The vibronic effects in the excited 11Eu state of (CO)4 dianion in terms of time dependent (TD) density functional theory: The resonance and pre-resonance Raman studies

Franck-Condon (FC) and Jahn-Teller (JT) effects in the 11Eu state of the (CO)4 dianion are examined in terms of time dependent density functional theory (TD DFT) at the B3LYP/6-31G+* level. The experimental absorption, pre-resonance and resonance Raman spectra associated with 1Ag → 11Eu(ππ*) transition are studied. The computations indicate considerable activities of ν5(b2g) = 1073 cm−1 (CC stretching) and ν10(b1g) = 624 cm−1 (ring bending) modes due to JT effect in the 11Eu(ππ*). Of two totally symmetric modes of the squarate dianion only single ν2(a1g) = 698 cm−1 (ring breathing) vibration is shown to exhibit the sizable Franck-Condon activity in the lowest energy 11Eu(ππ*) state of the squarate dianion.