Quantum chemical descriptions of FOOF: The unsolved problem of predicting its equilibrium geometry

Single determinant Moller-Plesset perturbation (MP) theory at second order (MP2), third order (MP3), and fourth order (MP4) with standard basis sets r anging from cc-pVDZ to cc-pVQZ quality predicts the equilibrium geometry of FOOF qualitatively incorrect. Sixth-order MP (MP6), CCSD(T), and DFT lead to a qualitatively correct FOOF equilibrium geometry r(e), provided a suffi ciently large basis set is used; however, even these methods do not succeed in reproducing an exact r(e) geometry. The latter can be achieved only by artificially increasing anomeric delocalization of electron lone pairs at t he O atoms into the o*(OF) orbitals by selectively adding diffuse basis fun ctions, adjusting exponents of polarization functions, or enforcing an incr ease of electron pair correlation effects via the choice of a rigid basis s et. DFT geometries of FOOF can be improved in a similar way and, then, DFT presents the best cost-efficiency compromise currently available for descri bing FOOF and related molecules. DFT and CCSD(T) calculations reveal that F OOF can undergo either rotation at the OO bond or dissociation into FOO and F because the corresponding barriers (trans barrier: 19.4 kcal/mol; dissoc iation barrier 19.5 kcal/mol) are comparable. Previous estimates as to the height of the rotational barriers of FOOF are largely exaggerated. Rotation at the OO bond raises the barrier to dissociation because the anomeric eff ect is switched off. The molecular dipole moment is found to be a sensitive antenna for probing the quality of the quantum chemical description of FOO F.