Pseudopotential SCF–MO studies of hypervalent compounds. IV. Structure, vibrational assignments, and intramolecular forces in IF7

Abstract

Ewig’s pseudopotential procedure, applied with a small basis set, yielded a minimum energy structure with D5h symmetry instead of the deformed pseudorotating structure proposed to account for electron diffraction and electric beam deflection experiments. Nevertheless, calculated restoring forces for e2 displacements were so feeble that a higher quality calculation making the postulated e2 deformation spontaneous is not at all precluded. Otherwise, calculated molecular characteristics of IF7 closely resembled those deduced from the diffraction and beam experiments, from extended Hückel calculations, and from the simple valence shell electron pair repulsion theory in a repelling points‐on‐a‐sphere (POS) variant. Axial bonds were shorter by 0.05 Å than equatorial bonds, and calculated mean bond lengths exceeded observed ones by only 0.03 Å. Furthermore, the calculated anharmonic coupling of e′1 polar displacements with large‐amplitude e2 puckering vibrations closely reproduced the coupling inferred from experiment. This favorable correspondance between theory and interpretation of experiment was found not to extend to vibrational assignments, however. Instead, the pseudopotential calculations were in suprisingly close agreement with the simple POS and EHT models predicting that equatorial in‐plane bends are far stiffer, due to the close lateral contacts, than the other bending modes. None of the six published vibrational assignments had been interpreted on this basis. An alternative assignment was found which is reasonably consistent with both the calculated force field and the observed infrared and Raman spectra. A similar assignment seems appropriate for ReF7. Calculations with and without d orbitals on iodine furnished evidence that the role of d orbitals in hypervalent compounds is less important than it is often considered to be. Neither the second‐order Jahn–Teller softening of the e2 force constants nor the covalent binding of seven fluorines to iodine was found to depend crucially upon valence d orbitals.