Accurate prediction of vertical electronic transitions of Ni(II) coordination compounds via time dependent density functional theory

Density functional theory (DFT) calculations have been completed for five Ni(II) complexes formed by polidentate peptide ligands to predict the electronic absorption spectrum. The ligands examined were glycyl-glycyl-glycine (abbreviated as GGG), glycyl-glycyl-glycyl-glycine (GGGG), glycyl-glycyl-histidine (GGH), glycyl-glycyl-cysteine (GGC) and triethylenetetramine (trien). Thirteen functionals (B3LYP, B3P86, CAM-B3LYP, ω-B97x-D, MPW1PW91, HSE06, PBE0, BHandHLYP, TPSSh, M06, M06-2X, BLYP and PBE) and two basis sets (LANL2DZ and def2-TZVP) were tested. On the basis of the mean absolute percent deviation (MAPD) the following ranking among the functionals was found: HSE06 ~ MPW1PW91 ~ PBE0 > ω-B97x-D ~ B3P86 ~ B3LYP ~ CAM-B3LYP > TPSSh > PBE ~ BLYP ~ BHandHLYP >> M06 >> M06-2X. Concerning the basis sets, the triple-ζ def2-TZVP performs better than the double-ζ LANL2DZ. With the functional HSE06 and the basis set def2-TZVP the MAPD with respect to the experimental λmax values is 1.59% with a standard deviation of 1.17%. The absorption electronic spectra were interpreted in terms of vertical excitations between occupied and virtual MOs based on Ni-d atomic orbitals. The electronic structure of the Ni(II) species was finally calculated and discussed.