Modelling the circularly and linearly polarised spectrum of [Ni(en)3]2+

Abstract

The relative intensity and band shapes of the low energy spin-allowed transitions in the linearly polarised and circular dichroism spectrum of [Ni(en)₃]²⁺ have been calculated using a time-dependent density functional theory approach. The effect of the trigonal ligand-field is minimal and no splitting of the bands is predicted by the simulations or observed experimentally. The ‘d–d’ transitions of the [Ni(en)₃]²⁺ ion are electric dipole allowed but gain much of their intensity through Herzberg–Teller vibronic coupling. Its CD spectrum is dominated by the low energy band, which gains its rotatory strength through the magnetic dipole-allowed character of the parent octahedral transition and the electric dipole character due to the trigonal field. The simulation of the spectrum incorporates the contribution from all inducing vibrational modes with significant involvement of the {NiN₆} unit. Vibrations which are centred on the chelate rings are not important in generating intensity, reflecting the localised d–d′ character of the transitions. Simulated linearly polarised and circular dichroism spectra of such an open-shell system are presented for the first time and predict the essential elements of the experimental spectra.