Quantum-chemical study of allylic compounds with two bonded heavy atoms

Abstract

It has been shown by the CNDO method that the bathochromic shift of the long-wave absorption band in the transition from allylstannane to compounds of the type C=C-C-Sn-X and C=C-Sn-X (where X is a heavy atom) is connected with the formation of a low-energy vacant σ*_S-X orbital, localized mainly in the region of the Sn-X chemical bond, and of an occupied σ_Sn-X orbital, the energy of which is somewhat higher than of the σ_C-Sn orbital. The dependence of the position of the long-wave absorbance region on conformation is related to the fact that, in planar and nonplanar conformers, the long-wave transitions are of a different type (σ → σ* and π → σ*, respectively); the$E_{\sigma \to \sigma ^ * } trans---E_{\pi \to \sigma ^ * }$ bathochromic shift is determined to a large degree by the difference in the energies of the highest occupied MO (ɛ_π - ɛ_σ) in the s-trans form. In the nonplanar conformers the heavy atom orbitals interact with the π-orbital of the ethylene moiety through the bridge group; this leads to a significant delocalization of the HOMO and to a considerable change in its energy. On the other hand, their interaction with the π*-orbital in compounds of the C=C-C-Sn-X type is very low and does not favor the delocalization of lower vacant MO. In vinyldistannane the π*-orbital is noticeably delocalized, due to the interaction with the π*_Sn-Sn orbital in planar and with the σ*_Sn-Sn orbital in nonplanar conformers.