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.