Ring Strain Energies of Three-Membered Homoatomic Inorganic Rings El 3 and Diheterotetreliranes El 2 Tt (Tt = C, Si, Ge): Accurate versus Additive Approaches.

Accurate ring strain energies (RSEs) for three-membered symmetric inorganic rings El 3 and organic dihetero-monocycles El 2 C and their silicon El 2 Si and germanium El 2 Ge analogues have been computed for group 14-16 "El" heteroatoms using appropriate homodesmotic reactions and calculated at the DLPNO-CCSD-(T)/def2-TZVPP//B3LYP-D4/def2-TZVP(ecp) level. Rings containing triels and Sn/Pb heteroatoms are studied as exceptions to the RSE calculation as they either do not constitute genuine rings or cannot use the general homodesmotic reaction scheme due to uncompensated interactions. Some remarkable concepts already related to the RSE such as aromaticity or strain relaxation by increasing the s-character in the lone pair (LP) of the group 15-16 elements are analyzed extensively. An appealing alternative procedure for the rapid estimation of RSEs using additive rules, based on contributions of ring atoms or endocyclic bonds, is disclosed.