The Microwave Spectrum and Molecular Structure of (Z)‑1-Chloro-2-Fluoroethylene–Acetylene: Demonstrating the Importance of the Balance Between Steric and Electrostatic Interactions in Heterodimer Formation

The structure of the gas-phase heterodimer formed between (Z)-1-chloro-2-fluoroethylene and acetylene is determined via Fourier transform microwave spectroscopy from 5.5 to 20.8 GHz. In the first instance where in the presence of both a fluorine atom and a chlorine atom on the haloethylene the protic acid binds to the chlorine atom, the acetylene adopts a configuration similar to that in the analogous complex with vinyl chloride. Positioned in a manner to interact favorably with both the chlorine atom and the hydrogen atom geminal to it, the acetylene molecule is able to maximize the overall electrostatic stabilization even though other regions of the haloethylene offer individual sites of greater positive or negative electrostatic potential. Detailed comparison with the vinyl chloride–acetylene complex suggests that the presence of the fluorine atom weakens the hydrogen bond but strengthens the interaction between the geminal hydrogen atom and the triple bond.