Improved point-charge model within the INDO/S-CI method for describing the ligand excited states of lanthanide coordination compounds

Due to the spatial extent of the 5s and 5p filled orbitals of lanthanide ions, the shielding of the nuclear charge is not perfect at close distances to the ion. Thus, ligand atoms should experience different effective charge depending upon their distance to the central ion. Using electrostatic arguments and ab initio calculations for Eu3+, we have proposed an improved model that describes the ion as an effective charge, q(r), whose value has the following radial dependence, q(r) = 3 + 14e(-Ar2). This functional form of the point charge has been implemented into the ZINDO program and INDO/S-CI calculations have been performed for the Eu(btfa)(3)bipy, Eu(btfa)(3) . 2H(2)O, Eu(bzac)(3)bipy, and Eu(bzac)3.2H(2)O compounds. The comparison with the experimental absorption spectrum was used to optimize the exponent A parameter. Application of this model to several other Eu3+ coordination compounds yielded better results for the calculated absorption spectra than the fixed 3+ point-charge model. (C) 2002 Wiley Periodicals, Inc.