Abstract
The behavior of the bond orders in symmetric model polymethine dyes has been investigated by the HMO, PPP, and CNDO/2 methods. It has been established that the greatest equalization of the bonds is achieved in the case of dyes with moderately basic terminal groups. In other cases, alternation of the bond orders, which diminishes with increasing length of the polymethine chain and attenuates with increasing distance from the end, has been discovered at the ends of the polymethine chain. The amplitude of the alternation of the bond orders in a polymethine chain is greater, the more the basicity of the terminal groups, as characterized by the electron-donor parameter Φo (0 ⩽ Φo ⩽ 90°), deviates in either direction from the mean (Φo = 45°). It is considerably smaller in the first excited state than in the ground state. The π-electronic approximation is a correct method for describing the electronic structure of a polymethine chain. The neglect of the σ electrons can distort the bond orders of the terminal groups.
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Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 2, pp. 138–146, March–April, 1985.
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Dyadyusha, G.G., Repyakh, I.V. & Kachkovskii, A.D. Quantum-chemical investigation of bond orders in symmetric polymethine dyes. Theor Exp Chem 21, 131–139 (1985). https://doi.org/10.1007/BF00945186
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DOI: https://doi.org/10.1007/BF00945186