Steric charge

Abstract

The steric effect is one of the most widely used chemical concepts in chemistry, yet a generally accepted implementation of its formulation and quantification from a theoretical viewpoint, if even possible, is still an unaccomplished task. Based on the energetics viewpoint of our earlier proposal using the Weizsäcker kinetic energy as a quantification of the energy contribution from the steric effect as well as our recent work using steric force as a reliable local descriptor to account for the origin of the stereoselective propensity in chemical reactions, in this work, we systematically examine the local behavior and general applicability of another closely related quantity, steric charge. To this end, its physical origin and physiochemical properties are formalized and highlighted, and are shown with numerical illustrations and verifications. To showcase its usefulness in appreciating chemical reactivity, we present three case studies of steric charge in this work. In the case of ethane flexible rotation, we found that the eclipsed conformer is more sterically charged than the staggered conformer, leading to the reduced stability and higher energy of the former. For the case of S_N2 reactions, a remarkable linear correlation has been obtained between the steric charge on the central atom in the transition state and the reaction barrier height of S_N2 reactions, in good agreement with experimental findings. In the case of stereoselectivity properties for the nucleophilic addition of carbonyl compounds, we found that steric charge is equally applicable as steric force to justify the stereoselective origin for the nucleophilic attack to the carbonyl carbon atom with different substituent groups. Put together, our results from the present study should pave the way towards the general use of steric charge, together with steric energy and steric force, as an insightful global and local descriptor to appreciate and quantify chirality and stereoselectivity related phenomena in chemical processes and transformations.