The varying nature of fluorine-oxygen bonds

The singles and doubles coupled-cluster method that includes a perturbational estimate of the effects of connected triple excitations, CCSD(T), together with a triple zeta double polarized (TZ2P) one-particle basis set is used to determine the geometries, harmonic frequencies, infrared intensities, and dipole moments of HOF, F2O, HOOF, FOOF, ClOOF. Agreement with experiment is very good, with the exception that the currently accepted experimental assignment of the symmetric and antisymmetric O-F stretches in FOOF is shown to be reversed (and to be consistent with an earlier experimental study). Very accurate heats of formation of HOOF, FOOF and ClOOF are also computed using the CCSD(T) method in conjunction with large atomic natural orbital basis sets. The F-O bond distances, quadratic force constants, bond energies, and fluorine and oxygen atomic charges from the above five molecules and six previously studied molecules (FONO2, trans-FONO, cis-FOND, FOCI, FOBr and FON) are compared and used to deduce a simple model of F-O bonding. The unusual relationship between the F-O bond distance and quadratic force constant shows that PO bonding is a function of at least three effects, which are degree of covalent character, degree of ionic character, and extent of lone electron-pair repulsions. All of the data are qualitatively consistent with this simple model. The bonding in cis-FONO is even more complicated, involving also dispersion interactions between fluorine and the terminal oxygen. It is suggested that the general importance of lone pair repulsions in F-O bonding and the additional importance of intra-molecular dispersion interactions explains why many density functionals have difficulty in describing the geometry of cis-FONO.