Structural aspects of aromaticity

Application of natural bond orbital analysis to delocalization and aromaticity in C-substituted tetrazoles

Energies of two tautomeric forms of 10 tetrazole derivatives substituted at C5 were established by DFT/B3LYP calculations carried out at the 6-311++G level. In each case the calculated energy of the 2H-tautomer was lower than that of the 1H. Furthermore, three geometric aromaticity indices of both forms were calculated, as were the values of nuclear independent nuclear shift and aromatic stabilization energy. The electronic properties were evaluated with the help of the natural bonding orbital theory. Following this a new pi-delocalization parameter, the root-mean square of pi-electron density localized on the atoms of the five-membered tetrazole ring, SDn, was introduced. It was concluded that the electronic delocalization can be described equally well by three different parameters: SDn, the extent of the transfer of electron density from the p(z) orbital of one nitrogen to the rest of the pi electron system, and population of two antibonding pi orbitals. Arguably, the information provided by the electronic parameters is similar to that contained in the geometric (structural) aromaticity indices except for tetrazole substituted by -BH(2).

The acenes: is there a relationship between aromatic stabilization and reactivity?

[reaction--see text] Despite the increasing reactivity from benzene to heptacene, the acene resonance energies per pi electron are nearly constant. The reactivities (computed activation energies) of the individual acene rings correlate with the reaction energies and depend on the product stabilities. Nucleus-independent chemical shifts (NICS; note the sizes of the red dots, above) indicate that the more reactive inner rings actually are more aromatic than the less reactive outer rings and even more aromatic than benzene itself.

Dissected Nucleus-Independent Chemical Shift Analysis of π-Aromaticity and Antiaromaticity

Analysis of the basic π-aromatic (benzene) and antiaromatic (cyclobutadiene) systems by dissected nucleus-independent chemical shifts (NICS) shows the contrasting diatropic and paratropic effects, but also reveals subtleties and unexpected details.