TD-DFT computations for trigonal silicon(IV) coordination compounds of rigid bidentates

Computations using standard time dependent DFT (B3LYP/6-31G) can neither reproduce experimental CD spectra of optically active tris(2,4-pentanedionato)silicon(IV), [Si(acac)3]+, nor the CD of a series of similar coordination compounds of Si(IV). These compounds are characterized by exciton coupled ligand transitions which are not influenced by orbitals on the central ion. We have found that TD DFT calculations using long range functionals can indeed reproduce the measured CD satisfactorily. The detailed interpretations make use of D3 point group symmetry and symmetry adapted excited state functions allowing for the electronic coupling of the excitation of the ligands. Computations of similar problems for transition metal ions coordinated to rigid π systems are common and probably also sensitive to the choice of functional.