Should contemporary density functional theory methods be used to study the thermodynamics of radical reactions?

ABSTRACT

The performance of a variety of DFT functionals (BLYP, PBE, B3LYP, B3P86, KMLYP, B1B95, MPWPW91, MPW1B95, BB1K, MPW1K, MPWB1K, and BMK), together with the ab initio methods RHF, RMP2, and G3(MP2)-RAD, and with ONIOM methods based on combinations of these procedures, is examined for calculating the enthalpies of a range of radical reactions. The systems studied include the bond dissociation energies (BDEs) of R-X (R = CH3, CH2F, CH2OH, CH2CN, CH2Ph, CH(CH3)Ph, C(CH3)2Ph; X = H, CH3, OCH3, OH, F), RCH(Ph)-X (R = CH3, CH3CH2, CH(CH3)2, C(CH3)3, CH2F, CH2OH, CH2CN; X = H, F), R-TEMPO (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3, CH2CH2CH3, CH2F, CH2OH, CH2CN, CH(CN)CH3, CH(Cl)CH3; TEMPO = 2,2,6,6,-tetramethylpiperidin-1-yloxyl) and HM1M2-X (M1, M2 = CH2CH(CH3), CH2CH(COOCH3), CH2C(CH3)(COOCH3); X = Cl, Br), the beta-scission energies of RXCH2* and RCH2CHPh* (R = CH3, CH2CH3, CH(CH3)2, C(CH3)3; X = O, S, CH2), and the enthalpies of several radical addition, ring-opening, and hydrogen- and chlorine-transfer reactions. All of the DFT methods examined failed to provide an accurate description of the energetics of the radical reactions when compared with benchmark G3(MP2)-RAD values, with all methods tested showing unpredictable deviations of up to 40 kJ mol-1 or more in some cases. RMP2 also shows large deviations from G3(MP2)-RAD in the absolute values of the enthalpies of some types of reaction and, although it fares somewhat better than the DFT methods in modeling the relative values, it fails for substituents capable of strongly interacting with the unpaired electron. However, it is possible to obtain cost-effective accurate calculations for radical reactions using ONIOM-based procedures in which a high-level method, such as G3(MP2)-RAD, is only used to model the core reaction (which should contain all substituents alpha to the reaction center), and the full system is modeled using a lower-cost procedure such as RMP2.