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 [D0(F2)] and, consequently, for the associated heats of formation of the
fluorine atom [ΔfH0°(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-ζ quality. Our best theoretical
results for D0(F2) and ΔfH0°(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 F2, 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 Å along the
dissociation curve with a height of 42 ± 10 μEh (∼0.11 ± 0.03 kJ/mol). Because of the neglect of this
effect during the evaluation of the raw experimental data used to
obtain D0(F2) = 154.52 ±
0.12 kJ/mol and ΔfH0°(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, D0(F2) = 154.92 ± 0.10 kJ/mol and ΔfH0°(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.