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 agg regates of the lithium ester enolate of methyl isobutyrate (MIB-Li) we re 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-li ke structure, The energies of the two MIB-Li dimers consisting of the planar and nonplanar MIB-Li monomers, respectively, were found to be n early the same. However, the experimental C-13 NMR shifts for the MIB- Li dimer in THF agree with those calculated for the planar stereoisome r. 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 tetra mer 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 TH F were carried out. It was found that the solvation effects decrease i n the series monomer > dimer > tetramer. The calculated total solvatio n energies for MIB-Li aggregates are used to estimate the solvent effe cts on the ab initio relative energies of the aggregates.