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Exploring the sterically disfavored binding of acetylene to a geminal olefinic hydrogen-fluorine atom pair: The microwave spectrum and molecular structure of cis-1,2-difluoroethylene–acetylene

Leung, Helen O. (Department of Chemistry, Amherst College)

The microwave rotational spectrum of the gas-phase bimolecular heterodimer formed between cis-1,2-difluoroethylene and acetylene is obtained using Fourier transform microwave spectroscopy from 5.9 to 21.2 GHz. Rotational constants derived from the analysis of the spectra for the normal isotopologue and singly substituted ¹³C isotopologues, obtained in natural abundance, allow the determination of the structure of the complex, which, in the absence of a fluorine-hydrogen atom pair located cis to each other, adopts a sterically disfavored geometry (“side-binding”) in which the acetylene interacts with a geminal fluorine-hydrogen atom pair. Structural details are found to be similar to those of previously studied heterodimers with side-binding of acetylene to fluorine while reflecting the degree of halosubstitution. A detailed comparison with the (Z)-1-chloro-2-fluoroethylene-acetylene complex reveals information regarding the relaxed steric requirements for hydrogen bonding to chlorine as opposed to hydrogen bonding to fluorine.

Exploring the sterically disfavored binding of acetylene to a geminal olefinic hydrogen-fluorine atom pair: The microwave spectrum and molecular structure of cis-1,2-difluoroethylene–acetylene
The microwave spectrum and molecular structure of vinyl chloride-acetylene, a side-binding complex

Marshall, Mark D. (Department of Chemistry, Amherst College)

The structure of the gas-phase bimolecular complex formed between vinyl chloride and acetylene is determined using a combination of broadband, chirped-pulse and narrow band, Balle-Flygare Fourier transform microwave spectroscopy from 5.8 to 20.7 GHz. Although all previous examples of complexes formed between protic acids and haloethylenes are observed to have similar modes of binding regardless of the specific identity of the acid, HF, HCl, or HCCH, the vinyl chloride-HCCH complex has HCCH located at one end of the vinyl chloride with the secondary interaction occurring with the geminal hydrogen atom as opposed to the “top” binding configuration found for vinyl chloride-HF. Nevertheless, the details of the structure, such as hydrogen bond length (3.01 Å) and amount of deviation from linearity (58.5°), do reflect the strength of the interaction and show clear correlations with the gas-phase acidity. Comparison with analogous complexes allows the determination of the relative importance of electrostatic interactions and steric requirements in leading to the observed structures.

The microwave spectrum and molecular structure of vinyl chloride-acetylene, a side-binding complex