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TY - JOUR
TI - 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
DO - 10.1021/jo8016555
AU - Foti, M. C.
AU - Daquino, C.
AU - Mackie, I. D.
AU - DiLabio, G. A.
AU - Ingold, K. U.
T2 - Journal of Organic Chemistry
SN - 0022-3263
VL - 73
IS - 23
SP - 9270
EP - 9282
KW - Activation energy
KW - Density functional theory
KW - Deuterium
KW - Hydrocarbons
KW - O rings
KW - Potential energy
KW - Probability density function
KW - Rate constants
KW - Reaction kinetics
KW - Sulfur compounds
KW - Apolar solvents
KW - Arrhenius
KW - Atom abstractions
KW - Complete basis sets
KW - Correlated wave functions
KW - Coupled cluster with single and double excitations
KW - Cyclohexadiene
KW - Deuterium kinetic isotope effects
KW - DFT calculations
KW - Electronic effects
KW - Exponential factors
KW - H-bonds
KW - Kinetic measurements
KW - Minimum energy structures
KW - Phenoxyl radicals
KW - Phenyl rings
KW - Reaction coordinates
KW - Reaction mechanisms
KW - Semiquinone
KW - Stacking interactions
KW - Steric effects
KW - Transition states
KW - Unsaturated hydrocarbons
KW - 1,1 diphenyl 2 picrylhydrazyl
KW - 1,4 cyclohexadiene
KW - alkene
KW - deuterium
KW - isotope
KW - mequinol
KW - phenol derivative
KW - solvent
KW - phenoxy radical
KW - density functional theory
KW - molecular dynamics
KW - reaction analysis
KW - conformation
KW - solubility
KW - thermodynamics
KW - Models, Chemical
KW - Models, Theoretical
KW - Molecular Conformation
KW - Nitrogen
KW - Phenols
AB - (Figure Presented) The formal H-atom abstraction by the 2,2-diphenyl-1-picrylhydrazyl (dpph•) 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• shows that the access to its divalent N is strongly hindered by an ortho H atom on 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) × 105 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, Ea,1 (range 2 to 10 kcal/mol). In phenols, electronic effects of the substituents and intramolecular H-bonds have a large influence on the activation energies and on the ArO-H BDEs. There is a linear Evans-Polanyi relationship between E a,1 and the ArO-H BDEs: Ea,1/kcal x mol-1 = 0.918 BDE(ArO-H)/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 on 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 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 • 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 as 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 π-π stacking interaction between the incipient cyclohexadienyl radical and a phenyl ring of the dpph• radical. © 2008 American Chemical Society.
PY - 2008
DA - 2008
LA - eng
C1 - Collection / Collection : NRC Publications Archive / Archives des publications du CNRC
C2 - Record identifier / Identificateur de l’enregistrement : 04d040c1-cc87-44d6-8f60-1d8717eaa3d7
ER -