The carbon-lithium electron pair bond in (CH₃Li)_n (n = 1, 2, 4)

The monomer, dimer, and tetramer of methyllithium, (CH₃Li)_n (n = 1, 2, 4), have been studied with use of density-functional (DFT) and conventional ab initio theory. The energy gain AE associated with the formation of (CH₃-Li)_n from n Li^• and n CH₃^• radicals is -45.5, -132.7, and -308.6 kcal/mol for n - 1, 2, and 4 using nonlocal density-functionals and a large, doubly polarized triple-ζ STO basis (NL-SCF/TZ2P). The corresponding dimerization and tetramerization energies for methyllithium are -41.7 and -126.6 kcal/mol, respectively. The 298 K heat of formation of CH₃Li(g) is calculated to be 29.2 kcal/mol, using experimental ΔH^f values for CH₃^•(g) and Li^•(g). The low-energy lithium 2p orbitals are shown to play an active role in the bonding of the methyllithium aggregates and can be viewed as valence orbitals. A detailed analysis of the carbon-lithium bonding mechanism highlights the significant role of covalent contributions. In CH₃Li, we find a strongly polar C-Li electron pair bond in which charge is donated from Li 2s to the CH₃ 2a₁ SOMO. The covalent character is indicated by 2s ± 2a₁ mixing and a sizable lithium 2p_z participation. In (CH₃-Li)₄ the carbon-lithium bond is provided by two distinct orbital interactions: (1) an essentially covalent electron pair bond between the strongly sp hybridized Li-Li and C-C bonding fragment orbitals of the lithium cluster and the methyl cage, respectively, in A₁ symmetry; (2) a strongly polar electron pair bond between the corresponding triply degenerate Li-Li and C-C antibonding fragment orbital sets in T₂ symmetry. The situation is similar for (CH₃Li)₂. The electron density is analyzed using atomic charges from the following: (1) the natural population analysis (NPA); (2) the Hirshfeld method; (3) the Mulliken method as well as a modification which we term Modified Mulliken; (4) a scheme which we designate Voronoi deformation density (VDD); the VDD charges monitor the shift of electron density out of (Q > 0) or into (Q < 0) the Voronoi cell of an atom upon formation of the molecule from the isolated atoms. The degree of ionicity of the carbon-lithium bond decreases from ca. "50" down to "30%" along CH₃Li, (CH₃Li)₂, and (CH₃Li)₄, according to the Hirshfeld charges. This agrees with a similar trend emerging from the VDD charges as well as with the results of the electronic structure analysis. The NPA charges suggest that the carbon-lithium bond is ca. "90%" ionic and that the degree of ionicity is independent of the size of the aggregate.