An electric field could make an unlikely nucleophilic substitution reaction feasible by fixing the reactants in place and lowering the reaction energy barrier, according to a theoretical study (J. Am. Chem. Soc. 2019, DOI: 10.1021/jacs.9b02174).
The reaction in question involves a complex held together by a halogen bond. These bonds are akin to hydrogen bonds and exist when a Lewis base coordinates with an electrophilic halogen atom. But an SN2-like reaction in which the base pulls off the halogen to form two ions is energetically unlikely in either the gas phase or in solvent, says study author Sason Shaik of The Hebrew University of Jerusalem. Shaik previously calculated that electric fields could drive Diels-Alder reactions (Chem. Phys. Chem. 2010, DOI: 10.1002/cphc.200900848), work that chemists later confirmed in the lab (Nature 2016, DOI: 10.1038/nature16989).
Now Shaik, along with graduate student Chao Wang, has shown that an electric field oriented along the direction of the halogen bond will weaken the halide’s bond, making it reactive. In a reaction involving ammonia and dichloride, a field of about 0.5 V/Å/angstrom lengthened the chloride bond and reduced the reaction energy barrier by more than 30 kcal/mol, making the reaction possible. At higher field strengths, the dichloride dissociates spontaneously. “The electric field is really helping the movement of electrons,” Shaik says.
Not only does the field stretch out the halide bond, Shaik explains, it also holds the molecules in the correct orientation to facilitate the reaction. The researchers liken the effect to holding the molecules in place with tweezers. In the applied electric field, the molecules need more than 25 kcal/mol to rotate, higher than the energy barrier of the reaction.
It remains to be seen if the reaction can happen in the laboratory. Jean-Sabin McEwen of Washington State University, who studies catalysis in electric fields, says the study’s calculated fields could be replicated in a scanning tunneling microscope, although the reactants may not stay in the gas phase in that environment. But Chérif F. Matta, a theoretical chemist at Mt. St. Vincent University, says whether or not these reactions are feasible, the work “may guide experiments in the future.”