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 symmetry on the average by 7.5° ring puckering displacements ( symmetry) and 4.5° axial bend displacements ( symmetry). The distortion from , 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 with 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 displacement coordinates) connecting 10 equivalent structures via intermediates. The observed pseudoradial displacement suggests a value of about 5 cm−1 for the pseudoangular rotation constant . 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 radial distribution peak (i.e., an “anharmonic shrinkage”) and also presumably imparts significant infrared activity to the 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.
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15 November 1970
Research Article|
November 15 1970
Structure, Pseudorotation, and Vibrational Mode Coupling in IF7: An Electron Diffraction Study
W. J. Adams;
W. J. Adams
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48104
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H. Bradford Thompson;
H. Bradford Thompson
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48104
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L. S. Bartell
L. S. Bartell
Department of Chemistry, The University of Michigan, Ann Arbor, Michigan 48104
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J. Chem. Phys. 53, 4040–4046 (1970)
Article history
Received:
May 26 1970
Citation
W. J. Adams, H. Bradford Thompson, L. S. Bartell; Structure, Pseudorotation, and Vibrational Mode Coupling in IF7: An Electron Diffraction Study. J. Chem. Phys. 15 November 1970; 53 (10): 4040–4046. https://doi.org/10.1063/1.1673876
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