Real space atomic decomposition of fundamental properties of carbon monoxide in the ground and the two lowest lying excited electronic states

The properties of carbon monoxide in its ground and two lowest lying excited states are investigated by conventional and time-dependent (TD) density functional theory (DFT). The dipole moment is decomposed into atomic polarisation (AP) and inter-atomic charge transfer (CT) contributions according to the quantum theory of atoms in molecules (QTAIM). Considerable AP and CT contributions cancel in the ground state (S0) resulting in its known negligible dipole moment. This balance is disturbed in the lowest triplet (T1) and singlet (S1) excited states, resulting in dipole moments of 1.57 D for T1 and 0.49 D for S1 . The AP decreases by 2.5 D on excitation to either state but the opposing CT is reduced by 0.8 D for T1 and by 1.8 D for S1, insufficient for cancellation. These excitations induce charge transfer from O to C which drives dipole changes accompanied by a weakening of the C–O bond, more pronounced in the S1 state, as trends in bond lengths, vibrational frequencies, and QTAIM properties sugges...