XANES Spectra of Actinide Complexes

The ground state of [NpClg] is an open-shell quartet (5f ) where the HOMO/LUMO character of the alpha electrons are dominated by Cl 3p (HOMO-3), Np 5f (HOMO-2), Np 5f + Cl 3p (HOMO-1), Np 5f + Cl 3p (HOMO), Np 5f (LUMO), Np 5f (LUMO+(l-3)). 

Experimentally, the axial U-0 and U-Cl bonds are 1.78 A and 2.43 A, and the equatorial U-Cl bonds are all 2.54 A, respectively. If the axial U-Cl bond is elongated by 0.25 A and two of the equatorial U-Cl bonds (opposite to each other) are elongated by 0.25 A, all the experimental features are reasonably captured including the shoulder. However, these bond length elongations ( 10%) are unphysical and cannot be accounted for by a weak host crystal field, dynamics and different oxidation states (which only accounts for 0.08 A per charge of charge). This feature may be related to spin-flip effects, which cannot be assessed because our implementation does not include these effects. 

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Nichols P, Govind N, Bylaska EJ and de Jong WA 2009 Gaussian basis set and planewave relativistic spin-orbit methods in nwchem. J. Chem. Theory Comput. 5(3), 491 99. 

Filatov M and Cremer D 2005b A gauge-independent zeroth-oider regular approximation to the exact relativistic hamiltonian—formulation and applications. The Journal of chemical physics 122, 044104. 

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