The Carbon−Lithium Electron Pair Bond in (CH3Li)n(n= 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 do...