Consider the two structures shown for kekulene, one...

Consider the two structures shown for kekulene, one suggesting inner and outer annulenes and the other a series of phenanthrene-like units. ${ }^{1} \mathrm{H}$-NMR and bond length data are given. The NICS values are calculated as $-4.3$ for the angularly fused rings and $-10.8$ for the linearly fused rings. Indicate properties that you would expect to be associated with each structure. Do you consider the properties to be more consistent with the double-annulene or the phenanthrenelike structures?

Key Concepts

Aromaticity and NICS Values

Aromaticity is a fundamental concept in organic chemistry referring to the delocalization of electrons in cyclic systems, which leads to enhanced stability. Nuclear independent chemical shift (NICS) values are computed magnetic properties used as indicators of aromaticity, where more negative values suggest greater aromatic character. In this context, the differing NICS values hint at a variation in the extent and localization of aromatic stabilization within the molecule.

1H NMR Chemical Shifts

1H NMR spectroscopy provides information about the electronic environment of hydrogen atoms. In aromatic systems, the ring current induced by circulating electrons causes deshielding, resulting in characteristic downfield chemical shifts. Analysis of these shifts helps infer the presence and degree of aromaticity in different parts of the molecule.

Bond Length Analysis

Bond lengths in conjugated systems reveal details about electron delocalization. In aromatic compounds, a uniform bond length suggests resonance stabilization, whereas significant alternation indicates localized double bonds. Comparing bond length data with expected values aids in determining whether electron delocalization is more consistent with a globally aromatic system or a series of localized aromatic units.

Double-Annulene and Phenanthrene-like Structural Models

Different structural interpretations, such as the double-annulene model and the phenanthrene-like model, emphasize either a global continuous conjugation (as in two interconnected annulenes) or localized aromatic units (each resembling a phenanthrene unit). The double-annulene model would imply distinct inner and outer aromatic circuits, while the phenanthrene-like model suggests aromatic stabilization in smaller, localized segments. The combined spectroscopic and computational data, including NICS, 1H NMR, and bond lengths, help determine which model better represents the molecule's real electronic structure.

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