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Article

Should Contemporary Density Functional Theory Methods Be Used to Study the Thermodynamics of Radical Reactions?

Supporting Info

Ekaterina I. Izgorodina,^† David R. B. Brittain, Jennifer L. Hodgson, Elizabeth H. Krenske, Ching Yeh Lin, Mansoor Namazian, and Michelle L. Coote*

ARC Centre of Excellence in Free-Radical Chemistry and Biotechnology, Research School of Chemistry, Australian National University, Canberra ACT 0200, Australia

J. Phys. Chem. A, 2007, 111 (42), pp 10754–10768

DOI: 10.1021/jp075837w

Publication Date (Web): September 22, 2007

†

Current address: School of Chemistry, Monash University, Victoria 3800, Australia.

Corresponding author. E-mail: mcoote@rsc.anu.edu.au. Fax: 61 2 6125 0750.

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 β-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 α to the reaction center), and the full system is modeled using a lower-cost procedure such as RMP2.