The unsubstituted benzylic radical (I) is an outlier in this stability/dimerization trend in that dimer is observed (11%), despite the equilibrium lying towards the Giese adduct (DG = -4.4 kcal mol -1 ). To explain this phenomenon, we noted that the spin density is much more localized at the benzylic position than that of the rest of the radicals under study (rs = 0.75, Figure ). More localized benzylic radicals may incur a lower penalty to dimerization due to non-perfect synchronization, 42 making unsubstituted radical I the most susceptible to dimerization of the series. On the other hand, the lower spin density of the cyclopropane radical II (rs = 0.67) increases the barrier to dimerization relative to the Giese addition. We propose that the extent of radical delocalization is controlled by the hybridization of the orbital of the benzylic carbon involved in the arene-to-benzylic-carbon s bond (C1-C2, Figure ), which in turn is determined by the hybridization of the orbitals involved in the C-C s bonds of the additional benzylic substituents (C1-C3 & C1-C4, Figure and). Substrates in which the orbitals making these benzylic C-C bonds require enhanced p-character due to the small internal angles of small rings, e.g. cyclopropane II (sp 3.75 ; Figure ), must assign greater s-character to the orbital of C1 involved in the C-C s bond with the arene (sp 1.25 ; Figure ). This is expressed in a shorter C1-C2 bond, increased radical delocalization and reduced benzylic spin density (Figure ; see Figure for our proposed model for variation in spin density). We propose that delocalization is enhanced in oxetane III compared to cyclobutane V due to Bent's rule 44 : the electronegative oxetane oxygen atom withdraws electron density from the ring C-C bonds, forcing a greater p-contribution on the orbitals forming these bonds in III than V (sp 3.40 vs sp 3.15 , respectively, Figure ). As a result, the orbital of the benzylic-carbon involved in the C-C bond with the arene in III is richer in s-character (sp ) and the bond is shorter (1.404 Å) than in V (sp 1.53 and 1.410 Å, respectively), thereby increasing 𝜋-delocalization and subsequently minimizing dimerization. Overall, dimerization will be minimized for substrates with destabilized benzylic radicals that are able to efficiently delocalize into the arene 𝜋 system. For example, the Giese addition for oxetane radical III is exergonic (∆G = -3.7 kcal mol -1 ), and the radical is sufficiently 𝜋-delocalized (rs = 0.67) due to the high p-character demanded within the oxetane ring (vide supra), such that no dimer is observed and the yield of Giese product is high in the reaction with acrylates. /def2-TZVP//ωB97X-D3 46 /def2-SVP level. Free energies calculated at 298.15 K and 1 M standard state. Hybridizations calculated using Natural Bond Orbital (NBO) theory. 47 a Yields were experimentally determined and as depicted in Scheme 4. DG calculated using methyl acrylate (Supporting Figure ). b DE (2) (p → p*) calculated using NBO second order perturbation theory.