Chemical origin of red shift of CO stretching vibration in acetone complexes with various metal cations

The structures, vibrational frequencies and interaction energies of acetone and its complexes with various metal cations (Li+, Na+, K+ , Rb+ Cs+, Mg+, Ca+, Al+, Mg2+, and Ca2+) have been determined using the hybrid three-parameter B3LYP density functional method. For all the studied complexes, the carbonyl (CO) stretch vibration is red-shifted by 8-181 cm−1 with respect to the CO vibrational frequency (1731 cm−1) in acetone. Our calculated shifts are in good agreement with available experiments. The chemical origin of the direction and the trend of the CO vibration shift was analyzed by using the nature bond orbital (NBO) theory. Lee's explanation [J.H. Song, J. Kim, G. Seo, J.Y. Lee, J. Mol. Struct. (Theochem) 686 (2004) 147], based on the electrostatic nature of the interactions, fails in some complexes. Instead, our explanation using electron density redistribution and rehybridization is more efficient and reasonable.