A computational study of the nondissociative mechanisms that interchange apical and equatorial atoms in square pyramidal molecules.

The lowest energy transition state for the nondissociative apical/equatorial atom exchange mechanism for three square pyramidal AEX5 molecular species was calculated (CCSD(T)/pVTZ; B3LYP/pVTZ, aug-cc-pV5Z) to have a hemidirected geometry with C(s) symmetry for BrF5, IF5, and XeF5+. In contrast, holodirected C2v-symmetric transition states for this process were located for the AEX5 square pyramidal molecules ClF5, ICl5, and IBr5. Imaginary frequencies were calculated and examined in a visual/dynamic fashion to gain insight into these fluxional processes. Although both mechanisms exchange one apical for one equatorial atom in each cycle of motion, processes that pass through C2v transition states have characteristic features of the well-known Berry pseudorotation and Lever mechanisms while those which pass through transition states of C(s) symmetry have features that are a mixture of Berry, Lever, and turnstile-like character. Two periodic trends are observed: as the atomic number on the central atom increases (same terminal atoms), the barrier for apical/equatorial exchange and the value of the imaginary frequency both decrease. Similarly, as the atomic number of the terminal atoms increase (same central atom), the barrier for apical/equatorial exchange decreases, as does the computed imaginary frequency.