Ring Strain Energies in Four-Membered Heterocycles Containing an Element of Groups 13-16

The ring strain energy (RSE) of four-membered rings (4MRs) incorporating only one element from groups 13-16, exhibit moderate values of approximately 25 kcal·mol^-1 for compounds containing second-row elements, and decreasing to 14-19 kcal·mol^-1 as one progresses down the periodic table. 4MRs containing Bi, S, Se, Te and Po, exhibited higher RSEs than their three-membered analogues, which seems to be related to enhanced σ-antiaromaticity. The relationship between RSEs and some structural and electronic parameters was investigated. The strain-relieving effect observed with an increase in heteroatom size in all groups, may also coexist with that provided by the increase in the p-character of the AO used by the heteroatom in groups 15 and 16 in the endocyclic bonds. Furthermore, the RSE is found to correlate linearly with the C-Z-C bond angles and Z-C bond distances, as well as with the corresponding relaxed force constants. There is also a strong correlation between the RSEs of 4MRs with elements of groups 15 and 16 and the HOMO-LUMO gaps, as well as with the electron density and its Laplacian, in the ring critical points. In addition, an effective additive method for rapid estimation of RSEs based on ring atoms or endocyclic bonds contributions is disclosed.