Precise PPP molecular orbital calculations of excitation energies of polycyclic aromatic hydrocarbons. Part 1. On the correlation between the chemical softness and the absolute hardness

By considering the character of π-conjugated systems of polycyclic aromatic hydrocarbons (PAHs), it has become apparent that a new two centre electron repulsion integral, new-γ, which takes the chemical softness of a π-conjugated system into account is useful for the Pariser–Parr–Pople molecular orbital (PPP MO) calculations of the excitation energy of the p-band of PAHs. Employing the correlation between the chemical softness and the absolute hardness, the method to evaluate the chemical softness parameter k was established. The calculated excitation energies for 38 PAHs were greatly improved compared with the calculated ones using the conventional Nishimoto·Mataga γ(N·M-γ) function, and were well correlated with the observed ones. This is the first example of precise PPP MO calculations of excitation energies of various PANS using new-γ.