Study of the Interconversion of Isomers of [16]annulene by the AM1 Method

Abstract

The AM1 method was used to carry out a theoretical study of different isomers of [16]annulene and of the reversible transformation between them preserving the molecular periphery. Transition states were located on the corresponding potential energy hypersurface.

Aromaticity with a Twist: Möbius [4n]Annulenes

“…The best one, 14 , is only 6.2 kcal/mol higher in energy than 13 and exhibits significant aromatic character (Table ). The relative energies of 13 − 16 correlate with the average deviations of the CCC angles from the strain-free 124.3° value . In accord with Heilbronner's expectation, 14 has the least amount of angle strain and (with the exception of 11 ) the greatest degree of bond-length equalization of all the Möbius annulenes reported here.…”

“…Clearly aromatic stabilization is not great enough to overcome the strain associated with the different conformations. In 1991, Hernando and co-workers found that “valence angle strain” determined the relative AM1 stabilities of the known [16]annulenes …”

Isotopic Perturbations in Aromatic Character and New Closely Related Conformers Found in [16]- and [18]Annulene

“…Confirming the intuitive observation, it was found that Hernando et al, using semiemperical calculations, had already predicted the existence of these isomers of [16]annulene. The aforementioned authors further indicate that the two isomers have slightly different energies, the most stable of each pair having relatively more ideal bond angles . As the temperature of the [16]annulene system is lowered, the energetically allowed geometry of the σ-framework approaches ideality (120° C−CC bond angles).…”

“…From the 1 H NMR data, it is apparent that in both the [16]- and [18]annulenes, there must exist such lower-energy structures having σ-frameworks that enforce greater π-delocalization and thus greater ring current (paratropic in the 4 n π-electron system and diatropic in the 4 n + 2 π-electron system).

3 The conformers (A, B, C, and D) for [16]annulene that were proposed by Hernando are shown (ref ). The plus signs indicate that the proton extends forward from the plane of the molecule, while a minus signifies that it extents back into the page.

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“…The conformers (A, B, C, and D) for [16]annulene that were proposed by Hernando are shown (ref ). The plus signs indicate that the proton extends forward from the plane of the molecule, while a minus signifies that it extents back into the page.…”