Spectroscopic characterization and Duschinsky analysis of the spin crossover in the cis-[Fe^II(bpym)₂(-NCSe)₂] and...

Computations at the DFT/UB3LYP/cc-PVDZ[LANL2DZ] level of theory are carried out to characterize spin crossover (SCO) phenomenon in six-coordinate iron complexes cis-[Fe^II/III(bpym)₂(-NCSe)₂]^0/+1 ([Fe^II/III]^0/+1) in terms of structural, energetics and spectroscopic characteristics. Geometrical parameters, energies, charge and spin density distributions, electric polarizabilities, frontier molecular orbitals (FMO), and the UV-Vis-NIR, ECD, vibrational (IR and Raman) and VCD spectra, as well as Duschinsky mixing matrix are calculated for the optimized structures of the [Fe^II/III]^0/+1 complexes in their high-spin (HS, most stable) and the low-spin (LS) states. The UV-Vis-NIR spectra show that the HS states have strong and broad absorption peaks over the visible region, predicting that they are colored (differently). Also, the UV-Vis peaks of the [Fe^II/III]^0/+1 complexes undergo large Δλ_max = 330 and 368 nm red shifts towards NIR due to the SCO HS → LS reaction. The calculated ECD spectra show that these complexes will have no significant nonlinear optical activities. Some of the IR bands of the two [Fe^II/III]^0/+1 complexes are shifted differently due to the SCO HS → LS transition. Also, Raman activities of the vibrational modes with high contributions from the SeCN ligands and iron atom are decreased significantly upon SCO, while those of the internal vibrational bands of the bpym ligands are not changed considerably. Furthermore, the two vibrational modes of the SeCN- ligands show the highest VCD, especially in the HS-[Fe^II] and LS-[Fe^III]⁺¹ complexes. Duschinsky analysis shows that a number of vibrational modes are mixed as a result of the HS → LS SCO reaction. This mixing for individual modes and the overall vibrational mode mixing (measured by the index f_tot) is larger for the [Fe^II] complex (as compared to [Fe^III]⁺¹), in accordance with larger increase in the asymmetry of the electron density distribution. Analysis of the FMOs shows that in both [Fe^II/III]^0/+1 complexes, the two SeCN^- ligands are more involved in the HS → LS SCO reaction and therefore, mixings of the vibrational modes having higher contributions from the atoms of these ligands are larger. Effect of the basis set for the iron ion is investigated by carrying out the same computations with the UB3LYP/cc-PVDZ[def2-TZVP] method (i.e. using def2-TZVP basis set for iron). Almost the same conclusions can be drawn from the results obtained with the def2-TZVP basis set for iron.