title: Ab initio coupled cluster determination of the heats of formation of C2H2F2, C2F2, and C2F4.
abstract: Heats of formation at 298.15 K were computed for cis- and trans-1,2-difluoroethylene, 1,1-difluoroethylene, difluoroethyne, and tetrafluoroethylene using conventional coupled cluster theory through perturbative triples and basis sets up through augmented 6ζ or 7ζ quality. As an independent check, calculations were also performed with the explicitly correlated F12b method and basis sets up through 4ζ quality. F12b calculations converge to the basis set limit much more rapidly than the conventional method. Both approaches were subsequently extrapolated to the complete basis set limit using a variety of simple formulas. After the 1-particle basis set limits were established, corrections were applied for core/valence, scalar relativistic, non-Born-Oppenheimer, and higher order correlation effects through explicit quadruple excitations. A final correction for the remaining error relative to full configuration interaction theory, the exact result for a given basis set, was included. Although this general approach produced excellent agreement with recently reported semiexperimental structures for the difluoroethylene compounds, the level of agreement with the available tabulated experimental heats of formation for 1,1-difluoroethylene is considerably poorer. The best theoretical ΔHf(0)(298K) estimates (with experimental values from tabulations given in parentheses) are 1,1-difluoroethylene = -84.1 ± 0.6 kcal/mol (-80.5 ± 0.2); cis-1,2-difluoroethylene = -74.4 ± 0.6 kcal/mol (-73.3 ± 1.2); trans-1,2-difluoroethylene = -73.5 ± 0.6 kcal/mol (-72.6 ± 1.2). In the other two cases, where Active Thermochemical Table values were available, the agreement was much better: C2F2 = 1.0 ± 0.3 (0.7 ± 0.4) and C2F4 = -161.3 ± 0.8 kcal/mol (-161.8 ± 0.2). Considering the demonstrated accuracy of the approach in previous studies for the three difluoroethylene equilibrium structures and its performance in predicting the enthalpies of formation of over 100 small-to-medium size chemical systems, we believe the magnitude of disagreement between theory and experiment for 1,1-difluoroethylene merits a re-examination of the experimental value.
authors.affiliation: Department of Chemistry, Washington State University , Pullman, Washington 99164-4630, United States.
authors.forename: David
authors.initials: D
authors.lastname: Feller
authors.forename: Kirk A
authors.initials: KA
authors.lastname: Peterson
authors.forename: David A
authors.initials: DA
authors.lastname: Dixon