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

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)(pi,zz) values (R-2 = 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.