The electronic spectrum of AgI2. Ab initio benchmark CASSCF + Averaged Coupled Pair Functional studies on the ligand-field states including spin-orbit couplings

Abstract

The X2Πg, 2Σg+ and 2Δg states of AgI2 have been studied through benchmark ab initio CASSCF + Averaged Coupled Pair Functional (ACPF) calculations using especially developed valence basis sets to study the transition energies and equilibrium geometries. The spin–orbit (SO) effects were included through the effective Hamiltonian formalism using the |ΛSΣ〉 ACPF energies as diagonal elements. AgI2 is a linear centrosymmetric molecule with a 2Πg ground state, in contradiction with ligand-field theory. The ACPF adiabatic excitation energies of the 2Σg+ and 2Δg states are 7344 and 26085 cm−1, respectively. A comparison of the present results with the previous ones for AgCl2 and AgBr2 shows a monotonic increase in the 2Πg-2Σg+ and 2Πg-2Δg transition energies due to a decrease of the ionic structures in the LF states as the atomic number of the halogen grows. The inclusion of the SO effects leads to a pure Ω = 3/2 X(2Πg) ground state, a Ω = 1/2 (83% 2Πg + 17% 2Σg+) A state, a Ω = 1/2 (17% 2Πg + 83% 2Σg+) B state, a Ω = 5/2 (2Δg) C state and a Ω = 3/2 (2Δg) D state. The X-A, X-B, X-C, X-D vertical transition energies are 4250, 11248, 27,127 and 30441 cm−1, respectively, nearly identical to the adiabatic Te values.