Quantum-chemical calculations on model ester molecules have been performed to study the interactions of carbonyl groups with lithium cation. The preferred conformations of the complexes and their stabilization energies have been determined. The largest complexation energy has been obtained for Li+ binding to four carbonyl groups. The vibrational frequency calculations have been used to predict the changes in the IR spectrum of polyester upon Li+ complexation. For the most stable complex the red-shift of about -29 to -24 cm(-1) in the frequency of the C = O stretching mode has been calculated. in a good agreement with the experimental value