Knowledge of the factors governing a molecule’s ability to form intermolecular interactions in simple cases forms a basis for understanding those in more complicated systems. For complexes formed between hydrogen chloride and haloethylenes, a delicate balance between electrostatics and sterics dictates the geometry of the complex. Hydrogen chloride interacts with the sterically accessible cis H-atom in vinyl fluoride as opposed to the electrostatically favorable geminal hydrogen. However, with 1,1,2-trifluoroethylene the opposite occurs, and electrostatics is favored over sterics. In cis-1,2-difluoroethylene, only electrostatically-equivalent geminal H-atoms are present. Rather than adopting the sterically unfavorable arrangement found in 1,1,2-trifluoroethylene-HCl, the hydrogen chloride in this complex instead forms a bifurcated hydrogen bond with the two F-atoms, and there is no secondary interaction involving the chlorine atom. We investigate this unexpected result through ab initio calculations at the MP2/6–311++G(2d,2p) level using GAUSSIAN 09. With the geometric parameters for the average, ground vibrational state structures of the cis-1,2-difluoroethylene and HCl molecules fixed at their literature values, the intermolecular potential energy surface is explored to determine the possible structures for the complex, to interpret the factors leading to the relative stabilities among them, and to understand the unusual experimental result.

Insights into the molecular structure of hydrogen chloride-cis-1,2-difluoroethylene from ab initio calculation of an intermolecular potential energy surface

254th National Meeting (2017)

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