Antiaromatic 1376 INDEX

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>. 

Proton chemical shifts are also a potential index of aromaticity. Thus, in pyrazine 7 the protons resonate at S = 8.6 ppm, whereas in its antiaromatic 162 and nonaromatic 163 derivatives the double bond protons show signals at 4.64 and 5.38 ppm, respectively <1983AGE171>. There is qualitative agreement with the ring current influencing... 

Other theoretical tests of aromaticity have been studied. The graph theory has predicted thiepine to be antiaromatic <76JA2750>. A new measure of aromaticity has been studied on the basis of absolute hardness >]) known to be an index of stability and reactivity of a molecule. The operation formula is the equation, tj = (/-/l)/2 where / is the ionization potential and A the electron affinity. Alternatively, the equation is rj = (Fhomo- lumo)/2. Absolute hardness has also predicted thiepine (1) and 2-benzothiepine (5) to be antiaromatic and 3-benzothiepine (6) to be nonaromatic. However, the prediction by this approach fails for 1-benzothiepine (2) <89JA737l>. 

It is thus possible to expect that the comparison of corresponding pair densities for individual electron states of the antiaromatic reference structure via the similarity index will make it possible to identify the electron state of the reference species which approximates the structure of the transition state the most closely. The values of the second order similarity index calculated for a series of several forbidden pericyclic reactions are collected in Table 10. Let us discuss now at least some conclusions which can be deduced from this Table. The most interesting in this... 

More recently, a new and widely used index of aromaticity, the nucleus-independent chemical shift (NICS), has originally been proposed by Schleyer and coworkers [44,45]. It is defined as the negative value of the absolute shielding computed at a ring center or at some other interesting points of the system. Negative NICS values correspond to aromaticity, while positive values are associated with antiaromaticity. There is a whole family of NICS methods, which include NICS computations at ring centers [43], NICS(O) and above [46], NICS(I) and NICS(2), dissected NICS values, i.e., the total NICS at a particular point in space may be dissected into paratropic and diatropic components, which mainly arise from the C-C cr and jt multiple bonds, respectively [46,47] and MO contributions to NICS,... 

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