RESUMO
Ab initio studies of 14 [N]phenylenes containing 12-membered rings furnish geometries, energies, standard heats of formation, NICS(1) values, and proton chemical shifts. The extent of double-bond localization, DeltaR (in A), for each type of the 58 unique six-membered rings-angular, branched, and terminal-varies linearly with both their NICS(1) values and proton resonances. Values of these parameters depend upon the number and type of neighboring rings. DeltaRtot, the molecular sum of the bond localizations, correlates linearly with the ab initio energy and heat of formation. Reactions that conserve the numbers of angular, branched, terminal, and 12-membered rings are thermoneutral, and their net DeltaRtot and DeltaNICS(1) are also nearly zero.
RESUMO
Geometries, energies and magnetic shieldings are reported at the ab initio B3LYP/6-31G* level for the phenylene cluster C120 (archimedene) and eight phenylene-based hydrocarbon bowls consisting of four-, six-, and ten-membered rings. The six-membered rings are branched, angular, or terminal. The latter are the most aromatic, based upon NICS criteria and lack of substantial bond alternation. At the other extreme are branched rings, having less negative NICS values. Four-membered rings, except those on a rim, are nearly square. Regularities are found in the ab initio energies, heats of formation (deltaH(o)f), and strain energies relative to those of hypothetical planar acyclic analogues. The bowls appear to have little aromatic character, and their interiors are but slightly shielded. Archimedene, with deltaH(o)f = 2191 kcal/mol, has energetic and structural properties akin to those of phenylene-based bowls.