Mechanistic Insight into the Decomposition of Eco-Friendly Dielectric Gas Heptafluoro-iso-butyronitrile in the Presence of Water Impurity

Decomposition of heptafluoro-iso-butyronitrile (C 3 F 7 CN), a novel eco-friendly dielectric gas for high voltage electrical applications, is investigated theoretically in the presence of water vapor using various computational methods including DFT, CBS-Q, G4, CCSD(T)-F12 and RRKM theory. Hydrolysis of C 3 F 7 CN proceeds dominantly via the stepwise addition/isomerization or elimination mechanisms to form amide or HF. The reaction pathways are bifurcated into cis and trans conformations. Activation barriers are strongly dependent on the number of water molecules in hydrolysis due to catalytic effect. The intermediate imidic acid prefers to produce formamido imine via recombination or secondary reaction with C 3 F 7 CN. Hydrolysis of CF 3 and CF 3 CFCN undergoes via the direct H-abstraction mechanism to form OH radicals enhancing hydrolysis of C 3 F 7 CN. Kinetic calculations predict that imidic acid is the major decomposition product below 500 K while amide is generated exclusively above 1000 K with a small fraction of HF. The present work not only provides a theoretical basis for the increasing risk of dielectric failure of C 3 F 7 CN with the increment of the high humidity levels, but also identifies the potential characteristic decomposition products to develop failure diagnostic techniques.