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

Conclusions

• 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 C_β atoms, corresponding to the concept of “interaction through space” (Li...C_β).

• 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.