Theoretical study of the agostic bond in Me _2Al(tBu _2pz) _2Li(THF)

摘要

A detailed theoretical study of the agostic bond CH?Li in Me _2Al(tBu _2pz) _2Li(THF) is performed at the HF and B3LYP levels. The interaction is characterized using structural, energetic, and spectroscopic information which shows an unusually short Li-H distance but a very small barrier for internal rotation of the methyl group indicating the weak nature of this interaction. The HF calculation is shown to be sufficient in reproducing the main geometrical and energetic parameters characterizing the agostic interaction pointing out to the importance of electrostatic/overlap effects over dispersion forces. This is confirmed by calculations using the M062X functional which has been shown to perform much better than B3LYP for noncovalent interactions. Topological analysis of the electron density shows a bond path between the Li and the H atoms bent strongly toward the C atom, the first ever reported for agostic interaction involving lithium. The bond is characterized as mainly ionic where analysis of the Kohn-Sham orbital components shows that the contribution arises largely from the 2s orbital of the lithium atom and the 1s of the hydrogen atom while the C atom plays a minor role with its interaction with Li secondary via the CH bond.