Density functional theory study on lactides: Geometries, IR, NMR and electronic spectra

Density functional theory (DFT) has been used to determine the equilibrium geometries, IR, total energies and NMR chemical shifts of various kinds of lactides. According to the calculations, besides l-lactide and d-lactide, three kinds of meso-lactides have been obtained: meso-i-lactide, meso-α-lactide and meso-β-lactide. The latter two are enantiomers. It indicates that meso-i-lactide with a symmetrical center is a transition state connecting meso-α-lactide and meso-β-lactide, with activation energy only about 6.77 kJ/mol. It is assumed that meso-lactide prepared by previous experiments may be a racemate of meso-α-lactide and meso-β-lactide. The energy of l(or d)-lactide is predicted to be about 4.83 kJ/mol lower than meso-α(or β)-lactide. Time-dependent DFT calculations show that the strongest absorption peaks of l(or d)-lactide and meso-α(or β)-lactide are at about 155 and 158 nm, respectively.