Project PTDC/QUI/65535/2006
The stability of selected carbon-centered radicals and the antioxidant role of some terpenes
PI: J. A. Martinho Simões
(2009-2012)
This project aims at investigating the thermodynamic stability of several selected carbon-centered radicals by time-resolved photoacoustic calorimetry (TR-PAC) and high-level quantum chemistry calculations. The comparison between the two sets of results will allow drawing conclusions on the solvation energetics of those radicals and gaining a better understanding of the reactivity of the free radicals in solution.
One of the radicals to be investigated will be pentamethylcyclopentadienyl, a widely used ligand in organometallic chemistry, whose thermodynamic comparison with the analogous cyclopentadienyl is relevant. Another example will be the determination of the three carbon-hydrogen bond dissociation enthalpies in norbornane. Only one of these measurements is amenable to TR-PAC experiments and the other two will be determined by quantum chemistry calculations. This system is particularly interesting because, due to structural constraints, the radicals cannot be stabilized by hyperconjugation and therefore the C-H bond dissociation enthalpies are expected to be rather high.
A very important class of organic reactions is the addition of oxygen radicals to double bonds of olefins. We will use TR-PAC to probe the energetics of this reaction in the case of norbornadiene and norbornene. These results will also be related to a fundamental concept in chemistry: the strength of a p-bond.
Finally, we will turn our attention to the thermochemistry of a series of terpenes with potential (or demonstrated) antioxidant properties. We will try to understand these properties by assessing the relevant carbon-hydrogen bond dissociation enthalpies. Before these studies, however, we will look into two compounds (1,3- and 1,4-cyclohexadiene), which are useful to understand the thermochemistry of some terpenes. The thermochemical knowledge on the addition of tert-butoxyl radicals to double bonds may also be useful in understanding the reactivity of terpenes.