Finding the Better Fit: The Microwave Spectrum and Sterically Preferred Structure of...

The rotational spectrum of the gas-phase bimolecular heterodimer formed between trans-1,2-difluoroethylene and hydrogen chloride is obtained using Fourier transform microwave spectroscopy from 5.6 to 20.6 GHz. Analysis of the spectrum provides rotational constants and nuclear quadrupole coupling constants that are used to determine the structure of the complex. The HCl molecule forms a hydrogen bond with one of the two electrostatically equivalent fluorine atoms of the difluoroethylene, and this hydrogen bond bends from linearity to allow a secondary interaction between the chlorine atom and the hydrogen atom located cis to the fluorine atom in the hydrogen bond. Because the two hydrogen atoms are likewise electrostatically equivalent, the structure indicates that this is the sterically preferred arrangement in HCl binding to a fluoroethylene rather than the one with the secondary interaction to the geminal hydrogen atom. Detailed comparisons among the geometries of the complexes formed between HCl and HF, on the one hand, and vinyl fluoride, 1,1-difluoroethylene, trans-1,2-difluoroethylene, 1,1,2-trifluoroethylene, and ( E)-1-chloro-2-fluoroethylene, on the other, reveal structural trends accompanying increasing fluorination and substitution of chlorine for fluorine.