Annulenes definition

Our definition does not take account of lone pairs on heteroatoms in the ring which do not contribute to the -system we are normally concerned here with pyridinoid nitrogen atoms, and their influence on aromaticity is discussed in Section V,B. It is also noteworthy that for molecules possessing an unsubstituted pyrrole-type nitrogen atom, the aromaticity of the anion produced on deprotonation can be significantly greater than that of the protonated form. This has been found in a number of cases with aromatic annulenes,15-18 and we will meet similar results with certain mesoionic azapentalenes (Section IV,C,l,a). 

The frequency exaltation of the Kekule-type b2u modes of the electronically excited l B state is not limited to benzene and its derivatives. A similar observation was made by Michl and co-workers238 for [ 14]-annulene, who explained the phenomenon in terms of the avoided crossing of the Kekule structures similar to the above. Other hydrocarbons such as naphthalene, anthracene, etc. have been reported to exhibit the same phenomenon. Thus, in naphthalene,239240 the Kekule-type mode undergoes a frequency exaltation of 189 cm 1 in the 11 B2u state relative to the ground state. In anthracene, two Kekule-type modes exist. One was assigned and undergoes an upshift of 231 cm-1.241-243 The second anthracene mode has not been definitely assigned yet. It is calculated to be exalted by 96 cm-1.243... 

We will not concern ourselves here with the attempt to discern a true definition of aromaticity, nor with the largely unproductive " desire to rank-order relative aromaticity. Rather, we will focus on how computational chemistry has been used to help define two of the criteria of aromaticity. We will discuss how to compute the stabilization energy of an aromatic species and then how we can compute magnetic properties that can be related to aromaticity. We will then conclude with a discussion of how these measures have been employed to investigate the nature of annulenes and the MiUs-Nixon effect. 

It seems clear that definitive evidence of a synthesized aromatic Mobius annu-lene has not been produced. We note in passing that a few examples of aromatic Mobius hexaporphyrins have been reported. Castro and Kamey have suggested that an aromatic Mobius annulene has been prepared, but as a transition state. The optimized transition state 54 for the twist-coupled bond shifting process then connects the tri-frani-[12]annulene 53 with the di-fra i-[12]annulene... 

Although we cannot as yet converge on a desired enthalpy of formation of gaseous [18]annulene, it is quite apparent that this last number is suspect in terms of at least two acyclic paradigms for aromaticity. Recall the Dewar-Breslow definition for aromaticity and antiaromaticity of an [njannulene in terms of the corresponding acyclic polyene with n/2 double bonds. There is no experimental measurement of the enthalpy of formation of all-( )-l,3,5,7,9,ll,13,15,17-octadecanonaene, species 151. However, we should be surprised if this value seriously differed from that of nine ethylenes and 8(5) kJmol , the 5 kJ mol being taken as the enthalpy of reaction 19 for unstrained olefins and dienes . The enthalpy of formation of 151 is thus ca 513 kJmol . This is somewhat less than the value for [18]annulene and so we would conclude that the cyclic species is essentially nonaromatic . Alternatively, consider the series of acyclic polyenes, ethylene, 1,3-butadiene, 1,3,5-hexatriene, The gas phase enthalpies of formation are respec-... 

They have reached similar conclusions concerning the two isomeric [14]annulenes. Isomer A (p. 146) definitely has the structure 9 as previously assigned, but Isomer B probably has the configuration 71. 

According to the definition, one has to draw a distinction between the benzoderivatives of the valence isomers of annulenes (a restricted class), and the valence iscuners of benzoannulenes (a much larger dass). The difierence arises because in the former case m pairs of vertices of degree 4 must be adjacent for the m-benzoderivatives of the valence isomers of [n]-annulene, e.g. cme pair for the monobenzoderivaUve, while in the latter... 

All (4A7 -I- 2) annulenes clearly exhibit aromatic character, whereas 4 A7 annulenes are definitely antiaromatic, with negative HOMA. 

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