Quantum-chemical study of the structure and electronic absorption spectrum of allyllithium

Abstract

1. The MNDO method has been used to calculate the geometrical parameters, the electronic structure, and the UV spectrum of π-allyllithium and of the hypothetical σ-allyl structures. It is shown that the diffuse nature of the 2s AO of Li is the most important factor which makes the π-complex the stable form. 2. The observed long-wave absorption band of the π-complex is attributed to the electronic transition π → π* in the allyl moiety; this assignment does not extend to the spectra of the allyl forms, where absorption in the long-wave region is probably due to the transition a σ → σ*, localized mainly on the Li-C bond. 3. In the gauche conformer of σ-allyllithium (in contrast to allylmercury) there should be a considerable overlap between the AO of the Li and Cβ atoms, corresponding to the concept of “interaction through space” (Li...Cβ). 4. The redistribution of electron density from the Li atom to the π-orbitals should be strengthened on passing from the planar s-trans-σ-allyl structure to the gauche conformer, and then to the π-complex.