Energetic Descriptors of Steric Hindrance in Real Space: An Improved IQA Picture**

Steric hindrance (SH) plays a central role in the modern chemical narrative, lying at the core of chemical intuition. As it however happens with many successful chemical concepts, SH lacks an underlying physically sound root, and multiple mutually inconsistent approximations have been devised to relate this fuzzy concept to computationally derivable descriptors. We here argue that being SH related to spatial as well as energetic features of interacting systems, SH can be properly handled if we chose a real space energetic stance like the Interacting Quantum Atoms (IQA) approach. Drawing on previous work by Popelier and coworkers (ChemistryOpen 8, 560, 2019) we build an energetic estimator of SH, referred to as E-ST. We show that the rise in the self-energy of a fragment that accompanies steric congestion is a faithful proxy for the chemist's SH concept if we remove the effect of charge transfer. This can be done rigorously, and the E-ST here defined provides correct sterics even for hydrogen atoms, where the plain use of deformation energies leads to non-chemical results. The applicability of E-ST is validated in several chemical scenarios, going from atomic compressions to archetypal S-N2 reactions. E-ST is shown to be a robust steric hindrance descriptor.

Automated summary

Steric hindrance is a fundamental concept in modern chemistry, but lacks a physically sound root. The use of a real space energetic stance like the Interacting Quantum Atoms approach can provide a more accurate handling of steric hindrance. An energetic estimator called E-ST has been developed to describe steric hindrance more faithfully, and has been validated in various chemical scenarios.