The problematic C2H4+F2 reaction barrier
The C(2)H(4)+F(2) 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 C(2)H(4)+F(2) 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, Delta(f)H(298) degrees(C(2)H(4)F)=-13.2+/-0.2 kcal mol(-1), whose uncertainty is an order of magnitude less than previous experimental values.