Accurate ring strain energy (RSE) data for parent (CH2)2X rings are reported, where X are group 13–16 elements (El) in their lowest oxidation state, from the second to the sixth row, with their covalence completed by bonds to H. They are obtained from appropriate homodesmotic and hyper-homodesmotic reactions at different levels up to the CCSD­(T) level, thus providing a benchmark of high-quality reference RSE values, as well as acceptably accurate faster lower-level options. Derivatives of indium, thallium, and lead cannot be properly described by a three-member ring connectivity, because they display a unique donor–acceptor structure from an ethylene π­(CC) orbital to an empty p orbital of a metallylene subunit. The RSE of groups 13 and 14 heterocycles increases on descending in the group (except for Ga and Ge), while it decreases for groups 15 and 16. The latter is presumably due to a strain-releasing mechanism favored by the increase of p-character at the sp3-type atomic orbital used by El in the endocyclic El–C bonds, %p­(El)El‑C, originated by the tendency of the El lone pairs in groups 15–16 to increase their s-character. This strain-releasing mechanism does not exist in heavier tetrels, which keep almost unchanged the p-character in the endocyclic bonds at El, whereas for triels the p-character is still lower owing to their sp2-like hybridization. Remarkable linear correlations were found between the RSE and either the above-mentioned %p­(El)El‑C, the distal C–C bond distance or the relaxed force constants for the endocyclic bond angles.

High-quality ring strain energy (RSE) data for 1H-unsaturated (CH)2X parent rings, where X is a group 13–16 element, are reported in addition to the 2H-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD­(T)/def2-TZVPP//B3LYP-D3/def2-TZVP­(ecp) level. 1H-Tallirene and 1H-plumbirene have unique donor–acceptor structures between an acetylene π­(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar–Chatt–Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1H-indirene and 1H-oxirene lack ring critical points and constitute borderline cases of pseudorings. 1H-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15–16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15–16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2H-Unsaturated rings were found only for group 15 elements (although only 2H-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1H-isomers.

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.

The full chemical space of the CHNO isomers and their related deprotonated CNO- anions is described for first time in terms of both minimum energy structures and interconversions among them...

Dedicated to Professor Christoph Janiak on the occasion of his 60th birthday. Due to the potential interest of oxaphosphiranes in ringopening polymerizations, accurate ring strain energies (RSEs) of a wide variety of oxaphosphirane derivatives was computed, after validation of the optimization method by comparison with reported X-ray structures. The parent oxaphosphirane exhibits a moderate RSE (22.44 kcal/mol) that is significantly enhanced by k-P-complexation (especially with boranes), the introduction of P=O or P +-Me groups as well as exocyclic double bonds at the ring carbon, such as 3-methylene, 3-imino and 3-oxo functionalities. However, C3 alkyl substitution or pentacoordination at P in σ 5 λ 5-oxaphosphirane does not lead to significant variation of RSE. Bicyclic spiro-oxaphosphirane derivatives show an RSE decrease when the size of the spiro ring increases. A moderate linear correlation between RSE and G(r)/1(r) values calculated at the ring critical points and also with the relaxed force constant (k 0) for the PÀ C bond is observed for most oxaphosphiranes. The possibility of ring-opening polymerization by using better (anionic) nucleophiles in the initiation step can be foreseen from the exergonicity and relatively low barrier of the endocyclic CÀ O cleavage by nucleophilic attack of methanol, thus underlining the effect of the RSE as driving force compared to acyclic analogs.