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)-R AD, 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 t o 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 reaso nable 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, CO OCH3, CN and NO2) and hyperconjugating groups (CH3, CH2CH3, CF3 and CF2CF3) . All substituents other than CF3 and CF2CF3 result in radical stabilizatio n, with the vinyl (CH=CH2), ethynyl (C drop CH) and phenyl (C6H5) groups pr edicted to give the laraest stabilizations of the pi -accept dr substituent s examined and the NH2 group calculated to provide the greatest stabilizati on of the lone-pair-donor groups. The substituents investigated in this wor k stabilize methyl radical centers in three general ways that delocalize th e odd electron: pi -acceptor groups (unsaturated substituents) delocalize t he unpaired electron into the pi -system of the substituent, lone-pair-dono r groups (heteroatomic substituents) bring about stabilization through a th ree-electron interaction between a lone pair on tile substituent and the un paired electron at the radical center, while alkyl groups stabilize radical s via a hyperconjugative mechanism. Polyfluoroalkyl substituents are predic ted to slightly destabilize a methyl] radical center by inductively withdra wing electron density from the electron-deficient radical center.