Bond Dissociation Energies and Radical Stabilization Energies Associated with Substituted Methyl Radicals

摘要

Bond dissociation energies (BDEs) and radical stabilization energies (RSEs) associated with a series of 22 monosubstituted methyl radicals (CH2X) have been determined at a variety of levels including, CBS-RAD, G3(MP2)-RAD, RMP2, UB3-LYP and RB3-LYP. In addition. WI values were obtained for a subset of 13 of the radicals. The WI’ 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 G3(MP2)-RAD produce good BDEs. A cancellation of errors leads to reasonable RSEs being produced from all the methods examined. CBS-RAD, WI and G3(MP2)-RAD petform best, while UB3-LYP performs worst. The substituents (X) examined include lone-pair-donors (X=NH_2,OH.OCH_3, F. PH_2,SH, Cl, Br and OCOCH_3). #pi#-acceptors (X = BH_2.CH=CH_2. C=CH,C_6H_5, CHO, COOH. COOCH_3. CN and NO_2) and hyperconjugating groups (CH_3, CH_2CH_3,CF_3 and CF_2CF_3).All substituents other than CF_3 and CF_2CF_3 result in radical stabilization, with the vinyl (CH=CH_2). ethynyl (C=CH) and phenyl (C_6H_5) groups predicted to give the largest stabilizations of the r-acceptor substituents examined and the NH_2 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.