Theoretical Investigation of the Pyridinium-Inspired Catalytic Dehydration of Heptafluoro-Iso-Butyramide for the Synthesis of Environmentally Friendly Insulating Gas Heptafluoro-Iso-Butyronitrile.

Abstract

Heptafluoro-iso-butyronitrile (i-C3F7CN) represents a feasible eco-friendly replacement gas for the most potent greenhouse gas sulfur hexafluoride in various high-voltage power transmission equipment. The reaction mechanisms for the in situ synthesis of i-C3F7CN from heptafluoro-iso-butyramide [i-C3F7C(O)NH2] in the presence of trifluoroacetic anhydride (TFAA) and pyridine (Py) in dimethylformamide solution have been studied within density functional theory with M06-2X exchange-correlation functional with the 6-311++G(d,p) basis set and the high-level ab initio complete basis set quadratic CBS-QB3 method. It is revealed that the unimolecular dehydration of i-C3F7C(O)NH2 can be catalyzed efficiently by TFAA in terms of both kinetic and thermodynamic aspects, producing i-C3F7CN and trifluoroacetic acid (TFA). Furthermore, Py is capable of reducing the energy barrier of the rate-determining step through hydrogen abstraction to form pyridinium hydrogen. The synergic effect of the TFAA/Py co-catalyst plays a pivotal role in the production of i-C3F7CN as the Gibbs free energy barrier can be lowered by more than 40 kcal/mol with the ratio of TFAA:2Py, in accordance with the experimental observation. The present theoretical work provides new insights into the rational design on the novel catalysts for large-scale synthesis of the perfluorinated nitriles.