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.
Metalloporphyrin assemblies such as Zn–porphyrins are significant photoactive compounds with a number of applications including molecular devices and dye-sensitized solar cells (DSSC).
Recent high-accuracy X-ray absorption measurements of the sandwich organometallics ferrocene (Fc) and decamethylferrocene (DmFc) at temperatures close to liquid helium are compared with new full-potential modeling of X-ray absorption fine structure (XAFS) covering the near-edge region (XANES) and above up to k = 7 Å(-1). The implementation of optimized calculations of the oscillatory part of the spectrum from the package FDMX allows detailed study of the spectra in regions of the photoelectron momentum most sensitive to differences in the molecular stereochemistry. For Fc and DmFc, this corresponds to the relative rotation of the cyclopentadienyl rings. When applied to high-accuracy XAFS of Fc and DmFc, the FDMX theory gives clear evidence for the eclipsed conformation for Fc and the staggered conformation for DmFc for frozen solutions at ca. 15 K. This represents the first clear experimental assignment of the solution structures of Fc and DmFc and reveals the potential of high-accuracy XAFS for structural analysis.
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