Theoretical molecular orbital modeling of organic free radical reactions and the polarized pi-frontier molecular orbitals method for predicting diastereofacial selectivities.
Item
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Title
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Theoretical molecular orbital modeling of organic free radical reactions and the polarized pi-frontier molecular orbitals method for predicting diastereofacial selectivities.
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Identifier
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AAI9315471
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identifier
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9315471
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Creator
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Huang, Xiao Ling.
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Contributor
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Adviser: Joseph J. Dannenberg
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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, Physical | Physics, Molecular
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Abstract
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AM1/CI calculations on the reactions paths for the coupling of organic free radicals are presented. The calculations are in good agreement with previously reported experimental results for benzyl radical coupling. Selectivities for combinations of methyl radicals with allyl azaallyl, benzyl, and variously cyano-substituted benzyl radicals, the coupling reactions of two tri-substituted methyl radicals, as well as seven different coupling reactions of two benzyl radicals are reported. Analyses of the results suggest the existence of an intrinsic barrier about 25 kcal/mol for the coupling of carbon-centered free radicals.;The AM1 molecular orbital method was used to compute the activation enthalpies for H-transfer between carbons as a function of C-H-C angle and size of the cyclic transition state for intramolecular H-transfer. In the case of intramolecular H-transfer, reactions of a primary radical site with primary, secondary, and tertiary C-H's were considered for H-shifts to C{dollar}\sb1{dollar} from C{dollar}\sb3{dollar}-C{dollar}\sb8.{dollar} The activation enthalpies are insensitive to C-H-C angle in the range 145-180{dollar}\sp\circ{dollar}. Activation enthalpies are lowest for intramolecular H-transfers involving 1-5 and 1-6 H-shifts. The higher activation enthalpies for the other internal H-transfers are attributed to C-H-C strain for 1-3 and 1-4 transfers only, and conformational effects other than C-H-C angle in the transition states.;A new method (polarized {dollar}\pi{dollar}-frontier molecular orbitals, PPFMO) is presented for predicting the electronic component of {dollar}\pi{dollar}-diastereofacial selectivities. The effects of the various substitutents adjacent to carbon {dollar}\pi{dollar}-systems cannot be categorized completely in terms of electron-withdrawing and electron-donation effects. The substitutent effects on the polarization of FMO's are shown to be quite different from those on their energies. The PPFMO theory is adapted to include both effects. A comparison of PPFMO theory with earlier proposals made by Anh and Cieplak are discussed in terms of the predicted effects on FMO's.;The polarized {dollar}\pi{dollar}-frontier molecular orbital (PPFMO) is used to analyze several experimental observations that have been previously discussed in terms of other theoretical models for predicting the electronic components of {dollar}\pi{dollar}-diastereofacial selectivities. In particular, the reductions of propanal, fluoroethanal, and R-fluoropropanal as a function of rotational angle are compared with other high level calculations. The reductions of many ({dollar}>{dollar}40 cases) substituted cyclohexanones, methoxydioxanes, 1,3-dioxin-4-ones, norbornanones and adamantanones are compared with other calculations and experimental data. The PPFMO polarizations are in qualitative agreement with the experimental results in all cases. Comparison with ab initio transition states leads to only one discrepancy, where an unusual repulsive factor in the TS seems to be responsible.
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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.
