Does a Second Halogen Atom Affect the Nature of Intermolecular Interactions in Protic Acid-Haloethylene Complexes? In (Z)-1-Chloro-2-Fluoroethylene-Hydrogen Chloride It Most Certainly Does!

Abstract

As part of a systematic study of the effect of chlorine substitution on the structures of protic acid-haloethylene complexes, the structure of the (Z)-1-chloro-2-fluoroethylene-hydrogen chloride complex has been investigated using ab initio quantum chemistry calculations and microwave spectroscopy. Although theory predicts a non-planar equilibrium structure for this species, it is only 6 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 unlike that of any previously characterized protic acid-haloethylene complex with a bifurcated primary interaction in which the hydrogen of the acid interacts with both the fluorine and the chlorine atoms on the haloethylene and there is no evidence for a secondary interaction involving the electron rich region of the acid. This structure can be contrasted to those of vinyl fluoride-hydrogen chloride (fluorine bound, planar "top-binding," across the double bond), vinyl chloride-hydrogen chloride (chlorine bound, non-planar) and (Z)-1-chloro-2-fluoroethylene-acetylene (chlorine bound, planar "side-binding," at one end of the double bond).

Exploring the Forces Contributing to Noncovalent Bonding by Microwave Spectroscopy and Structural Characterization of Gas-Phase Heterodimers of Protic Acids with Haloethylenes

“…The lowest-energy isomer for each is a side binding configuration, with HCl forming a hydrogen bond to fluorine with ( E )-1-chloro-1,2-difluoroethylene and chlorine with 2-chloro-1,1-difluoroethylene. The higher-energy structure, once again for each complex, is a bifurcated structure, with the H atom of HCl interacting with the F, F pair and the F, Cl pair, respectively, similar to the motifs observed for cis -1,2-difluoroethylene–HCl and ( Z )-1-chloro-2-fluoroethylene–HCl …”

“…Once again, HCCH appears to have a steric preference for side binding to chlorine rather than to fluorine. An entirely different mode is adopted by HCl when binding to this haloethylene; it adopts the bifurcated motif, interacting with the fluorine and chlorine atoms …”

“…An entirely different mode is adopted by HCl when binding to this haloethylene; it adopts the bifurcated motif, interacting with the fluorine and chlorine atoms. 78 The Journal of Physical Chemistry A…”

“…When the haloethylene contains a chlorine atom, the spectrum of its HCl-containing complex is complicated, because each rotational transition is extensively split into many hyperfine components. We are in the process of finishing collecting and analyzing the spectra of two species: the HCl complexes of 1chloro-1-fluoroethylene 72 and (Z)-1-chloro-2-fluoroethylene 78 and their preliminary structures are reported in a previous section. There remain three HCl-containing complexes to be investigated.…”

“…The higher-energy structure, once again for each complex, is a bifurcated structure, with the H atom of HCl interacting with the F, F pair and the F, Cl pair, respectively, similar to the motifs observed for cis-1,2-difluoroethylene− HCl 58 and (Z)-1-chloro-2-fluoroethylene−HCl. 78 (Z)-1-Chloro-1,2-difluoroethylene has only recently become commercially available. Now that it is, we will able to study its heterodimers with HF, HCl, and HCCH.…”