Characterizing Complexes with F-Li•••N, H-Li•••N, and CH3Li•••N 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 center dot center dot center dot N bonds with F-Li, H-Li, and CH3Li as the Lewis acids. Complexes of these acids with the nitrogen bases N-2, HCN, 1,3,5-triazine, pyrazine, 1,2,3-triazine, pyridine, and NH3 have linear X-Li center dot center dot center dot 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 center dot center dot center dot N bonds with each acid. Except for complexes with N-2, 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 (1)J(F-Li) may increase or decrease upon complexation, but (1)J(H-Li) and (1)J(C-Li) decrease significantly. These coupling constants have their smallest values in complexes with nonlinear X-Li center dot center dot center dot N bonds. No correlations appear to exist between (1)J(X-Li) and the X-Li distance and (1li)J(Li-N) and the Li-N distance. Values of the two-bond coupling constants (2li)J(X-N)are extremely small. Comparisons of (2li)J(F-N) With (2h)J(F-N) for coupling across a hydrogen bond and (2x)J(F-N) for coupling across a halogen bond suggest that the extremely small values of (2li)J(X-N) are not due to long X-N distances but to the low valence electron density on Li in lithium-bonded complexes.