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 n-conjugated systems of polycyclic aromatic hydrocarbons (PAHs), it has become apparent that a new two centre electron repulsion integral, new-y, which takes the chemical softness of a n-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 NishimotoeMataga y (NeM-y) function, and were well correlated with the observed ones. This is the first example of precise PPP MO calculations of excitation energies of various PAHs using new-y.Paper 4/07756C

Chiroptical study and conformation analysis of helical polymers surrounded by helical hydrogen-bonding strands

“…It has been reported that the l max of conjugated compounds predicted by the INDO/S calculation tends to be shorter than the experimental value as shown in Fig. 6(A) [20]. The difference between the predicted and observed l max of polyacetylenes reaches 30-40 nm at the degree of polymerization is 20.…”

Molecular Design of Polycyclic Aromatic Hydrocarbons Based on Precise PPP Molecular Orbital Calculations.

“…We defined acenes as aromatic sextet resonance system (ASKS) based on the essential character of their mobile ~t-electrons [7]. On the contrary, the absorption wavelengths of the p-band of zig-zag phenes (phenanthrene, chrysene, picene, and so on) shift toward longer wavelengths more slightly compared with those of acenes, along with the annellation.…”

“…On the contrary, the absorption wavelengths of the p-band of zig-zag phenes (phenanthrene, chrysene, picene, and so on) shift toward longer wavelengths more slightly compared with those of acenes, along with the annellation. We defined zig-zag phenes as aromatic sextet alternation system (ASAS), because their ac-electrons trend to localize at the particular positions of the molecular framework [7].…”

“…Nishimoto proposed a novel two center electron repulsion integral new-y, which takes the spectrochemical softness into account, for the PPP MO calculations [5]. The new-y was successfully applied for the precise calculations of absorption wavelengths of the p-band, which especially affects the visual color, of polycyclic aromatic hydrocarbons (PAHs) [6][7][8][9]. The calculated results using new-y were more accurate than the ones using the conventional Nishimoto • Mataga y (N • M-y) function [10].…”

Improved CNDO/S Calculation of Electronic Spectra of Organic Compounds. I. New CNDO/S Calculation by Using an Improved Method of One-Center Electron Repulsion Integral

“…Moreover, they reported that this error produced by the conventional PPP method could be greatly improved by the reevaluation of the twocenter electronic repulsion integral (New-g), thus introducing the concept of chemical softness (referred to as New-g PPP). [6][7][8][9][10][11][12] Furthermore, Nakayama and co-worker 13,14) also reported that the shortcomings of NM-g CNDO/S calculations are appreciably absent among the results of the modified CNDO/S and ZINDO (referred to as New-g CNDO/S and New-g INDO/S) calculations, which introduce a New-g using the concept of chemical softness for a number of polycyclic aromatic hydrocarbons. However, the New-g CNDO/S and New-g INDO/S methods cannot be applied to compounds other than p-conjugated aromatic hydrocarbons, because the New-g values have not yet been determined with respect to the p-conjugated compounds containing various hetero atoms, namely, there is a variable k-function for each bonds C-N, C-O, N-N, etc.…”