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.

Benchmark Energetic Data in a Model System for Grubbs II Metathesis Catalysis and Their Use for the Development, Assessment, and Validation of Electronic Structure Methods

“…However, evidence delineating poor performance of popular density functionals in several areas in chemistry has been presented by many research groups. [36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52] Indeed, BP86 and B3LYP are not accurate for the description of Grubbs metathesis catalysis, as shown by Tsipis et al 23 and us; 30,53 both functionals fail to predict the trend of the phosphine binding energies between the first-and second-generation Grubbs' ruthenium precatalysts for olefin metathesis. 30,31,53 Moreover, Piacenza et al 31 assessed the performance of five density functionals against benchmark energetic data for RuCl 2 (PH 3 ) 2 CH 2 , a small model system for the firstgeneration Grubbs catalysts.…”

β‐Fragmentation of Tertiary Alkoxyl Radicals: G3(MP2)‐RAD and Natural Bond Orbital Investigations

“…It is known that the fragmentation requires the TS geometry to exhibit good overlapping between the SOMO and the bonding s CÀY or the antibonding s* CÀY orbitals of the cleaved CÀY bond (Figure 1 a). Such molecular orbital interactions lead to the weakening of the bond, which is observed through both its lengthening and the diminution of its electron population.[15] In a recent work on the application of natural bond orbital (NBO) analysis to the b-fragmentation of alkoxyl radicals, [16] we observed in starting materials 1 c that the expected interactions between the SOMO and the s and/or the s* orbitals did not occur with the experimentally observed CÀOOEt bond cleavage despite that an early TS is observed for such a reaction (reactant-like from the Hammond's postulate).[17] This puzzling result led us to investigate, using the recommended G3(MP2)-RAD method, the b-fragmentation of the CÀY (left pathway 1, Scheme 1) and CÀMe (right pathway 2, Scheme 1) bonds for alkoxyl radicals 1 a-d, both in its thermodynamical aspects [18] (reaction entalphies DH r , and reaction free Gibbs energies, DG r ) and kinetic aspects [19] (activation barriers DG ¼ 6 ), and some of our results are summarised in Table 1. For comparison, thermodynamic data were calculated using G3 MP2B3 methods.…”

Assessing the performances of some recently proposed density functionals for the description of bond dissociations involving organic radicals

“…Consistently with Refs. [7,8], and to perform meaningful comparisons with previously published results, the reported energies have been obtained by single-point calculations using the B3LYP/6-31G(d) optimized geometries, the 6-311þG(3df,2p) basis set, and a tight convergence criterion for the SCF procedure. Moreover, to deal ''with pure'' DFT considerations, the zero-point energies have not been included, as suggested in Ref.…”

“…To this aim, benchmarking databases providing us with sufficiently accurate reference values are required. Recently, Izgorodina et al [4][5][6] have performed a series of systematic studies of bond-dissociation energies, leading to the definition of three representative sets [7] that revealed to be well-suited for functionals comparison. For instance, Zhao and Truhlar have used these sets [8] to test their M0x [9] functional family obtaining quite good results, whereas some of the most popular functionals exhibit important failures.…”