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 th e lithium ester enolate of methyl isobutyrate (MIB-Li) in THF solution, in the presence of TMEDA, dimethoxyethane (DME), crown ether 12-crown-4, and c ryptand-2,1,1, as electron donor ligands (a-ligands). Both specific solvati on with THF and/or ligand molecules: and nonspecific solvation by the solve nt continuum were taken into account. The possibility of ligand-separated i on pair formation was analyzed for each of the ligands, including THF alone . In most cases peripherally solvated dimers are the most stable species. O nly in the presence of cryptand-2,1,1 was a ligand-separated triple ion pai r, (MIB-Li-MIB)(-)(THF)(2),Li(2,1,1)(1)(+), shown to be comparable in stabi lity to the THF-solvated dimer, (MIB-Li)(2)(THF)(4). These results are in a greement with experimental NMR data on the structure of MIB-Li in the prese nce of DME, 12-crown-4, and cryptand-2,1,1. An upfield shift of the C-13 NM R signal of the alpha -carbon of MIB-Li observed in the presence of cryptan d-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 pred icted ligand-separated triple ion pair.