The Carbon−Lithium Electron Pair Bond in (CH3Li)n(n= 1, 2, 4)

Abstract

The monomer, dimer, and tetramer of methyllithium, (CH 3 Li) n ( n = 1, 2, 4), have been studied with use of density-functional (DFT) and conventional ab initio theory. The energy gain Δ E associated with the formation of (CH 3 −Li) n from n Li • and n CH 3 • 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 CH 3 Li(g) is calculated to be 29.2 kcal/mol, using experimental Δ H f values for CH 3 • (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 CH 3 Li, we find a strongly polar C−Li electron pair bond in which charge is donated from Li 2s to the CH 3 2a 1 SOMO. The covalent character is indicated by 2s ± 2a 1 mixing and a sizable lithium 2p z participation. In (CH 3 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 A 1 symmetry; (2) a strongly polar electron pair bond between the corresponding triply degenerate Li−Li and C−C antibonding fragment orbital sets in T 2 symmetry. The situation is similar for (CH 3 Li) 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 CH 3 Li, (CH 3 Li) 2, and (CH 3 Li) 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.