Ab initio molecular-orbital study of the trichlorine radical, Cl3

Abstract

We report a rigorous ab initio study of the ground and low-lying excited-state potential-energy surfaces (PESs) of the Cl3 radical at CASSCF, CASPT2, and MRSDCI levels of theory. The ground state has two Cl⋯Cl2 van der Waals complexes, X̃L and X̃′B. The linear asymmetric minimum (X̃L) is 2Π, with a Cl–Cl distance r=3.90 bohr, and a Cl–M (M: the Cl2 center-of-mass) distance R=8.70 bohr. The bent asymmetric minimum (X̃′B) is of 2A′ symmetry, with r=3.90 bohr, R=6.85 bohr, and the angle between r̂ and R̂, γ=68.4°. Spin–orbit CI (configuration interaction) predicts that the global minimum is linear X̃L (2Π3/2) with a bond dissociation energy of De(Cl2(X)-Cl) of 280 cm−1. Low-lying doublet excited states have only one strongly bound structure, a linear symmetric ÃL (1 2Πg) state with a bond distance of 4.67 bohr. This state is bound by ∼4300 cm−1 with respect to the Cl2(3Πu)+Cl asymptote, and its minimum lies about 8700 cm−1 above the X̃L van der Waals minimum. Transition dipole moment calculations show that the ÖX̃ transition is fully allowed. Two bound quartet minima were located. The most deeply bound was QD3h (1 4A1′) with a D3h equilibrium geometry (r=5.00 bohr) about 11 300 cm−1 above X̃L. The other state, QC2v (1 4A2) had a C2v equilibrium geometry (r1=4.83 bohr and θ=101.7°) and an energy of about 13 500 cm−1 relative to X̃L. Although Cl3(X̃) is shown to be unstable, the present results support the notion that Cl3 participates in Cl atom recombination processes. However, the energies and transition moments of the low-lying excited states are not consistent with electronic spectra that have been tentatively assigned to Cl3.