Accurate X-ray Absorption Spectra of Dilute Systems: Absolute Measurements and Structural Analysis of Ferrocene and Decamethylferrocene

Abstract

X-ray absorption fine structure (XAFS) of ferrocene (Fc) and Decamethylferrocene (DmFc) have been determined on an absolute scale using transmission measurements of multiple solutions of differing concentrations (15 mM, 3 mM, pure solvent) at operating temperatures of 10–20 K. Mass attenuation coefficients and photoelectric absorption cross sections are measured and tabulated for both molecules for an extended energy range in excess of 1.5 keV from the Fe K-shell absorption edge. At these temperatures, the minimization of of dynamic disorder has enabled a critical determination of the oscillatory absorption structures created by multiple-scattering paths of the excited photoelectron. These oscillatory structures are highly sensitive to the local conformation environment of the iron absorber in organometallic structures. Crystallographic and scattering studies have reported both structures characterized by staggered cyclopentadienyl rings, in contrast with low temperature crystallography and recent density functional theoretical predictions. Phase changes in the crystallographic space groups are reported for Fc at different temperatures, raising the possibility of alternative conformation states. Robust experimental techniques are described which have allowed the measurement of XAFS spectra of dilute systems by transmission at accuracies ranging from 0.2% to 2%, and observe statistically significant fine structure at photoelectron wavenumbers extending to >12 Å–1. The subtle signatures of the conformations are then investigated via extensive analysis of the XAFS spectra using the full multiple scattering theory as implemented by the FEFF package. Results indicate a near-eclipsed D 5h geometry for low-temperature Fc, in contrast with a staggered D 5d geometry observed for DmFc. The ability of this experimental approach and data analysis methodology combined with advanced theory to investigate and observe such subtle conformational differences using XAFS is a powerful tool for future challenges and widens the capacity of advanced XAFS to solve a broad range of challenging systems.

A comparative study of energetics of ferrocenium and ferrocene

“…5 Such the discovery of Fc conformers stimulates a number of significant experimental and theoretical studies. 3,8,9,20,26 For example, a recent highly accurate X-ray absorption fine structures (XAFS) study of ferrocene (Fc) confirmed a near-eclipsed D5h geometry at low-temperature, 7,9 in agreement with our earlier theoretical results. 5 The recent self-consistent temperature and environment sensitive IR spectral study of Fc, by a series of Synchrotron-based high-resolution IR spectral measurements provided insight on the electronic structure changes of Fc with temperature.…”

“…5 Despite significant advances in experimental techniques such as XAFS which has been employed to study Fc, 7 and the first comprehensive study of Fc dynamics based on theory and a series of theory-guided infrared (IR) spectral measurements, 8 it concluded that the Fc is in eclipsed form at low temperature (e.g. 0 K) and becomes a mixture of both conformers at higher temperatures such as room temperature 300 K. 7,9 Ferrocene has been studied extensively in the literature. It is found that many methods such as (u)HF and (u)MP2 with a variety of basis sets were unable to describe transition metalcontaining complexes like Fc appropriately.…”

Molecular dynamics study of ferrocene topology under various temperatures

“…[24][25][26] It was discovered that Fc inferred (IR) spectra allow for structural assignment in the region of 400 to 600 cm −1 in the gas phase, [25] which later guided and was confirmed by temperature-dependent Fourier-transform infrared spectroscopy (FTIR) measurements and assisted the X-ray absorption fine structure (XAFS) measurements in frozen solutions, showing small statistical preference for the eclipsed rotamer. [24,27] It is also important to understand the dynamics of the Fc rotameric configurations under various temperatures, which is almost not possible for direct observation. [28] A small number of molecular dynamic (MD) studies of ferrocene has been reported, [28][29][30] using empirical force field methods, [29,30] the Car-Parrinello molecular dynamics (CPMD) method, [28] and the first-principle MD with the projector augmented wave basis set.…”

Structure of Diferrocenyl Thioketone: From Molecule to Crystal

“…This is in agreement with the conformation of FcH in solutions. Experimental studies suggested the dominance of eclipsed conformer for FcH in solutions [57,62,63].…”