Structure and Dynamics of the Methane-Propane van der Waals Complex.

Microwave transitions in the region 7-26 GHz were measured for the methane-propane van der Waals complex. The nearly free internal rotation of methane within the complex gives rise to three states that do not relax even in a 5 K supersonic expansion. Eighteen lines have been assigned to the lowest state and are well fitted to a semirigid rotor model, with rotational constants A = 7553.8229 (24) MHz, B = 2483.9200 (8) MHz, and C = 2041.8692 (5) MHz, and six distortion constants. The structure has the methane positioned above the plane defined by the propane carbon atoms with a center-of-mass van der Waals bond distance of 3.98 Å. This is significantly larger than the equilibrium value of 3.71 Å found with ab initio calculations done at the CCSD(T)-F12a/aug-cc-pVTZ level of theory. Further calculations encompassing a large range of angular orientations of the methane subunit indicate that angular motion produces a large zero-point contribution to the energy, which not only lowers the effective barrier to internal rotation of the methane but also increases the radial distance between subunits. Therefore, although in the lowest energy structure the methane can get close to the propane by interdigitating the hydrogens atoms, the zero-point energy effectively flattens out the potential so that the hydrogens become less restricting.