Theoretical Investigations on the Mechanisms for Dielectric Failure of Heptafluoro-iso-butyronitrile in the Presence of Water Vapor

Heptafluoro-iso-butyronitrile
$(\mathrm{i}-\mathrm{C}_{3}\mathrm{F}_{7}\text{CN}$
, in abbr., C4), a potential eco-friendly alternative to
$\text{SF}_{6}$
, has attracted considerable interests for high voltage electrical applications. It is well recognized that the humidity is a failure mechanism of the gas insulated equipment, responsible for serious technical problems of the electrical circuit. Mechanisms for the hydrolysis of C4 were calculated theoretically using the high-level ab initio quantum chemistry methods. All the intermediates and transition states were optimized at the M06-2X1AVTZ level and the accurate energetics were obtained with the CBS-Q and Gaussian-4 composite methodologies. The reaction of C4 with water takes place via the complex addition/elimination mechanism. The rate-determining step involves the four-center barrier to form imidic acid and subsequently isomerize to amide. Further degradation of amide leads to
$\text{NH}_{3}$
and carboxylic acid, which decomposes to form various by-products including C02,
$\mathbf{C}_{3}\mathbf{F}_{7}\mathbf{H}, \mathbf{CF}_{3}\mathbf{H}, \mathbf{C}_{2}\mathbf{F}_{4}, \mathbf{C}_{3}\mathbf{F}_{6}$
, HF, etc., as indicative of the potential characteristic gas molecules for the failure diagnostic technique. Moreover, the hydrolysis of C4 could be effectively catalyzed by one or more water molecules, implying the increasing risk of dielectric failure of C4 with the increment of the high humidity levels. The yield of imidic acid is significant at temperatures below 500 K. The recombination of imidic acid and the addition reaction of imidic acid with C4 can produce formamido imine. The present work provided a comprehensive theoretical picture for the effect of moisture on C4.