Unravelling the Strain: Accurate Ring Strain Energies in Chalcogeniranes and Predictive Models for Most p-Block Three-Membered Rings

State-of-the-art single reference computational methods were utilized to accurately determine the ring strain energy (RSE) for all three-membered rings (3MRs) containing the <i>El</i>₂<i>Ch</i> core, where <i>Ch</i> is a chalcogen atom and <i>El</i> is any element of groups 13 to 16, from the second to the sixth period. Only thallium was excluded as <i>El</i> element because <i>Tl</i>₂<i>Ch</i> energy minima were demonstrated to constitute pseudorings (absence of ring critical point). The p-character of the atomic orbitals employed in endocyclic bonding, the destabilizing repulsion of electron clouds in chalcogen-chalcogen bonds in trichalcogeniranes and the existence of 2π-electron Hückel-type aromaticity in Tr₂<i>Ch</i> rings were identified as key electronic factors influencing RSE. Finally, the increased number of 3MRs for which accurate RSE has been reported so far allows widening the additive methodology for the estimation of RSE to an extended set of one hundred and sixty-one endocyclic bonds with remarkable accuracy (root-mean-square error 1.21 kcal/mol).