DOI:10.1021/jp9035919

Characterizing Complexes with F-Li...N, H-Li...N, and CH3Li.. Lithium Bonds: Structures, Binding Energies, and Spin-Spin Coupling Constants

Ab initio calculations have been carried out to determine the structures, binding energies, and spin−spin coupling constants of complexes stabilized by X−Li···N bonds with F−Li, H−Li, and CH₃Li as the Lewis acids. Complexes of these acids with the nitrogen bases N₂, HCN, 1,3,5-triazine, pyrazine, 1,2,3-triazine, pyridine, and NH₃ have linear X−Li···N bonds. Methylamine forms a nonlinear lithium bond only when F−Li is the lithium donor. Two bases, HN═CH₂ and aziridine, form nonlinear X−Li···N bonds with each acid. Except for complexes with N₂, which have small binding energies of about 5 kcal/mol, the binding energies of lithium-bonded complexes are appreciable, varying between 15 and 23 kcal/mol. The one-bond coupling constant ¹J(F−Li) may increase or decrease upon complexation, but ¹J(H−Li) and ¹J(C−Li) decrease significantly. These coupling constants have their smallest values in complexes with nonlinear X−Li···N bonds. No correlations appear to exist between ¹J(X−Li) and the X−Li distance and ^1liJ(Li−N) and the Li−N distance. Values of the two-bond coupling constants ^2liJ(X−N) are extremely small. Comparisons of ^2liJ(F−N) with ^2hJ(F−N) for coupling across a hydrogen bond and ^2xJ(F−N) for coupling across a halogen bond suggest that the extremely small values of ^2liJ(X−N) are not due to long X−N distances but to the low valence electron density on Li in lithium-bonded complexes.