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Title
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In the image and likeness of benzene: Variable-temperature X-ray studies of substituted barbaralanes and semibullvalenes. (Volumes I and II).
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Identifier
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AAI9413697
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identifier
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9413697
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Creator
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Todaro, Louis J.
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Contributor
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Adviser: Klaus G. Grohmann
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Date
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1993
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Language
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English
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Publisher
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City University of New York.
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Subject
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Chemistry, Organic | Chemistry, Physical | Physics, Radiation
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Abstract
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The Cope rearrangement involves the breaking of a {dollar}\sigma{dollar} bond, formation of a new {dollar}\sigma{dollar} bond, and migration of {dollar}\pi{dollar} bonds. A fast Cope rearrangement is a consequence of a low activation energy. Molecules, which rapidly change back and forth from one structure to an entirely equivalent structure via the Cope rearrangement, are called fluxional or valence tautomers. Among these molecules, semibullvalene, one of the fastest valence tautomer, and its derivatives have received particular attention:(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI)The existence of the Cope rearrangement in the solid state can be detected with various spectroscopic and diffraction methods. Solid-state {dollar}\sp{13}{dollar}C-NMR, {dollar}\sp1{dollar}H-NMR, and single-crystal X-ray diffraction have proved to be powerful tools in probing the nature of fluxional molecules.;We have undertaken a systematic, variable-temperature single-crystal X-ray study of substituted barbaralanes and semibullvalenes, which have a low activation energy for the Cope rearrangement. The compounds for this study are: 2,4,6,8-tetracarbomethoxybarbaralane (barbaralane-(COOMe){dollar}\sb4{dollar}), octacarbomethoxysemibullvalene (Sb-(COOMe){dollar}\sb8{dollar}), 1,5-cyclohexano-2,4,6,8-tetracarbomethoxysemibullvalene (Sb-6), 1,5-cycloheptano-2,4,6,8-tetracarbomethoxysemibullvalene (Sb-7), 1,5-dimethyl-2,4,6,8-tetracarbomethoxysemibullvalene (Sb-Me{dollar}\sb2{dollar}), and 1,5-cyclooctano-2,4,6,8-tetracarbomethoxysemibullvalene (Sb-8).;The data clearly indicate that these six molecules approach the bishomoaromatic transition state with increasing temperature (from 20 to 360 K), that is, they become more and more like benzene. In solution, these compounds undergo Cope rearrangement. In the solid state, however, no significant structural alterations take place in barbaralane-(COOMe){dollar}\sb4{dollar} with temperature changes; gradual alterations creep in Sb-(COOMe){dollar}\sb8{dollar}; and spectacular transformations occur in Sb-8. These six compounds may be classified in an order of increasing dynamic behavior in the solid state: barbaralane-(COOMe){dollar}\sb4{dollar} being nearly static, while Sb-8 is the most dynamic. The rest fell between these two extremes. Stepping over one candidate to the next, we see that the trend toward the bishomoaromatic state intensifies. The climax is reached when Sb-8 achieves at room temperature the delocalized bishomoaromatic structure, where the single bonds in the semibullvalene ring, C2-C3 and C7-C8, and double bonds, C3-C4 and C6-C7 are all equal, along with the interatomic distances, C2-C8 and C4-C6.(DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI).;For the first time, we have characterized a neutral bishomobenzene molecule of the type which Hoffmann and Dewar had predicted more than twenty years ago. Even though Sb-8 at room temperature has the delocalized structure expected for a neutral bishomoaromatic compound, it is, nevertheless, static at temperatures below 90 K. We conclude that Sb-8 has not crossed the barrier between valence tautomerism ({dollar}\Delta G\sp\ddagger >0{dollar}) to resonance ({dollar}\Delta G\sp\ddagger <0{dollar}) as in benzene. The significance of low-temperature work is clear. It is indispensable in preventing misconceptions, which may be construed from examining just room-temperature data, when characterizing compounds claimed to exhibit benzene-like behavior.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
