Theoretical Calculations of Carbon-Oxygen Bond Dissociation Enthalpies of Peroxyl Radicals Formed in the Autoxidation of Lipids
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- 作者：Derek A. Pratt ; Jeremy H. Mills ; and Ned A. Porter
- 刊名：Journal of the American Chemical Society
- 出版年：2003
- 出版时间：May 14, 2003
- 年：2003
- 卷：125
- 期：19
- 页码：5801 - 5810
- 全文大小：222K
- 年卷期：v.125,no.19(May 14, 2003)
- ISSN：1520-5126
文摘
Theoretical calculations were carried out to provide a framework for understanding the free radicaloxidation of unsaturated lipids. The carbon-hydrogen bond dissociation enthalpies (BDEs) of organic modelcompounds and oxidizable lipids (R-H) and the carbon-oxygen bond dissociation enthalpies of peroxylradical intermediates (R-OO
) have been calculated. The carbon-hydrogen BDEs correlate with the rateconstant for propagation of free radical autoxidation, and the carbon-oxygen BDEs of peroxyl radicalscorrelate with rate constants for 
-fragmentation of these intermediates. Oxygen addition to intermediatecarbon radicals apparently occurs preferentially at centers having the highest spin density. The calculatedspin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals.Where the C-H BDEs are a function of the extent of conjugation in the parent lipid and the stability of thecarbon radical derived therefrom, C-OO BDEs are also affected by hyperconjugation. This gives way todifferent rates of -fragmentation of peroxyl radicals formed from oxygen addition at different sites alongthe same delocalized radical. We have also studied by both theory and experiment the propensity for benzylicradicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalentunpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicalsin these cases have negative C-OO BDEs and, thus, have rate constants for -fragmentation that exceedthe diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.
) have been calculated. The carbon-hydrogen BDEs correlate with the rateconstant for propagation of free radical autoxidation, and the carbon-oxygen BDEs of peroxyl radicalscorrelate with rate constants for -fragmentation of these intermediates. Oxygen addition to intermediatecarbon radicals apparently occurs preferentially at centers having the highest spin density. The calculatedspin distribution therefore provides guidance about the partitioning of oxygen to delocalized carbon radicals.Where the C-H BDEs are a function of the extent of conjugation in the parent lipid and the stability of thecarbon radical derived therefrom, C-OO BDEs are also affected by hyperconjugation. This gives way todifferent rates of -fragmentation of peroxyl radicals formed from oxygen addition at different sites alongthe same delocalized radical. We have also studied by both theory and experiment the propensity for benzylicradicals to undergo oxygen addition at their ortho and para carbons which, combined, possess an equivalentunpaired electron spin density as the benzylic position itself. We find that the intermediate peroxyl radicalsin these cases have negative C-OO BDEs and, thus, have rate constants for -fragmentation that exceedthe diffusion-controlled limit for the reaction of a carbon-centered radical with oxygen.