Microwave Spectrum, Dipole Moment, and Internal Dynamics of the Methyl Fluoride−Carbonyl Sulfide Weakly Bound Complex

Rotational spectra for the normal and four isotopically substituted species of the 1:1 complex between methyl fluoride (H₃CF) and carbonyl sulfide (OCS) have been measured using Fourier-transform microwave spectroscopy in the 5−16 GHz frequency region. The observed spectra fit well to a semirigid Watson Hamiltonian, and an analysis of the rotational constants has allowed a structure to be determined for this complex. The dipole moment vectors of the H₃CF and OCS monomers are aligned approximately antiparallel with a C···C separation of 3.75(3) Å and with an ab plane of symmetry. The values of the P_cc planar moments were found to be considerably different from the expected rigid values for all isotopologues. An estimate of ∼14.5(50) cm^-1 for the internal rotation barrier of the CH₃ group with respect to the framework of the complex has been made using the P_cc values for the H₃CF−OCS and D₃CF−OCS isotopic species. Two structures, very close in energy and approximately related by a 60° rotation about the C₃ axis of the methyl fluoride, were identified by ab initio calculations at the MP2/6-311++G(2d,2p) level and provide reasonable agreement with the experimental rotational constants and dipole moment components.