The Carbon−Lithium Electron Pair Bond in (CH₃Li)<i>_n</i>(<i>n</i>= 1, 2, 4)

Abstract

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 ΔE 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 ΔHf 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 2pz participation. In (CH3Li)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.