Structure, Pseudorotation, and Vibrational Mode Coupling in IF7: An Electron Diffraction Study

Free vapor-phase molecules of iodine heptafluoride are pentagonal bipyramids with axial bonds (1.786 ± 0.007 Å esd) shorter than equatorial bonds (1.858 ± 0.004 Å). They are deformed from D5h symmetry on the average by 7.5° ring puckering displacements (e2″ symmetry) and 4.5° axial bend displacements (e1″ symmetry). The distortion from D5h, interpreted in terms of the points-on-a-sphere variant of the valence-shell electron-pair theory, is compatible with an effective force law between electron pairs of Vij ∼ rij−n with n in the broad vicinity of 3.5. Expressing forces harder than simple Coulomb repulsions and much softer than conventional atom–atom nonbonded repulsions, the potential-energy law is in a range consistent with Gillespie's bond–bond repulsion theory. The simplest interpretation of the diffraction intensities is that the molecules undergo essentially free pseudorotation along a pathway (predominantly e2″ displacement coordinates) connecting 10 equivalent C2 structures via Cs intermediates. The observed pseudoradial displacement suggests a value of about 5 cm−1 for the pseudoangular rotation constant h / 8π2cIeff. The appreciable axial bend induced by the ring pucker is correlated in phase with the pucker displacement. This correlation is responsible for introducing a pronounced skewing of the Fax···Feq radial distribution peak (i.e., an “anharmonic shrinkage”) and also presumably imparts significant infrared activity to the e2″ modes in overtones and combination bands. Furthermore, the axial bend gives the molecule a dipole moment which may explain recent molecular-beam experiments by Klemperer et al.