Experimental methods for measuring BDEs

Table 2.1 A Summary of Main Experimental Methods for Measuring BDEs...

Comparison of the BDEs and enthalpies of formation obtained from different experimental approaches can be used to assess the quality of the measurements. Berkowitz, et al. have compared the results of positive ion and negative ion methods for determining BDEs of stable molecules with those obtained from kinetics studies. They found that for most systems, the agreement in the results from the three approaches was excellent. In this section, the results that have been obtained for the enthalpies... 

Some empirical methods have provided useful correlations concerning the effects of substituents on thermochemical properties of phenols. Griller and coworkers developed a photoacoustic method for measuring bond dissociation energies (BDE) of phenols and showed for the first time a linear relationship between the Hammett a+ para-substituent constant and BDEs. Wayner and coworkers found a correlation between experimental ABDE values for a series of substituted phenols compared with phenol and the Hammett a+ constants (equation 58). 

This relationship was then used to calculate the BDE for a-Toc, giving a value of 77.2 kcalmor, in excellent agreement with the experimental value of 77.3 kcalmoP. This empirical method does depend on the electronic effects of groups (methyls and para-ether) around the phenyl ring and provides some confirmation of the role these play in weakening the O—H bond and thus raising the antioxidant activity. In a similar manner, Jovanovic and coworkers obtained a correlation between the measured reduction potentials and the a+ constants for twenty-one substituted phenols at pH 7 (equation 59) and pH 0 (equation 60) ". ... 

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