Reaction of Phenols with the 2,2-Diphenyl-1-picrylhydrazyl Radical. Kinetics and DFT Calculations Applied To Determine ArO-H Bond Dissociation Enthalpies and Reaction Mechanism

The formal H-atom abstraction by the 2.2-diphenyl-1-picrylhydrazyl (dpph(center dot)) radical from 27 phenols and two unsaturated hydrocarbons has been investigated by a combination of kinetic measurements in apolar solvents and density functional theory (DFT). The computed minimum energy Structure of dpph(center dot) shows that the access to its divalent N is strongly hindered by an ortho H atom oil each of the phenyl rings and by the o-NO2 groups of the picryl ring. Remarkably small Arrhenius pre-exponential factors for the phenols [range (1.3-19) x 10(5) M-1 s(-1)] are attributed to steric effects. Indeed, the entropy barrier accounts for up to ca. 70% of the free-energy barrier to reaction. Nevertheless, rate differences for different phenols are largely due to differences in the activation energy, E-a,E-1 (range 2 to W kcal/mol). In phenols, electronic effects of the substituents and intramolecular H-bonds have a large influence oil the activation energies and oil the ArO-H BDEs. There is a linear Evans-Polanyi relationship between E-a,E-1 and the ArO-H BDEs: E-a,E-1/kcal x mol(-1) = 0.918 BDE(ArOH)/kcal x mol(-1) - 70.273. The proportionality constant, 0.918, is large and implies a "late" or "product-like" transition state (TS), a conclusion that is congruent With the small deuterium kinetic isotope effects (range 1.3-3.3). This Evans-Polanyi relationship, though questionable oil theoretical grounds. has profitably been used to estimate several ArO-H BDEs. Experimental ArO-H BDEs are generally in good agreement with the DFT calculations. Significant deviations between experimental and DFT calculated ArO-H BDEs were found, however, when an intramolecular H-bond to the O-center dot center was present in the phenoxyl radical, e.g., in ortho semiquinone radicals. In these cases, the Coupled cluster with single and double excitations correlated wave function technique with complete basis set extrapolation gave excellent results. The TSs for the reactions of dpph(center dot) with phenol, 3- and 4-methoxyphenol, and 1, 4-cyclohexadiene were also computed. Surprisingly, these TS structures for the phenols show that the reactions cannot be described its occurring exclusively by either a HAT or a PCET mechanism, while with 1.4-cyclohexadiene the PCET character in the reaction coordinate is much better defined and shows a strong pi-pi, stacking interaction between the incipient cyclohexadienyl radical and a phenyl ring of the dpph(center dot) radical.