Effect of solvent molecules on negative charge development on α-carbon of Li-enolate from acetaldehyde: A computational study

Tetrameric Li-enolate (TLE) molecules from acetaldehyde are studied by attaching solvent (guest) molecules like benzene, trimethylamine (TMA), tetrahydrofuran (THF), dimethyl ether (DME), ethanol and water to one of the Li atoms in order to examine the development of negative charge on the α-carbon of the enolate and stabilization of the TLE-guest complex. The density functional calculations at the B3LYP/6-311++G** level suggest that the charge on the α-carbon (-0.5 a.u.) is almost the same for most solvents that form σ-bonded TLE-guest complex, and is larger for a π-bonded TLE-benzene complex (-0.7 a.u.). Interestingly, each enolate unit in the TLE-complex has primarily a CC double bond and a CO single bond, and contradicts what one expects from a valence bond resonance structure that has a negative charge on the α-carbon.