O-H bond dissociation energies in phenols

In order to estimate the radical stabilization elfect on the BDE, we decided to study how the spin delocalization of the phenoxyl radical varies with the substituent. For this purpose, we computed the surface maxima in the spin density associated with the oxygens of the substituted phenoxyl radicals. The molecular surface was defined by the 0.002 a.u. contour of the electron density. By calculating the spin density on the molecular surface rather than at the nuclei, the spin density will emphasize the spin delocalization of the valence electrons, which is expected to be the most important for reactivity. For example, the spin density at the nuclei does not reflect the spin delocalization of the 7t-electrons, since these generally have zero densities at the nuclear positions. 

On the basis of the linear correlations between Vmin and ABDE for electron-accepting substituents and p max and ABDE for electron-donating substituents, we decided to investigate if a dual parameter relationship of the following type could correlate the ABDE of all phenols  

In Table 6 are listed the calculated APSE and ARSE, together with computed and predicted ABDEs for the substituted phenols. According to the derived stabilization energies, the ABDEs of the phenols with electron-withdrawing substituents are mainly determined by the polar stabilization of the parent molecules. The polar effect is less important for the phenols with electron-donating substituents, but is in all cases destabilizing. For these substituents, it is instead the spin delocalization that has the greatest effect on 

Finally, we like to point out that our results can explain the observations that 0-H BDEs in phenols correlate with ap+. Because of the direct conjugation between the oxygen lone pair and the substituent, the polar stabilization of the phenol can be expected to follow a linear relationship with Op rather than with ap+. This is also consistent with our computed APSE which correlates linearly with Op with a correlation coefficient of 0.984. Since the Up and the CTp+ scales differ in that Gp+ predicts much larger substituent effects for resonance donors (e.g. 0CH3 oh and NH2) and relatively smaller substituent effects for resonance attractors (e.g. CN and NO2), the overall relationship between ABDE and Jp+ can be explained by the observed extra stabilization of the radical by electron donating substituents. Thus to understand the substituent effects on the 0-H 

B3LYP/6-31G computed molecular properties, 0-H bond dissociation energies and stabilization energies for some phenols  

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Bosque, R. and Sales, J. (2003) A QSPR study of O-H bond dissociation energy in phenols. J. Chem. Inf Comput. Sci., 43, 637-642. 

We have also discussed the use of the electrostatic potential for the analysis of substituent effects in aromatic systems. Substituent effects on gas phase and solution acidities of benzoic acids and phenols are dominantly determined by the relative stabilization of the negative charge in the ionized forms of these systems. The oxygen Vmin is an excellent tool for the analysis of this stabilization effect. On the other hand, we have found that the homol5dic O-H bond dissociation energy in phenols depends both on the substituent s ability to stabilize the parent molecule (the phenol) and the radical. The relative stabilization energies of the parent molecule and the radical can be estimated from their computed Vmin and surface maxima in the spin density, respectively. 

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