Molecular Energetics Group

PAC is probably the most reliable technique for the determination of homolytic bond dissociation enthalpies (BDEs) in solution. A BDE can be defined as the minimum enthalpy required to break a given bond. BDEs are of fundamental importance because they allow to decide whether or not a particular reaction will be enthalpically favoured. In many cases this is all we need to predict if a reaction will be efficient or not. For example, the determining step of the antioxidation mechanism by which phenolic compounds effectively break the autoxidation (or lipid-peroxidation) chain reaction, is:

ROO^• + ArOH → ROOH + ArO^•

ROO^• represents a peroxyl radical, resulting from the autoxidation of, for example, a cellular membrane. ArOH represents a phenolic antioxidant, such as, for example, α-tocopherol, the main and most active compound in vitamin-E, which in turn is the most important antioxidant in humans.

Being a simple hydrogen abstraction, the entropy of the above reaction is expected to be negligible. Therefore, the more exothermic, the more favorable it will be.

The homolytic O—H BDEs in the compounds ArOH and ROOH are identified with the enthalpies of the following reactions:

ArOH → ArO^• + H^•

ROOH → ROO^• + H^•

If these BDEs are both available, then, from their difference it is possible to derive the enthalpy of the antioxidation reaction above,

Δ_rH = DH(ArO—H) - DH(ROO—H)

and assess the effectiveness of a phenol antioxidant (ArOH). A weak ArO—H bond leads to a more exothermic reaction. Therefore, phenolic compounds with low O—H BDEs should have better antioxidant properties. Photoacoustic calorimetry has been extensively used to determine bond dissociation enthalpies in several compounds, including O—H bonds in phenols, S—H bonds in thiophenols, and C—H bonds in alkylbenzenes. It has contributed significantly to the development of a database of reliable BDE values, which can then be used to test theories and help us understand why a given bond is "weak" or "strong". This knowledge is very important. For the examples just mentioned, the BDEs are affected by the nature and position of substituent groups in the benzene ring. In the case of phenols, for instance, knowing which substituent groups lead to a decrease in the O—H BDE allows to design efficient antioxidants for the pharmaceutical, food, and cosmetic industries.