DFT study of the effect of sigma-ligands on the structure of ester enolates in THF, as models of the active center in the anionic polymerization of methyl methacrylate.

A Density Functional Theory (DFT) study was carried out on structures of the lithium ester enolate of methyl isobutyrate (MIB-Li) in THF solution, in the presence of TMEDA, dimethoxyethane (DME), crown ether 12-crown-4, and cryptand-2,1,1, as electron donor ligands (sigma-ligands). Both specific solvation with THF and/or ligand molecules and nonspecific solvation by the solvent continuum were taken into account. The possibility of ligand-separated ion pair formation was analyzed for each of the ligands, including THF alone. In most cases peripherally solvated dimers are the most stable species. Only in the presence of cryptand-2,1,1 was a ligand-separated triple ion pair, (MIB-Li-MIB)(-)(THF)(2),Li(2,1,1)(1)(+), shown to be comparable in stability to the THF-solvated dimer, (MIB-Li)(2)(THF)(4). These results are in agreement with experimental NMR data on the structure of MIB-Li in the presence of DME, 12-crown-4, and cryptand-2,1,1. An upfield shift of the (13)C NMR signal of the alpha-carbon of MIB-Li observed in the presence of cryptand-2,1,1, originally attributed to a ligand-separated monomer, MIB(-),Li(2,1,1)(+), was well reproduced by Hartree--Fock calculated NMR shifts for the predicted ligand-separated triple ion pair.