Abstract
The rotational spectra for the normal isotopic species and for six C-13 singly substituted isotopologues (in natural abundance) of the fluorobenzene center dot center dot center dot acetylene (C6H5F center dot center dot center dot HCCH) weakly bound dimer have been measured in the 6.5-18.5 GHz region using chirped-pulse Fourier-transform microwave spectroscopy. The HCCH molecule interacts with the fluorobenzene via a CH center dot center dot center dot pi contact and is determined to lie almost over the center of, and approximately perpendicular to, the aromatic ring, with an H center dot center dot center dot pi distance (perpendicular distance from the H atom to the ring plane) of around 2.492(47) angstrom a slight tilt of HCCH towards the para carbon atom of the fluorobenzene is evident. Binding energies of this complex and related benzene and fluorobenzene dimers obtained from the pseudodiatomic approximation are compared and indicate that fluorobenzene center dot center dot center dot acetylene lies among the more weakly bound of the complexes exhibiting some type of CH center dot center dot center dot pi interaction.