Multireference configuration-interaction studies on the UV and photoelectron spectrum of phosgene, Cl2CO Original Research Article

Multireference configuration-interaction (MR-CI) calculations, using basis sets between double to triple-zeta quality with added polarization and Rydberg functions, have been performed on the ground state and excited singlet states of phosgene, Cl2CO, in C2v symmetry. Vertical excitation energies and CO potential functions for the four lowest 1A1, 1B1, 1B2 and 1A2 states were calculated. On the basis of the CO potentials, the A ← X system corresponds to 11A2 ← X (nO → π∗). The B ← X system can clearly be assigned to 21A1 ← X, 3b1 → π∗, where 3b1 is a Cl nonbonding MO at the ground state equilibrium geometry, but changes into πCO at larger CO separations. Contrary to the situation in formaldehyde, the 3b1 → π∗ (π → π∗) potential is well separated from the higher-lying nO → Rydberg potentials. On the basis of vertical excitation energies, other calculated valence transitions (nO → σCCl ∗, 3b1 → σCCl ∗ and nCl → π∗) lie (vertically) in the range of the B ← X and C ← X systems. Several higher valence states are expected, but have not been found among the four lowest roots of each symmetry species calculated. The nO → 4s Rydberg transition is best associated with the D ← X system, and the nO → 4p transitions with E ← X. MR-CI calculations were also performed on low-lying states of Cl2CO+. The maximum deviation of calculated vertical ionization energies of phosgene from those obtained via the photoelectron spectrum is 0.5 eV, much smaller than deviations based on orbital energies. The third photoelectron band is now assigned to 2a2, and the fourth to 3b1, opposite to the Koopmansʹ ordering.