The Importance of How the Pieces Fit Together: The Microwave Spectrum and Molecular Structure of 2-Chloro-1,1-Difluoroethylene-Acetylene.

Fourier transform microwave spectroscopy is used to obtain the rotational spectrum of the gas-phase heterodimer formed between 2-chloro-1,1-difluoroethylene and acetylene between 5.1 and 20.0 GHz. Rotational constants derived from the analysis of the spectra for the normal isotopologue, the singly substituted 37Cl isotopologue, observed in natural abundance, and two isotopologues singly substituted with 13C, obtained using an isotopically enriched HC13CH sample, are used to determine the structure of the complex. Although the formation of a hydrogen bond to fluorine, considered in isolation, would be electrostatically favored, the angle strain induced in forming the secondary interaction between the acetylene triple bond and the hydrogen atom on 2-chloro-1,1-difluoroethylene coupled with the relaxed steric requirements of hydrogen bonding to chlorine lead to the heterodimer adopting what we have previously termed the side-binding configuration as the lowest energy structure. In this arrangement, the acetylene forms a hydrogen bond with the chlorine atom and a secondary interaction with the hydrogen atom, which is geminal to the chlorine. Comparisons with acetylene complexes of (Z)-1-chloro-2-fluoroethylene and vinyl chloride show the effects of increasing fluorine substitution on this bonding motif.