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Abstract A new empirical electrophilicity reactivity parameter,E_RB, was developed based on the rotational barriers of a series ofN‐phenylimide molecular rotors containing various electrophilic groups. In the bond rotation transition state, these electrophilic groups form close contact with an electronegative C═O oxygen. Thus, strong electrophilic groups significantly lowered the rotational barrier. As a result, the rotational barriers were inversely correlated with the strengths of the electrophiles. The rotational barriers were measured by dynamic NMR (EXSY), enabling the quantification across a wide range of types of electrophiles. Computational analysis confirmed that the observed variations arose from intramolecular interactions in the transition state, where the C═O oxygen served as a probe of both the electrophilic group's electrostatic potential and steric accessibility. By simultaneously capturing attractive and repulsive transition state interactions,E_RBprovides an effective means of predicting electrophilicity and reactivity trends across a broad range of electrophiles and reaction types. The utility ofE_RBwas initially validated using a series of rotors containing Michael addition electrophiles, followed by broader application to a diverse array of reactions involvingsp³andsp²electrophiles, including S_N2, S_NAr, Pd‐oxidative addition, and Sonogashira reactions. 

Abstract Non‐covalent chalcogen bond (ChB) interactions have found utility in many fields, including catalysis, organic semiconductors, and crystal engineering. In this study, the transition stabilizing effects of ChB interactions of oxygen and sulfur were experimentally measured using a series of molecular rotors. The rotors were designed to form ChB interactions in their bond rotation transition states. This enabled the kinetic influences to be assessed by monitoring changes in the rotational barriers. Despite forming weaker ChB interactions, the smaller chalcogens were able to stabilize transition states and had measurable kinetic effects on the rotational barriers. Sulfur stabilized the bond rotation transition state by as much as −7.2 kcal/mol without electron‐withdrawing groups. The key was to design a system where the sulfur ‐hole was aligned with the lone pairs of the chalcogen bond acceptor. Oxygen rotors also could form transition state stabilizing ChB interactions but required electron‐withdrawing groups. For both oxygen and sulfur ChB interactions, a strong correlation was observed between transition state stabilizing abilities and electrostatic potential (ESP) of the chalcogen, providing a useful predictive parameter for the rational design of future ChB systems.