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

Abstract

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.