Rotational study of the CH₄–CO complex: Millimeter-wave measurements and ab initio calculations

The rotational spectrum of the van der Waals complex CH₄–CO has been measured with the intracavity OROTRON jet spectrometer in the frequency range of 110–145 GHz. Newly observed and assigned transitions belong to the K = 2–1 subband correlating with the rotationless j_CH4 = 0 ground state and the K = 2–1 and K = 0–1 subbands correlating with the j_CH4 = 2 excited state of free methane. The (approximate) quantum number K is the projection of the total angular momentum J on the intermolecular axis. The new data were analyzed together with the known millimeter-wave and microwave transitions in order to determine the molecular parameters of the CH₄–CO complex. Accompanying ab initio calculations of the intermolecular potential energy surface (PES) of CH₄–CO have been carried out at the explicitly correlated coupled cluster level of theory with single, double, and perturbative triple excitations [CCSD(T)-F12a] and an augmented correlation-consistent triple zeta (aVTZ) basis set. The global minimum of the five-dimensional PES corresponds to an approximately T-shaped structure with the CH₄ face closest to the CO subunit and binding energy D_e = 177.82 cm⁻¹. The bound rovibrational levels of the CH₄–CO complex were calculated for total angular momentum J = 0–6 on this intermolecular potential surface and compared with the experimental results. The calculated dissociation energies D₀ are 91.32, 94.46, and 104.21 cm⁻¹ for A (j_CH4 = 0), F (j_CH4 = 1), and E (j_CH4 = 2) nuclear spin modifications of CH₄–CO, respectively