, 1979; Bourke et al., 2016). The stable form and the energy difference between the eclipsed and staggered forms can be obtained from analysis of the IR spectrum (Best et al., 2016; Trevorah et al., 2020; Islam et al., 2017) and synchrotron based X-ray absorption fine structure (XAFS) (Bourke et al., 2016; Chantler et al., 2012; Islam et al., 2016). The small calculated energy difference between the D5h and D5d forms of Fc is a feature of published ab initio studies and the value of 0.58 kcal⋅mol−1, obtained from DFT calculations (B3LYP/m6-31G(d)) (Mohammadi et al., 2012) is consistent with experimental estimates for the gas-phase species (Haaland and Nilsson, 1968).
Dominance of eclipsed ferrocene conformer in solutions revealed by the IR spectra between 400 and 500 cm-1
2021, Radiation Physics and ChemistryCitation Excerpt :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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