Semiempirical calculation of electronic spectra of organic compounds by using the improved method of new- γ electron repulsion integral

Abstract

Electronic spectra of organic compounds were calculated by incorporating the improved method of New- γ for the two-center electron repulsion integral into two semiempirical molecular orbital methods (CNDO/S and INDO/S). In this method we improved the previously reported New- γ approximation by determining a parameter k for each bond. This method significantly improved the calculated wavelengths of linear polyenes obtained by using New- γ. We further improved this by determining an expression for a variable k in which k is a function of the bond length of C–C. This variable method well reproduced experimental electronic spectra of linear polyenes, toluene, xylene, styrene, and polyacenes. These results show that this variable method can be applied to the C–C bond in other organic compounds, such as polycyclic aromatic hydrocarbons (PAHs).