Project PTDC/QUI-QUI/110542/2009
Thermochemistry of oxygen addition to carbon-centered radicals
PI: R. M. Borges dos Santos (FCUL)
(2011-2013)
This project aims at investigating the reactivity of carbon-centered radicals towards oxygen, specifically the reaction R•+ O2 = ROO• (where R• denotes the carbon radical and ROO• the product peroxyl radical). This reaction is a key step in the interaction mechanism of oxygen with organic matter, hence its enormous importance. For instance, it is essential in the production of thermal energy, but it is also responsible for the unwanted deterioration of cellular membranes. To fully describe its thermodynamics, we need information regarding the stability of both the carbon and peroxyl radicals. Investigation of the energetics of carbon radicals has been the subject of another ongoing project. We now wish to probe the thermochemistry of the peroxyl radicals by using photoacoustic calorimetry and quantum chemistry methods, to obtain the complete thermodynamic characterization of the title reaction.
The thermochemistry of carbon radicals has attracted much attention recently thanks to the new capabilities of theory and experiment, yielding an increasingly accurate database on radical thermochemistry, allowing a (re) evaluation of the factors responsible for the stability of hydrocarbons, and leading to what has been called a renaissance in hydrocarbon thermodynamics studies. We have been involved in these studies, by using experimental (time-resolved photoacoustic calorimetry, TR-PAC) and theoretical chemistry methods (from density functional theory to complete basis-set extrapolated coupled-cluster) to determine thermochemical stabilities of selected hydrocarbons with chemical accuracy (uncertainties of ca. 5 kJ/mol). Our findings allowed a detailed discussion of the observed trends in terms of the factors referred above. From a more applied point of view, these studies are important because they allow understanding the properties of antioxidants based on carbon radicals. Phenolic antioxidants (such as g-tocopherol), which form oxygen radicals, have been widely studied and used. The discussion of the use of antioxidants based on carbon radicals (such as •-terpinene) is, however, relatively recent. Unlike a-tocopherol, they have the considerable advantage of not becoming 'pro-oxidants' at higher concentrations. As such, there has been a considerable effort in the design of novel antioxidants based on carbon radicals, for which the thermochemical knowledge is fundamental.
Photoacoustic calorimetry allows the direct determination of the enthalpy and rate constants of radical reactions in solution, and is therefore particularly well suited for these studies. TR-PAC is a development of the original technique that presents several advantages. We have contributed to its establishment as a tool for obtaining high quality thermochemical data for radical species. Complementing our investigations with quantum chemical studies also allowed us to assess various theoretical models and devise the methodologies that best balance accuracy with computational cost for the molecules studied.
Based on our previous experience studying carbon radicals, we are now prepared to systematically investigate the product of their addition with oxygen, the peroxyl radicals. As mentioned above, these radicals take part in a wide-range of important mechanisms. Besides the free radical oxidation/antioxidation mechanism, they play a key role in the oxidation of fuels at cool-flame temperatures. In this regard, the stability of peroxyl radicals is central to an understanding of combustion in novel internal combustion engines, which are under active development to meet the increasingly stringent requirements of efficiency and emission performances.
The present knowledge on the stability of organic free radicals is far from satisfactory. But while for carbon radicals the situation has improved significantly in recent years, for peroxyl radicals the data are still both scarce and uncertain, and controversies abound. A birds-eye view of the pertinent literature can reveal either a perfect, or even a non-existent correlation between the stabilities of the carbon radicals and of the corresponding peroxyl radicals; therefore, those stabilities are either related, or largely unrelated; alkyl substitution shows either a little effect on the stability of peroxyl radicals or a substantial effect, revealing stabilization by hyperconjugation; stabilization of the peroxyl radical by benzylic and allylic groups is either very similar, or the effect of the latter is more powerful due to better spin density distribution. With this project we will try to provide answers to these and other questions. Due to the importance of the title reaction, it is also hoped that our data will be further helpful in understanding a broad range of reactions in chemistry, biology, and materials science.