Calculations of derivatives of porphin and porphyrazine with an annulated five-membered heteroaromatic ring using a modified INDO method |
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Authors: | D I Volkovich V A Kuzmitsky P A Stuzhin |
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Institution: | (1) B. I. Stepanov Institute of Physics, National Academy of Sciences of Belarus, 68 Nezavisimosti Ave., Minsk, 220072, Belarus;(2) Ivanovo State University of Chemical Technology, Ivanovo, Russia |
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Abstract: | Quantum-chemical calculations for series of porphin (H2P) and porphyrazine (H2PA) derivatives whose molecules contain a 3,4-annulated five-membered heteroaromatic ring of pyrrole, furan, and thiophene
as well as 1,2,5-triazole, 1,2,5-oxadiazole, and 1,2,5-thiadiazole have been carried out by the AM1 and INDO/Sm methods (m
means “modified parametrization”). As follows from the unrestricted Hartree-Fock AM1 calculations, the ground state energy
of the a isomer is lower than that of the b isomers for all compounds, the a and b isomers being NH isomers with the additional five-membered ring fused to a pyrrolenine or pyrrole ring, respectively. Condensation
of the five-membered ring for the b isomers of the H2P derivatives is shown to extend the main conjugation path from 18-membered to 21-membered cyclopolyene. However, the 18-membered
cyclopolyene for the a isomers of the H2P derivatives and 16-membered cyclopolyene for both isomers of the H2PA derivatives prove to be isolated from the five-membered ring carrying six π-electrons. Based on INDO/Sm computations of
electronic absorption spectra of the H2P and H2PA derivatives, it is shown that the Qx level of the a isomers is sensitive to the nature of the annulated ring whereas the Qy level is practically not shifted. On the contrary, the Qx level of the b isomers is relatively insensitive to the nature of the annulated ring while the Qy level decreases monotonously in the H2P and H2PA series, being the first excited state level for the majority of the derivatives (X is along the NH-HN axis; Y, perpendicular
to it). The electronic absorption spectra at the border of the visible and near-UV regions are determined mainly by two high-intensity
G → Bx and G → By transitions for the H2P derivatives. However, four intense G → Bx, G → By, G → Nx, and G → Ny transitions are characteristic of the spectra of the H2PA derivatives.
Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 5, pp. 606–622, September–October, 2008. |
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Keywords: | quantum-chemical calculations AM1 and INDO/Sm methods porphyrins porphyrazines five-membered heteroaromatic rings excited electronic states |
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