Microwave spectra and structure of the fluorobenzene-carbon monoxide van der Waals complex

The rotational spectra of the fluorobenzene–CO complex and four of its isotopomers have been measured between 5–18GHz with a pulsed nozzle Fourier transform microwave spectrometer. An analysis with a rigid asymmetric rotor model revealed that CO must be located above the ring plane with CO at an angle to the symmetry plane. A second set of transitions indicated that CO is executing large amplitude motions. The rotational transitions of two states were analysed simultaneously using an effective model Hamiltonian which accounts for an internal rotation between CO and fluorobenzene hindered by a low barrier (below 25cm^-1).