Electronic spectrum of boron dichloride. Theoretical study of vibronic levels of the ground and first excited states

Abstract As expected by the Walsh diagram of AB2 molecules, the electronic spectrum of BCl2 radical involves bent-linear transitions between the ground and first excited states that form the two degenerate components of the 2Π electronic state of the linear molecule. A high level ab-initio theoretical study on BCl2 radical has been undertaken by constructing potential energy surfaces (PES) using the coupled-cluster singles and doubles with perturbative triples (CCSD(T)) with aug-cc-pVQZ for boron and aug-cc-pV(Q+d)Z for chlorine atoms. Vibronic energy levels were determined variationally, from the obtained PES, up to ∼35000 cm−1 above the relative minima of the ground and excited states. Low-lying vibrational levels of the ground and first excited electronic states were obtained, which showed a very complex structure which is known to be as a result of large Renner-Teller vibronic coupling. For the most abundant isotopologue, 11B35Cl2, the ground state fundamental frequencies are; ω 1 0 = 696.0 cm−1, ω 2 0 = 281.0 cm−1 and ω 3 0 =981.8 cm−1. For the same isotopologue, vibrational fundamentals of the excited state were estimated by fitting Σ vibronic levels to be ω 1 0 = 463.2 cm−1, ω 2 0 = 281.7 cm−1 and ω 3 0 =1245.1 cm−1.