Département de chimie moléculaire

This work presents a detailed evaluation of the performance of d. functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in Mn(II) coordination complexes. Eighteen exptl. well characterized four-, five-, and six-coordinate complexes of the general formula [Mn(L)nL'2] with L' = Cl, Br, I, NCS, or N3 (L = an oligodentate ligand) are considered. Several DFT-based approaches for the prediction of the ZFSs are compared. For the estn. of the spin-orbit coupling (SOC) part of the ZFS, it was found that the Pederson-Khanna (PK) approach is more successful than the previously proposed quasi-restricted orbitals (QRO)-based method. In either case, accounting for the spin-spin (SS) interaction either with or without the inclusion of the spin-polarization effects improves the results. This argues for the phys. necessity of accounting for this important contribution to the ZFS. On av., the SS contribution represents ∼30{%} of the axial D parameters. In addn. to the SS part, the SOC contributions of d-d spin flip ($\alpha$$\beta$) and ligand-to-metal charge transfer excited states ($\beta$$\beta$) were found to dominate the SOC part of the D parameter; the obsd. near cancellation between the $\alpha$$\alpha$ and $\beta$$\alpha$ parts is discussed in the framework of the PK model. The calcns. systematically (correlation coeff. ∼0.99) overestimate the exptl. D values by ∼60{%}. Comparison of the signs of calcd. and measured D values shows that the signs of the calcd. axial ZFS parameters are unreliable once E/D {\textgreater} 0.2. Finally, we find that the calcd. D and E/D values are highly sensitive to small structural changes. It is obsd. that the use of theor. optimized geometries leads to a significant deterioration of the theor. predictions relative to the exptl. geometries derived from X-ray diffraction. The std. deviation of the theor. predictions for the D values almost doubles from ∼0.1 to ∼0.2 cm-1 upon using quantum chem. optimized structures. We do not find any noticeable improvement in considering basis sets larger than std. double- (SVP) or triple-$\zeta$ (TZVP) basis sets or using functionals other than the BP functional. [on SciFinder(R)]