Dissociation of the Fluorine Molecule

The primary purpose of the present study is to resolve the discrepancy that exists between the two most recently published dissociation energies for the fluorine molecule [D-0(F-2)] and, consequently, for the associated heats of formation of the fluorine atom [Delta H-f(0)degrees(F)]. We hope to provide a reliable, well-established theoretical estimate for these thermochemical quantities. To this end, a high-accuracy coupled-cluster-based composite ab initio model chemistry has been utilized. The protocol involves contributions of up to pentuple excitations in coupled-cluster theory augmented with basis set extrapolation techniques and additional corrections beyond the nonrelativistic and Born-Oppenheimer approximations. The augmented core-valence correlation consistent basis set families, aug-cc-pCVXZ, have been successively used, in some cases, up to octuple-zeta quality. Our best theoretical results for D-0(F-2) and Delta H-f(0)degrees(F) obtained in this study are 154.95 +/- 0.48 and 77.48 +/- 0.24 kJ/mol, respectively. Because conflicting theoretical results are also reported about the existence of a barrier along the dissociation curve of F-2, extensive multireference configuration interaction and coupled-cluster calculations have been performed using reference orbitals taken from all-electron complete active space self-consistent field computations. Extrapolations from the results obtained with the aug-cc-pCVXZ (X = T, Q, 5) basis sets clearly indicate that the barrier indeed exists. It is located at 3.80 +/- 0.20 angstrom along the dissociation curve with a height of 42 +/- 10 mu E-h (similar to 0.11 +/- 0.03 kJ/mol). Because of the neglect of this effect during the evaluation of the raw experimental data used to obtain D-0(F-2) = 154.52 +/- 0.12 kJ/mol and Delta H-f(0)degrees(F) = 77.26 +/- 0.06 kJ/mol [Stevens; et al. J. Phys. Chem. A 2010, 114, 13134], an additional error should be attached to these latter values. Obviously, the barrier does not affect either the experimental results, D-0(F-2) = 154.92 +/- 0.10 kJ/mol and Delta H-f(0)degrees(F) = 77.46 +/- 0.05 kJ/mol [Yang; et al. J. Chem. Phys. 2005, 122, 134308; 2007, 127, 209901], which are based on the ion-pair dissociation of the molecule, or the data calculated theoretically. It is also noteworthy that our best estimates are in excellent agreement with those obtained from the ion-pair dissociation experiment.