Ab initio potential and rotational spectra of the CO-N₂ complex

Ab initio calculations of the intermolecular potential energy surface (PES) of CO-N₂ have been carried out using the closed-shell single- and double-excitation coupled cluster approach with a non-iterative perturbative treatment of triple excitations method and the augmented correlation-consistent quadruple-zeta (aug-cc-pVQZ) basis set supplemented with midbond functions. The global minimum (D_e = 117.35 cm^-1) of the four-dimensional PES corresponds to an approximately T-shaped structure with the N₂ subunit forming the leg and CO the top. The bound rovibrational levels of the CO-N₂ complex were calculated for total angular momenta J = 0-8 on this intermolecular potential surface. The calculated dissociation energies D₀ are 75.60 and 76.79 cm^-1 for the ortho-N₂ (A-symmetry) and para-N₂ (B-symmetry) nuclear spin modifications of CO-N₂, respectively. Guided by these bound state calculations, a new millimeter-wave survey for the CO-N₂ complex in the frequency range of 110-145 GHz was performed using the intracavity OROTRON jet spectrometer. Transitions not previously observed were detected and assigned to the subbands connecting the K = 0 and 1, (j_CO, j_N ) = (1, 0) states with a new K = 1, (j_CO, j_N ) = (2, 0) state. Finally, the measured rotational energy levels of the CO-N₂ complex were compared to the theoretical bound state results, thus providing a critical test of the quality of the PES presented. The computed rovibrational wave functions were analyzed to characterize the nature of the different bound states observed for the two nuclear spin species of CO-N₂.