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

We report a rigorous ab initio study of the ground and low-lying excited-state potential-energy surfaces (PESs) of the Cl-2 radical at CASSCF, CASPT2, and MRSDCI levels of theory. The ground state has two Cl ... Cl-2 van der Waals complexes, (X) over tilde L and (X) over tilde'B. The linear asymmetric minimum ((X) over tilde L) is (II)-I-2, with a Cl-Cl distance r = 3.90 bohr, and a Cl-M (M: the Cl-2 center-of-mass) distance R = 8.70 bohr. The bent asymmetric minimum ((X) over tilde'B) is of (2)A' symmetry, with r = 3.90 bohr, R = 6.85 bohr, and the angle between (r) over cap and (R) over cap, gamma = 68.4 degrees. Spin-orbit CI (configuration interaction) predicts that the global minimum is linear (X) over tilde L ((II3/2)-I-2) with a bond dissociation energy of D-e(Cl-2((X) over tilde)-Cl) of 280 cm(-1). Low-lying doublet excited states have only one strongly bound structure, a linear symmetric (A) over tilde L (1 (IIg)-I-2) state with a bond distance of 4.67 bohr. This state is bound by similar to 4300cm(-1) with respect to the Cl-2((II3/2)-I-3)+Cl asymptote, and its minimum lies about 8700 cm(-1) above the (X) over tilde L van der Waals minimum. Transition dipole moment calculations show that the (A) over tilde-(X) over tilde transition is fully allowed. Two bound quartet minima were located. The most deeply bound was QD3h (1 (4)A(1)') With a D-3h equilibrium geometry (r = 5.00 bohr) about 11 300 cm(-1) above (X) over tilde L. The other state, QC2v (1 (4)A(2)) had a C-2v equilibrium geometry (r(1) = 4.83 bohr and theta = 101.7 degrees) and an energy of about 13 500 cm(-1) relative to (X) over tilde L. Although Cl-3((X) over tilde) is shown to be unstable, the present results support the notion that Cl-3 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 Cl-3. (C) 1998 American Institute of Physics.