Straining to Put the Pieces Together: The Molecular Structure of (E)-1-Chloro-1,2-difluoroethylene-Acetylene from Microwave Spectroscopy.

The microwave, rotational spectrum between 5.6 and 19.7 GHz of the gas-phase heterodimer formed between acetylene and (E)-1-chloro-1,2-difluoroethylene is obtained using both broadband, chirped-pulse and narrow band, Balle-Flygare Fourier transform microwave spectrometers. The structure of the complex is determined from the rotational constants obtained via the analysis of the spectra for the normal isotopologue of the complex and three isotopically substituted species: the singly substituted 37Cl isotopologue, obtained in natural abundance, and two isotopologues singly substituted with 13C, obtained using an isotopically enriched HC13CH sample. The acetylene forms a hydrogen bond with the fluorine atom on singly halogenated carbon and a secondary interaction with the hydrogen atom on that same carbon. The angle strain induced in forming the secondary interaction is offset by the favorable electrostatics of the hydrogen bond to fluorine. Comparisons with acetylene complexes of 1,1,2-trifluoroethylene and cis-1,2-difluoroethylene show the effects of halogen substitution at the remote carbon on this bonding motif.