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 jCH4 = 0 ground state and the K = 2–1 and K = 0–1 subbands correlating with the jCH4 = 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-di...