Circular dichroism of tris chelates with planar conjugate ligands described by angular overlap model and calculated by time dependent density functional theory

Abstract

The main theme is coordination entities having trigonal dihedral point group symmetry. The geometric parameters used to describe such entities are reviewed and inter-related. The symmetry aspects of planar, symmetric, bidentate ligands are explained and the angular overlap model's (AOM) formulation of tris-coordination entities is described. The full use of symmetry makes it an advantage to use an approach where pi-electron systems from the three ligands are coupled to the metal electron system. The construction of complete molecular orbitals is demonstrated. We emphasize the original ideas of the AOM and show how it is useful to consider overlap conditions before dealing with the one-electron energies. To demonstrate the generality of this remark we also include a few other geometries in the discussion.The general use of density functional theory (DFT) to model structures of coordination entities of trigonal dihedral symmetry is reviewed. The output from such computations is used as input for time dependent DFT (TD-DFT) resulting in computed rotatory strengths of Λ absolute configurations. These rotatory strengths were transformed into a circular dichroism (CD) curve using a constant spectral band width. Eight such calculated CD-curves were compared with the experimental ones. Little correlation between spectral characteristics and electronic parameters were found. As a second order effect CD gives a sensitive measure of the validity of TD-DFT calculations.The computations also lead to drawings of molecular orbitals which clearly demonstrate that the electronic structure is made up according to the concept of molecules-in-molecules as discussed in the review. A detailed understanding of the computational results is possible using this concept and the language of AOM. A closer look on the drawings also reveals interesting features of overlaps, but unfortunately they also show that full trigonal dihedral symmetry is not always maintained during diagonalization. This may happen because this point group is a non-Abelian group.