To directly compare the impact of delocalization on activation barriers, we examined changes in barrier (∆∆H ‡ ) for the test set relative to bicyclo[2.2.0]hexane (G, Figure ), which exhibits a moderate strain release value (-52.5 kcal mol -1 ) but has a small 2-Nocc value (0.045 e). Among the test set, delocalization, not strain release emerged as the primary cause of reactivity difference for seven of the twelve members relative to G (denoted by asterisks). In four cases (D, F, H and lobes). Similar behavior is observed with cyclobutane, with a node between the bridging methylenes indicating a lack of through-bond communication. However, for cyclopropane, a buildup of electron density on the bridging methylene indicates stabilizing delocalization.