Quantum Research on the Aromaticity of the Polycyclic Aromatic Hydrocarbons

The molecular aromaticity and the loss of conjugation in the polycyclic aromatic hydrocarbons (PAHs) are investigated by using the restricted Hartree Fock and multi-reference configuration interaction method. As usual the aromaticity of the PAHs may be defined as the average two-center indices (ATI) obtained from the electron delocalization. It is shown that the ATI values of the outer symmetry-unique rings are greater than those of the inner ones both in the cata-condensed and peri-condensed PAHs in the ground states. However, in the excited states the outer ring is greater (smaller) than that of the inner ring for the cata-condensed (peri-condensed) PAHs. By comparing the aromaticities of the PAHs in the ground states and in the excited states, we can find that the ATI values in the ground states are larger than those in the excited states for the cata-condensed PAHs. However, for the peri-condensed PAHs, the aromaticities of the outer rings in the ground states are larger than those in excited states, and those of the inner rings in the ground states are smaller than those in excited states. In addition, it has been found that the ATI value of the benzene aromaticity will be very close to zero when it is in the superimposed state composed of the ground state, and the third and fourth excited states. This means that in this state the benzene molecule shows the loss of its conjugation. This work should be an useful reference for preparing the nonaromatic superposed states for the PAHs to realize the application in the industrial fields of PAHs cracking.