Bond dissociation energies and radical stabilization energies associated with substituted methyl radicals

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with a series of 22 monosubstituted methyl radicals ((CH2X)-C-.) have been determined at a variety of levels including, CBS-RAD, G3(MP2)-RAD, RMP2, UB3-LYP and RB3-LYP. In addition, Wl' values were obtained for a subset of 13 of the radicals. The Wl'BDEs and RSEs are generally close to experimental values and lead to the suggestion that a small number of the experimental estimates warrant reexamination. Of the other methods,CBS-RAD and C3(MP2)-RAD produce good BDEs. A cancellation of errors leads to reasonable RSEs being produced from ail the methods examined. CBS-RAD, Wl' and G3(MP2)-RAD perform best, while UB3-LYP performs worst. The substituents (X) examined include lone-pair-donors (X = NH2, OH, OCH3, F, PH2, SH, Cl, Br and OCOCH3). pi -acceptors (X = BH2, CK=CH2, C drop CH, C6H5, CHO. COOH, COOCH3, CN and NO2) and hyperconjugating groups (CH3, CH2CH3, CF3 and CF2CF3). All substituents other than CF3 and CF2CF3 result in radical stabilization, with the vinyl (CH=CH2), ethynyl (C drop CH) and phenyl (C6H5) groups predicted to give the laraest stabilizations of the pi -accept dr substituents examined and the NH2 group calculated to provide the greatest stabilization of the lone-pair-donor groups. The substituents investigated in this work stabilize methyl radical centers in three general ways that delocalize the odd electron: pi -acceptor groups (unsaturated substituents) delocalize the unpaired electron into the pi -system of the substituent, lone-pair-donor groups (heteroatomic substituents) bring about stabilization through a three-electron interaction between a lone pair on the substituent and the unpaired electron at the radical center, while alkyl groups stabilize radicals via a hyperconjugative mechanism. Polyfluoroalkyl substituents are predicted to slightly destabilize a methyl] radical center by inductively withdrawing electron density from the electron-deficient radical center.