Nonaromatic molecules

The presence of a ring-current is characteristic of all Hiickel aromatic molecules and is a good test of aromaticity. For example, benzene, a six-7r-clectron aromatic molecule, absorbs at 7.37 5, but cyclooctatetraene, an eight--iT-electxon nonaromatic molecule, absorbs at 5.78 8. 

The aromaticity of 1,2,4-triazoles has been investigated and quantified using the harmonic oscillator model of aromaticity (HOMA) index, where a value of 1 is assigned to a molecule that is fully aromatic, 0 for a nonaromatic molecule, and a negative value for a molecule that is antiaromatic the data obtained were compared to other small-molecule heteroaromatics. It was determined that different tautomers of substituted and unsubstitued 1,2,4-triazoles have individual HOMA indices <2000JST(524)151>. 

For convenience in application we have obtained both atom and bond values. In the atom approach we have assumed that an atom in a particular bonding situation (particular hybridization) will always contribute the same amount to the molecular susceptibility. This contribution consists of the three principal components as shown in Table 8 under atom susceptibilities. To evaluate the molecular susceptibility, the atom or bond values in Table 8, which are principal values, are rotated into the principal inertial axis system (a, b, and c) of the molecule. The atom and bond susceptibilities were determined by least squares fitting the experimental molecular susceptibility components of the 14 common nonstrained, nonaromatic molecules shown in Table 7. 

Linus Pauling and E. Hiickel tried to explain aromaticity using quantum mechanics. Within a semiemperical framework Hiickel provided a rationale for the extra stability of benzene through his famous (4n + 2) Jt-electron rule. Any planar monocyclic system with (4 + 2) ti electrons are more stable than the corresponding nonaromatic molecules. Consequently, a similar system with 4n (ny O) number of n- electrons is more reactive... 

Hgure 15-17 The molecular stmcture of 1,3,5,7-cyclooctatetraene. Note the alternating single and double bonds of this nonplanar, nonaromatic molecule. 

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