Effect of Chlorine Substitution in Modulating the Relative Importance of Two Intermolecular Interactions: The Microwave Spectrum and Molecular Structure of (E)-1-Chloro-2-fluoroethylene-HCl.

Fourier transform microwave rotational spectroscopy is used to determine the structure of the gas-phase bimolecular complex formed between (E)-1-chloro-2-fluoroethylene and hydrogen chloride. Extensively split by nuclear quadrupole hyperfine structure and isotopic dilution, the spectrum is first identified via weak features observed using a broadband chirped pulse spectrometer in the 5.6-18.1 GHz range and studied in detail with greater sensitivity and resolution over 6.0-20.0 GHz with a Balle-Flygare, narrowband instrument. The complex has a geometry similar to that of vinyl fluoride-HCl, with HCl binding across the C═C double bond, forming a hydrogen bond to the fluorine atom of the haloethylene and bending to allow a secondary interaction to develop with the hydrogen atom in the cis position. Further consideration of structural details among the complexes of hydrogen fluoride and hydrogen chloride with (E)-1-chloro-2-fluoroethylene and vinyl fluoride suggests that the addition of a trans Cl atom in vinyl fluoride enhances the significance of the secondary interaction while deemphasizing that of the hydrogen bond.