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A Density Functional Study of Substituent Effects on the O−H and O−CH3 Bond Dissociation Energies in Phenol and Anisole
journal contribution
posted on 1996-11-01, 00:00 authored by Yun-Dong Wu, David K. W. LaiThe substituent effects on O−H and O−CH3 bond
dissociation energies for a series of 18 para-substituted phenols (p-XC6H4OH)
and 11 para-substituted anisoles have been studied using
the
density functional method in order to understand the origin of these
effects. The calculated
substituent effects agree well with experimental measurements for
phenols but are substantially
larger than the reported values for anisoles. Both ground-state
effect and radical effect contribute
significantly to the overall substituent effect. An
electron-donating group causes a destabilization
in phenols or anisoles (ground-state effect) but a stabilization in the
phenoxy radicals (radical effect),
resulting in reduced O−R bond dissociation energy. An
electron-withdrawing group has the opposite
effect. In most cases, the radical effect is more important than
the ground-state effect. There is
a good correlation between the calculated radical effects and
calculated variations in charge and
spin density on the phenoxy oxygen. This supports the concept that
both polar and spin
delocalization effects influence the stability of the phenoxy radical.
While almost every para-substituent causes a stabilization of the phenoxy radical by spin
delocalization, electron-donating
groups stabilize and electron-withdrawing groups destabilize the
phenoxy radical by the polar effect.