Aromaticity and antiaromaticity in monoheterocyclic three‐membered rings: Application of natural bond orbital theory

• Published in: Journal of physical organic chemistry Vol. 36; no. 3
• Main Authors: Brydon, Samuel C., White, Jonathan M.
• Format: Journal Article
• Language: English
• Published: Bognor Regis Wiley Subscription Services, Inc 01.03.2023
• Subjects: Aromaticity; Benzene; Chemical bonds; Chemical equilibrium; natural bond orbital theory; nucleus‐independent chemical shift; Orbital stability; three‐membered ring
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.4463

Using natural bond orbital theory, aromatic stabilization energies (NBO‐ASEs) were calculated for neutral and cationic monoheterocyclic three‐membered rings C2H2X(R)n (X = group 14–17 elements of rows 3–5; R = F, H or SiH3; and n = 0–2). Generally, there was a decrease in the (anti)aromatic nature of the molecule moving down the group as either the stabilizing orbital overlap (aromaticity) or destabilizing orbital interactions (antiaromaticity) were reduced due to larger atoms inducing longer bonds. (Anti)aromatic trends were supported with dissected nucleus independent chemical shift NICS(0)π,zz values (R2 = 0.94) and also by comparison of NBO‐ASEs to literature values for a selection of larger systems including benzene, cyclobutadiene, and the cyclopentadienyl anion. Comparisons are made to other studies that have used a range of approaches to study (anti)aromaticity in some of these molecules, and it was found that non‐dissected NICS values and homodesmotic equations often gave significantly overestimated if not misleading results. This study has applied natural bond orbital (NBO) theory to determine the aromatic stabilization energies (ASEs) for a series of monoheterocyclic three‐membered ring neutrals and cations across groups 14–17. Hyperconjugative (anti)aromaticity in these systems was shown to be sensitive to both the nature of heteroatom and its exocyclic substituents if present with reduced (anti)aromatic character moving down the group. A strong empirical correlation between the NBO‐ASEs with dissected nucleus nucleus‐independent chemical shifts (NICS(0)π,zz) supported these conclusions with R2 = 0.94.