“X-ray diffraction spectroscopy” – Refinement of dispersion correction parameters for XRD measurements of Mo(CO)6 at the Mo K-edge

Correcting for anomalous dispersion is part of any refinement of an X-ray
diffraction crystal structure determination. The procedure takes the inelastic
scattering in the diffraction experiment into account. This X-ray absorption effect
is specific to each chemical compound and is particularly sensitive to radiation
energies in the region of the absorption edges of the elements in the compound.
Therefore, the widely used tabulated values for these corrections can only be
approximations as they are based on calculations for isolated atoms. Features of
the unique spatial and electronic environment that are directly related to the
anomalous dispersion are ignored, although these can be observed spectroscopically.
This significantly affects the fit between the crystallographic model and
the measured intensities when the excitation wavelength in an X-ray diffraction
experiment is close to an element’s absorption edge. Herein, we report on
synchrotron multi-wavelength single-crystal X-ray diffraction, as well as X-ray
absorption spectroscopy experiments which we performed on the molecular
compound Mo(CO)6 at energies around the molybdenum K edge. The dispersive
(f 0) and absorptive (f 00) terms of the anomalous dispersion can be refined as
independent parameters in the full-matrix least-squares refinement. This procedure
has been implemented as a new feature in the well-established OLEX2 software
suite. These refined parameters are in good agreement with the independently
recorded X-ray absorption spectrum. The resulting crystallographic models show
significant improvement compared to those employing tabulated values.