The problematic C2H4+F2 reaction barrier

The C2H4+F2 reaction is investigated through the most rigorous electronic structure methods currently feasible, using a focal point approach to converge toward the ab initio limit. Explicit computations were executed with basis sets as large as aug-cc-pV5Z and correlation treatments as extensive as coupled cluster through full triples with a perturbative inclusion of quadruple excitations [CCSDT(Q)]. Auxiliary core correlation, diagonal Born–Oppenheimer, and first-order relativistic corrections were included. All optimized geometries and vibrational frequencies were determined completely at the CCSD(T)/aug-cc-pVQZ level. The final C2H4+F2 reaction barrier from theory (8.0 kcal mol−1) is significantly higher than the recently reported experimental barrier (5.5±0.5 kcal mol−1). Our computations also yield a new enthalpy of formation of the fluoroethyl radical, ΔfH298°(C2H4F)=−13.2±0.2 kcal mol−1, whose uncertainty is an order of magnitude less than previous experimental values.