High-Accuracy Theoretical Thermochemistry of Fluoroethanes

A highly accurate coupled-cluster-based ab initio model chemistry has been applied to calculate the thermodynamic functions including enthalpies of formation and standard entropies for fluorinated ethane derivatives, C2H x F6–x (x = 0–5), as well as ethane, C2H6. The invoked composite protocol includes contributions up to quadruple excitations in coupled-cluster (CC) theory as well as corrections beyond the nonrelativistic and Born–Oppenheimer approximations. For species CH2F–CH2F, CH2F–CHF2, and CHF2–CHF2, where anti/gauche isomerism occurs due to the hindered rotation around the C–C bond, conformationally averaged enthalpies and entropies at 298.15 K are also calculated. The results obtained here are in reasonable agreement with previous experimental and theoretical findings, and for all fluorinated ethanes except CH2FCH3 and C2F6 this study delivers the best available theoretical enthalpy and entropy estimates.