Quantum-chemical study of the structure, aggregation, and NMR shifts of the lithium ester enolate of methyl isobutyrate

Ab initio HF-SCF calculations of the equilibrium geometries, energies, and C-13 NMR shifts of various monomeric, dimeric, and tetrameric aggregates of the lithium ester enolate of methyl isobutyrate (MIB-Li) were performed. It is shown that an enolate-like nonplanar structure of the MIB-Li monomer, in which a pi-interaction between the Li atom and C=C double bond is realized, is more stable than the planar enolate-like structure, The energies of the two MIB-Li dimers consisting of the planar and nonplanar MIB-Li monomers, respectively, were found to be nearly the same. However, the experimental C-13 NMR shifts for the MIB-Li dimer in THF agree with those calculated for the planar stereoisomer. For the MIB-Li tetramer, two structures were found, namely, a cubic and a nearly flat eight-membered-ring structure. The calculated C-13 NMR shifts for the two tetramers are very similar, but the cubic tetramer is considerably more stable than the flat one at the highest level of theory. Semiempirical MNDO and PM-3 calculations of both specific and nonspecific solvation energies of MIB-Li aggregates solvated by THF were carried out. It was found that the solvation effects decrease in the series monomer > dimer > tetramer. The calculated total solvation energies for MIB-Li aggregates are used to estimate the solvent effects on the ab initio relative energies of the aggregates.