Theoretical and structural studies of lithium cyclic amide conformations. Monomers and aggregates

High level ab initio calculations on the conformations of unsolvated and so lvated lithium piperidide, 1, and lithium morpholide, 2, were carried out. It was found that both monomers exhibit a global minimum for a chair struct ure with a planar nitrogen, and 2 shows an additional stable pseudo boat co nformation. Dimers and amine-lithium amide mixed aggregates were also calcu lated including discrete solvation; the role of aggregation is clearly show n both by the changes in geometries and in the stabilization energies. Semi empirical calculations carried out on a recently synthesized tetrameric mix ed aggregate give a geometry very similar to the structure determined by X- ray diffraction. The present calculations very usefully confirm the likelih ood of mixed aggregates of morpholine-lithium morpholide predicted by the c arbonylation reactions and not attainable in solid forms.