Characterization of the [Xtilde] 1Σ+ à 3Π and à 1Π electronic states of BBO

Abstract

The three lowest-lying electronic states, [Xtilde] 1Σ+, à 3II and à 1II, of the linear BBO molecule have been systematically investigated using ab initio electronic structure theory. The equilibrium structures and physical properties including dipole moments, vibrational frequencies and associated infrared intensities, Renner parameters and energetics for the three states of BBO have been determined employing SCF, CISD, CCSD and CCSD(T) levels of theory and a wide range of basis sets. The ground state of BBO presents a degenerate real bending frequency, while the à 3II and à 1II states show two distinct real bending frequencies due to the Renner-Teller interaction. The bending motion of the à 1II state was analysed using the equation-of-motion (EOM)-CCSD and EOM-CC3 techniques in order to avoid possible variational collapse to a lower-lying state. The [Xtilde] 1Σ+-à 3II separation was predicted to be T 0 = 16.6 kcal mol−1 (5800 cm−1, 0.719 eV) at the cc-pVQZ CCSD(T) level of theory. With the cc-pVQZ EOM-CC3 method the [Xtilde] 1Σ+-à 1II splitting was predicted to be T 0 = 48.0 kcal mol−1 (16 800 cm−1, 2.08 eV), which is in good agreement with the experimental value of T 0 = 46.6 kcal mol−1 (16 300 cm−1, 2.02 eV). The Renner parameters and averaged harmonic frequencies of the bending mode were determined to be ε = 0.184 and ω2 = 363 cm−1 for the à 3II state, and ε = 0.246 and ω2 = 383cm−1 for the à 1II state. The theoretical [Xtilde] 1Σ+ state harmonic B-B stretching frequency ω3 = 636 cm−1 is somewhat higher than the experimental estimate of 582 cm−1 and the predicted à 1II state harmonic B-B stretching frequency ω3 = 861 cm−1 is significantly higher than the experimental estimate of 440 cm−1