The structure and reactivity of hypervalent anions
Abstract
The intrinsic diastereoselectivity of the reduction of a series of cyclic ketones by hypervalent silicon hydride ions was investigated in the gas phase with the use of a flowing afterglow-triple quadrupole apparatus. The remarkable consistency of the results obtained in the gas phase and in solution with simple, alkyl-substituted cyclohexanones suggests that environmental effects are either unimportant or cancel out. The strong departure from condensed-phase behavior exhibited by other cyclohexanone derivatives containing oxygen and sulfur heteroatoms in the six-membered ring is rationalized in terms of a competition between steric, torsional, and electrostatic effects. The concept of transition state stabilization by hyperconjugative electron release need not be invoked. The bond dissociation energy in trifluoride ion (F3 −), a 22-electron hypervalent species, was determined from energy-resolved, collision-induced dissociation cross-section measurements. The gas phase F 2-F− bond dissociation energy was measured to be 1.02 ± 0.11 eV, and the energy for dissociation to F + F2 − is 0.28 ± 0.02 eV higher. An examination of relevant solvation energies revealed that F3− should not be stable with respect to dissociation in aqueous solution. Molecular orbital calculations predict a predominant formation of F2 − over F− at high energies, in agreement with experimental results. Finally, the structure and reactivity of the bis(pyridinium)phenide ion were examined. This species was prepared in a flowing afterglow reactor by reaction of N-(3,5didehydrophenyl)-4-tert-butylpyridinium ion with 4-tert-butylpyridine. Energy-resolved, collision-induced dissociation experiments revealed that this species is not an electrostatic cluster, but a true reactive intermediate that is weakly covalent. It also exhibited nucleophilic addition reactivity that is characteristic of negative ions, despite the fact that it possesses a net positive charge. Molecular orbital calculations suggest that this unusual behavior is due to highly localized electron density (a formal negative charge) at the phenide moiety.
Degree
Ph.D.
Advisors
Kenttamaa, Purdue University.
Subject Area
Organic chemistry|Chemistry
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