Does a second halogen atom affect the nature of intermolecular interactions in protic acid-haloethylene complexes? In (E)-1-chloro-2-fluoroethylene-hydrogen chloride it depends on how you look at it

As part of a systematic study of the effect of chlorine substitution on the structures of protic acid haloethylene complexes, the structure of the ({\it E})-1-chloro-2-fluoroethylene-hydrogen chloride complex has been investigated using {\it ab initio} quantum chemistry calculations and microwave spectroscopy. Although theory predicts a non-planar equilibrium structure for this species, it is only 7 cm$^{-1}$ lower in energy than the planar geometry connecting the two equivalent minima on either side of the haloethylene plane, and the observed spectrum is consistent with a planar, average structure, likely the result of zero-point averaging. The geometry is very similar to the fluorine binding, vinyl fluoride-hydrogen chloride complex, suggesting that the substitution of chlorine for a hydrogen {\it trans} to the fluorine atom has very little effect on intermolecular interactions in this case. On the other hand, vinyl chloride-hydrogen chloride adopts a non-planar, chlorine binding configuration so that alternatively one could say that the presence of fluorine has a large effect on protic acid-chlorine interactions