Biomedical subjects

Edward D Lorance

Publications and source records attributed to Edward D Lorance.

5 recordsLinked to original sources

PubMed2005

A quantitative curve-crossing model for radical fragmentation.

The kinetics of bond fragmentation for a series of N-methoxypyridyl radicals are analyzed in terms of a simple curve-crossing model that includes bond stretching and bond bending coordinates. The model accurately reproduces the reaction surfaces calculated using density functional theory (DFT) and also the experimental reaction energy barriers. The reactions proceed on the ground state surface by avoidance of a conical intersection, which is clearly illustrated by the model. A value for the electronic coupling matrix element responsible for splitting the upper and lower surfaces of 0.9 eV is obtained. The model illustrates the molecular features that allow barrierless fragmentation from a formally pi radical.

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PubMed2005

Density functional theory predicts the barriers for radical fragmentation in solution.

[reaction: see text] N-Methoxypyridyl radicals formed by one-electron reduction of the corresponding cationic heterocycles undergo N-O bond cleavage. Experimental activation free energies for a series of these bond fragmentations are compared to corresponding barriers determined from electronic structure calculations. The DFT barriers agree well with those from experiment, being smaller than the latter values by an average value of ca. 1 kcal/mol, for rate constants varying over almost 3 orders of magnitude, or within ca. 3 kcal/mol over 8 orders of magnitude of rate constant. For a model compound, the B3PW91/6-31+G hybrid density functional method is also found to be in good agreement with the MCSCF-MRMP2 method. One of the reactions is found by DFT to have no minimum for the reactant radical, consistent with a truly barrierless reaction.

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PubMed2004

Barrierless electron transfer bond fragmentation reactions.

The ultrafast N-O bond fragmentation in a series of N-methoxypyridyl radicals, formed by one-electron reduction of the corresponding N-methoxypyridiniums, has been investigated as potentially barrierless electron-transfer-initiated chemical reactions. A model for the reaction involving the electronic and geometric factors that control the shape of the potential energy surface for the reaction is described. On the basis of this model, molecular structural features appropriate for ultrafast reactivity are proposed. Femtosecond kinetic measurements on these reactions are consistent with a kinetic definition of an essentially barrierless reaction, i.e., that the lifetime of the radical is a few vibrational periods of the fragmenting bond, for the p-methoxy-N-methoxypyridyl radical.

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PubMed2003

Synthesis, electrochemistry, and gas-phase photoelectron spectroscopic and theoretical studies of 3,6-bis(perfluoroalkyl)-1,2-dithiins.

3,6-bis(trifluoromethyl)- and 3,6-bis(pentafluoroethyl)-1,2-dithiin (1a,b), the first known perfluoroalkyl-substituted 1,2-dithiins, were synthesized from (Z,Z)-1,4-bis(tert-butylthio)-1,3-butadiene (2) to evaluate the effects of electron-withdrawing groups on the ionization and oxidation potentials of 1,2-dithiins. Analysis of the photoelectron spectra of 1a and 1b provided a basis for assigning orbital compositions. Ab initio calculations on these compounds showed that they adopt a twist geometry as does 1,2-dithiin (1c) itself. Cyclic voltammetric studies on 1a and 1b revealed a reversible oxidation followed by an irreversible oxidation at much more positive potentials than for 1,2-dithiin and 3,6-dimethyl-1,2-dithiin (1d). The oxidation potentials determined electrochemically do not correlate with the ionization potentials determined by photoelectron spectroscopy. This result supports the previously advanced hypothesis that there is a geometry change on electrochemical oxidation leading to a planar radical cation.

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PubMed2002

Kinetics of reductive N-O bond fragmentation: the role of a conical intersection.

N-alkoxyheterocycles can act as powerful one-electron acceptors in photochemical electron-transfer reactions. One-electron reduction of these species results in formation of a radical that undergoes N-O bond fragmentation to form an alkoxy radical and a neutral heterocycle. The kinetics of this N-O bond fragmentation reaction have been determined for a series of radicals with varying substituents and extents of delocalization. Rate constants varying over 7 orders of magnitude are obtained. A reaction potential energy surface is described that involves avoidance of a conical intersection. A molecular basis for the variation of the reaction rate constant with radical structure is given in terms of the relationship between the energies of the important molecular orbitals and the reaction potential energy surface. Ab initio and density functional electronic structure calculations provide support for the proposed reaction energy surface.

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