(1) Many recent studies have shown that the thermodynamic stability of such radicals can be predicted with high accuracy using DFT calculations. In this study, we performed DFT calculations to evaluate the thermodynamic stability of radicals based on the BDE (for the calculation results, see Table ). The obtained results suggest that the BDE increases and that the radical species become thermodynamically unstable when there are a total of three or more electron-donating methoxy groups on the aromatic rings, or, conversely, when there are no electron-donating groups. On the other hand, when the aromatic rings have one or two electron-donating groups or one electron-donating group and one electron-withdrawing group, the BDE decreases, suggesting that the radical species become thermodynamically stable. The results of these calculations thus reflect the captodative effect. In addition, the BDE tends to increase when there is a methoxy group at the ortho-position. This result implies that the DAAN derivatives with substituents at the ortho-position show deviation from ideal molecular planarity due to steric repulsion, commensurate with lower thermodynamic stabilization compared to those without substituents at the ortho-position. Figure , which summarizes the calculation results for DAAN derivatives with substituents at only the para-position, predicts the highest conversion efficiency to DAAN radicals for SMe/OMe. Moreover, the calculations predict that derivatives from the blue and red groups can be expected to have a higher conversion efficiency to DAAN radicals than the derivatives from the gray group. Furthermore, Figure shows that, compared to the derivatives with substituents at only the para-position, the derivatives in the green group have higher BDE values and generate radical species with lower thermodynamic stability, albeit that they have a higher sC-H level and higher reactivity with mechanoradicals. Evaluation of the Amount of DAAN Radicals. After the ball-milling of the mixtures of polystyrene (Mn = 66.0 kg mol - 1 ; Mw/Mn = 1.46, 100 mg) and the DAAN derivatives (50 µmol), the amount of generated DAAN radicals relative to the corresponding DAAN derivative was evaluated using EPR measurements. The observed DAAN radicals are shown in Figure (for the EPR spectra, see Figures ). Among the DAAN derivatives with substituents at only the para-position, H/H, F/F, and H/Me furnished only a small amount of DAAN radicals, while SMe/OMe afforded a large amount of DAAN radicals. This result is consistent with the DFT-based prediction that the conversion efficiency to DAAN radicals increases with decreasing BDE value and increasing sC-H energy level. However, the amount of DAAN radicals with methyl groups was smaller than that predicted based on its BDE. This is probably due to side reactions, such as the reaction of the methyl group with the mechanoradicals to produce benzyl radicals (Figure ). Furthermore, in the DFT calculations, the thermodynamic stability of the DAAN radicals was predicted to be lower when substituents were present at the ortho-position. Nevertheless, with increasing number of substituents at the ortho-position, the amount of observed DAAN radicals also increased (Figure ). This result should most likely be attributed to the protection of the central (or a) carbon atom, which is the atom with the highest spin density, due to the introduction of a bulky methoxy group at the ortho-position. In other words, the low thermodynamic stability of the ortho-substituted DAAN derivatives is counterbalanced by their kinetic stability. To evaluate the steric hindrance of the DAAN radicals, their percentages of buried volume (%VBur) were calculated. For this purpose, a sphere was set up with the carbon-centered radical at the origin and a 6 Å radius; these dimensions are the most suitable to consider the spatial steric protection of the radical by the surrounding atoms. The buried volumes and steric maps presented in this work were obtained using the SambVca 2 Web application. According to the topography (Figure ), the methoxy groups at the meta-position of the DAAN radicals protect the carbon-centered radicals well (Figure ). These results suggest that small BDE values and high sC-H energy levels are important for improving the amount of generated DAAN radicals. In addition, the steric protection of the carbon-centered radicals overcomes the low thermodynamic stability of radicals and increases the amount of generated DAAN radicals.