The electronic, structural, and magnetic properties of the chromium dihalides - from the gas-phase to the solid-state : a thesis submitted in partial fulfillment of the requirements of the degree of Doctor of Philosophy in Theoretical Chemistry at Massey University, Albany, New Zealand
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Date
2008
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Massey University
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Abstract
Unrestricted Kohn-Sham (broken symmetry) density functional calculations have been
used to determine the low-energy geometries of the chromium dihalide molecules (CrX2)
and their clusters, Cr2X4, Cr3X6, and Cr4X8. The monomers are also investigated at
a higher level, including coupled-cluster and state-average CASSCF computations. Our
calculations show that the monomers have a 5B2 ground state arising from the Renner-Teller distorted 5IIg transition state, leading to a bent geometry. The global minima of the
gas-phase clusters of CrF2 and CrCl2 consist of two-dimensional, anti-ferromagnetically
coupled chains of CrX2 units forming four-membered, doubly bridged Cr2X2 rings, closely
resembling their solid-state structures. The global minima of the CrBr2 and CrI2 clusters
consist of the same two-dimensional chain-like structures for their dimers, but their
trimers and tetramers consist of three-dimensional ’triangular’ structures which contain
two capping ligands bound to three chromium atoms along with a Cr-Cr bond. Each
Cr atom within these clusters has spin quantum number S=2. There is approximately
a constant change in energy, between 45-55 kcal/mol, with every new CrX2 unit during
cluster formation.
Information about the structure of the CrCl2 clusters is used in the reanalysis of
high-temperature electron diffraction data. The vapor at 1170 K contains about 77%
monomeric molecules, 19% dimers, and a small amount of trimers. Monomeric CrCl2 is
found to be bent with a bond angle of 149(10)degrees, in good agreement with our computations.
Solid-state DFT calculations are performed on alpha-CrCl2 to determine the lattice structure and spin-coupling constants for the Cr atoms within the crystals. The GGA (PW91)
method produces a structure in good agreement with the literature. In the lowest energy
structure, the spins of the Cr atoms within the chains along the crystallographic c-axis
are anti-ferromagnetically coupled with four parallel spins situated almost exclusively in
the d-bands of Cr along these chains. This anti-ferromagnetic coupling is also seen in the
CrX2 clusters.
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Keywords
Chromium dihalides, Physical properties