Multireference configuration-interaction studies on the UV and photoelectron spectrum of phosgene, Cl 2CO

Abstract

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, Cl 2CO, in C 2v symmetry. Vertical excitation energies and CO potential functions for the four lowest 1A 1, 1B 1, 1B 2 and 1A 2 states were calculated. On the basis of the CO potentials, the A ← X system corresponds to 1 1 A 2 ← X ( n O → π ∗) . The B ← X system can clearly be assigned to 2 1 A 1 ← X, 3 b 1 → π ∗ , where 3b 1 is a Cl nonbonding MO at the ground state equilibrium geometry, but changes into π CO at larger CO separations. Contrary to the situation in formaldehyde, the 3 b 1 → π ∗ (π → π ∗) potential is well separated from the higher-lying n O → Rydberg potentials. On the basis of vertical excitation energies, other calculated valence transitions ( n O → σ CCl ∗, 3 b 1 → σ CCl ∗ and n Cl → 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 n O → 4s Rydberg transition is best associated with the D ← X system, and the n O → 4p transitions with E ← X. MR-CI calculations were also performed on low-lying states of Cl 2CO +. 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 2a 2, and the fourth to 3b 1, opposite to the Koopmans' ordering.