Bartell mechanism

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The Bartell mechanism is a pseudorotational mechanism similar to the Berry mechanism. It occurs only in molecules with a pentagonal bipyramidal molecular geometry, such as IF7. This mechanism was first predicted by H. B. Bartell. The mechanism exchanges the axial atoms with one pair of the equatorial atoms with an energy requirement of about 2.7 kcal/mol. Similarly to the Berry mechanism in square planar molecules, the symmetry of the intermediary phase of the vibrational mode is "chimeric"[1][2][3] of other mechanisms; it displays characteristics of the Berry mechanism, a "lever" mechanism seen in pseudorotation of disphenoidal molecules, and a "turnstile" mechanism (which can be seen in trigonal bipyramidal molecules under certain conditions).
References

WJ Adams, HB Thompson, LS Bartell, 1970, J. Chem. Phys. 53:4040-6.
LS Bartell, MJ Rothman & A Gavezzotti, 1982, , J. Chem. Phys. 76:4136-4413.

M Cass, KK Hii & HS Rzepa, 2005, "Mechanisms that interchange axial and equatorial atoms in fluxional processes: Illustration of the Berry pseudorotation, the turnstile and the lever mechanisms via animation of transition state normal vibrational modes", J. Chem. Educ. (online), 2005; see [1] Archived 2019-10-19 at the Wayback Machine, accessed 28 May 2014