Density Functional Calculations of Methyllithium nt Butyllithium and Phenyllithium Oligomers Effect of Hyperconjugation on Conformation

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Oligomers of methyllithium and tert -butyllithium (R n Li n, n = 1−4; R = Me, t -Bu) as well as phenyllithium (Ph n Li n, n = 1,4) have been studied by using density functional theory (DFT). Possible conformers of methyllithium and tert -butyllithium oligomers were optimized at the B3LYP/6-31+G* level, and relative energies were evaluated at the B3LYP/6-311+G(2d,p)+ZPC//B3LYP/6-31+G* level. Optimized geometric parameters of MeLi and t -BuLi tetramers are in good agreement with available experimental and previous computational results. Atomic charges from natural population analysis (NPA) indicate that Li−C bonds show dominant ionic character for methyl, tert -butyl, and phenyllithium oligomers. Comparison of atomic charges among the oligomers indicates that lithium charges are almost independent of the size of the oligomer or the identity of the ligand. NBO second-order perturbation energy analyses for the T d geometries of methyllithium and tert -butyllithium tetramers indicate that the hyperconjugative interaction (σ(C−H) → σ*(Li)) favors the eclipsed conformer relative to the staggered conformer. In particular, t -Bu 4 Li 4 shows significant contribution to the hyperconjugative interaction from C β −H bonds as well as C α −C β bonds. On the other hand, the phenyllithium tetramer prefers a staggered orientation of the phenyl ring to the C−Li bond due to similar hyperconjugative interactions in both orientations. Aggregation energies, computed at the B3LYP/6-311+G(2d,p)+ZPC//B3LYP/6-31+G* level, for the tetramers of methyllithium, t- butyllithium, and phenyllithium are −124.4, −108.6, and −117.2 kcal/mol, respectively.