Overestimation of the stability of the π-delocalized versus the σ-localized configuration in radicals by current density functionals: the case of vinylacyl radicals

Abstract

Calculations with the density functional theory (DFT) method using the most popular functional, Becke's three parameter hybrid with the Lee, Yang and Parr correlation functional, predict the p-delocalized con®guration of the vinylacyl radical, CH 2 @CHA C(·)@O, to be more stable than the r-localized con®g-uration in contrast with ab initio unrestricted quadratic con®guration interaction with single and double excitations calculations as previously found for the isoelectronic vinyl radical, Y-C(·)@CH 2 , bearing p-type a substituents. Experimental evidence on the electronic con®guration adopted by vinyl radicals is contrasting. In the present case comparison with experiment indicates ®rmly that the currently available density functionals overestimate the stability of p-delocalized versus r-localized con®gurations in radicals since they favor the p con®guration for the c-methylvinylacyl radical, CH 3 ACH@CHAC(·)@O, in contrast with unequivocal electron spin resonance data. This failure is mainly due to an incorrect estimate of dynamic correlation energy with DFT functionals.

A Computational and Experimental Investigation of the Origin of Selectivity in the Chiral Phosphoric Acid Catalyzed Enantioselective Minisci Reaction

“…While these computational methods have been shown to give reliable results in a large variety of systems, we were mindful of the possibility for systematic error in this particular case. It has been shown that DFT methods can overestimate the stability of delocalized radicals compared to localized radicals . This could impact our results, as in the proposed reaction mechanism a localized radical in I is transformed into a highly delocalized radical cation in II .…”

“…It has been shown that DFT methods can overestimate the stability of delocalized radicals compared to localized radicals. 22 This could impact our results, as in the proposed reaction mechanism a localized radical in I is transformed into a highly delocalized radical cation in II. Hence we were aware that direct comparison of A thorough exploration of the possible deprotonation (II− III) modes was then undertaken, with the minimal assumption that the TRIP-phosphate acts as the base.…”

“…In the reaction mechanism, a localized radical in the starting complex I is converted into a highly delocalized aromatic radical in the addition ( II ) and deprotonation product ( III , via IV ). As mentioned previously, DFT methods are known to overestimate the stability of delocalized radicals compared to localized radicals . This means that the energies of all of the deprotonation steps should be trustworthy when compared with each other, but the deprotonation process may require a systematic correction to be applied in order to compare the calculated energies to those for the addition process.…”

“…As mentioned previously, DFT methods are known to overestimate the stability of delocalized radicals compared to localized radicals. 22 This means that the energies of all of the deprotonation steps should be trustworthy when compared with each other, but the deprotonation process may require a systematic correction to be applied in order to compare the calculated energies to those for the addition process. To test this, we explored the reaction mechanism using a higher level double-hybrid B2PLYPD3 method.…”

Hydrogen Atom Transfer Driven Enantioselective Minisci Reaction of Alcohols

“…In our previous studies of the amide Minisci reaction we encountered a systematic error when using standard DFT methods, which problematically overestimated the stability of delocalized radicals when compared with localized ones. [13,29] In the current model system we observed the same effect-M06-2X results erroneously suggested that radical addition might be the selectivity-determining step (see Supporting Information, Figure S9 for details). However, energies obtained using the significantly more expensive double-hybrid B2PLYPD3 functional clearly showed that the deprotonation (II-III) is the selectivity-determining step, with a significantly higher barrier than radical addition (I-II) (Figure 2).…”