Accurate multireference configuration interaction calculations on the lowest 1Σ+ and 3Π electronic states of C2, CN+, BN, and BO+

Abstract

Using a series of correlation consistent basis sets from double to quintuple zeta in conjunction with large internally contracted multireference configuration interaction (CMRCI) wave functions, potential energy functions have been computed for the X 1Σ+g and a 3Πu states of C2 and the 1 1Σ+ and 1 3Π states of CN+, BN, and BO+. By exploiting the regular convergence behavior of the correlation consistent basis sets, complete basis set limits have been estimated that led to accurate predictions for the electronic excitation energies, dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies. The 1 1Σ+ states of CN+ and BO+ are predicted to be the electronic ground states of these species with predicted equilibrium excitation energies (Te) to the low-lying a 3Π states of 880±100 cm−1 and 5000±200 cm−1, respectively. A 3Π ground state of BN is predicted with an excitation energy to the low-lying a 1Σ+ state of just 190±100 cm−1. Identical calculations on the singlet–triplet splitting of C2 yielded a prediction of 778 cm−1 for Te, which was just 62 cm−1 above the experimental value. Accurate equilibrium bond lengths and fundamental frequencies are also predicted for BN, BO+, and the a 3Π state of CN+. Dipole moment functions have been computed by CMRCI for the ground and excited electronic states of the three heteronuclear diatomics, and these have been used to derive accurate microwave and infrared transition probabilities for these species. A dipole moment in v=0 of 5.42 D is calculated for the X 1Σ+ state of BO+, which should lead to an intense microwave spectrum. While the X 3Π ground state of BN is predicted to have a very weak infrared spectrum, this species should be observable in the microwave region since the predicted μ0 is 1.98 D. Both the microwave and infrared spectra of X 1Σ+ CN+ should be of moderate intensity.