Double-Layered Composite Methods Extrapolating to Complete Basis-Set Limit for the Systems Involving More than Ten Heavy Atoms: Application to the Reaction of Heptafluoroisobutyronitrile with Hydroxyl Radical.

Two versions of the double-layered composite methods, including the restricted open-shell model chemistry based on the complete basis set quadratic mode (DL-ROCBS-Q) and the extrapolated CBS limit of electronic energy on the basis of the coupled cluster with single, double, and noniterative triple excitations with the hierarchical sequence of the correlation-consistent basis sets (DL-RCCSD(T)/CBS), were developed to calculate the energetic reaction routes for the systems involving 13/14 heavy atoms with good balance between efficiency and accuracy. Both models have been employed to investigate the oxidation reactions of heptafluoroisobutyronitrile ((CF3)2CFCN) with hydroxyl radical. The (CF3)2CFCN + OH reaction is dominated by the C-O addition/elimination routes as bifurcated into trans- and cis-conformations. Although the formation of isocyanic acid or hydrogen fluoride is highly exothermic, the major nascent product was predicted to be the less exoergic cyanic acid. Preference of the product channels could be tuned by the single water molecule in the presence of the H2O-HO complex. The production of amide compound was found to be the most significant route accompanied by the OH regeneration. Moreover, the OH radical could be an efficient catalyst for the hydrolysis of (CF3)2CFCN. Implication of the current theoretical results in the chemistry of (CF3)2CFCN for both atmospheric sink and potential dielectric replacement gas was discussed.