Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13–16 Element

Abstract

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.

Ring Strain Energies of Three-Membered Homoatomic Inorganic Rings El3 and Diheterotetreliranes El2Tt (Tt = C, Si, Ge): Accurate versus Additive Approaches

“…On descending the groups, the atom-strain contributions decrease for elements having LPs (groups 15 and 16), especially for pnictogens, whereas they increase for those lacking LPs (groups 13 and 14), the highest values being observed for triels that bear an empty p orbital ( Figure 9 ). This effect is most likely related to the already reported LP strain releasing effect, 29 , 51 in turn affecting the p-character of the endocyclic bonds (see Figure 4 ).…”

“…As observed for monoheteroatomic saturated 29 and unsaturated 51 3MRs, one of the main mechanisms of strain relaxation is the effect of lone electron pair (LP) hybridization. According to this effect, an increase in the s-character of the LP-containing AO enhances the p-character of the AOs used by El in the endocyclic bonds, giving rise to a sp n -type hybridization with n > 3 (“high” p-character, beyond 75%) which fits better to small endocyclic bond angles ( Figure 4 ).…”

“…These are generally used as numerically stable and fully transferable parameters . The four ring types 1 – 4 show a remarkable decrease in the El–El bond strength, k 0 El–El , for El heteroatoms from the third row onward, in line with an increase in the p-character, especially for pnictogen and chalcogen El elements (Figure ) where LPs strain relaxation , is possible. As already observed for the p-character, for 1 El and 2 El rings, the expected decrease in k 0 El–El on moving down the group does not lead to a significant relaxation of the ring strain, except for 2 Pn to some extent (Pn = pnictogen atoms).…”

Stability of Carbocyclic Phosphinyl Radicals: Effect of Ring Size, Delocalization, and Sterics

“…However, the s-contribution is larger for the strained species 2 (83.1%) than for the remaining cases (66.3, 65.0, and 63.8% in 8 , 12 , and 21 , respectively). The increased s-character of the lone pair in radical 2 is in accordance with similar observations on three-membered rings containing pnictogens …”

“…The increased s-character of the lone pair in radical 2 is in accordance with similar observations on three-membered rings containing pnictogens. 60 2.5. Steric Effects.…”