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-Bu4Li4 shows significant
contribution to the hyperconjugative interaction from Cβ−H bonds as well as Cα−Cβ 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.
The reactions of CH(2), CHCl, and CCl(2) with cyclopropane, 1, have been examined computationally. In all cases the lowest energy reaction between the carbene and 1 is predicted to be C-H insertion. In the reaction of CH(2) with 1, the transition state for C-C insertion leading to cyclobutane is 1.7 kcal/mol higher in enthalpy than the transition state for C-H insertion at the G3B3 level. A pathway higher in energy than C-H insertion in the reactions of CHCl and CCl(2) with 1 involves two-bond cleavages generating ethylene along with chloro and dichloroethylene, respectively.
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