Correlation between the Vibrational Frequencies of the Carboxylate Group and the Types of Its Coordination to a Metal Ion:  An ab Initio Molecular Orbital Study

The structures and vibrational frequencies of the acetate ion interacting with a metal ion (Na +, Mg 2+, and Ca 2+ ) in the unidentate, bidentate, bridging, and pseudobridging forms are studied by ab initio molecular orbital calculations. Effects of a water molecule coordinating to either the acetate ion or the metal ion are also examined. The calculations are carried out by using the self-consistent reaction field method at the Hartree−Fock level with the 6-31+G** basis set. For the species interacting with a divalent metal cation, the lengths of the two CO bonds of the acetate ion are nearly equal in the bidentate form but are significantly different in the unidentate form. The frequency of the COO - antisymmetric stretch of the unidentate species is higher than that of the ionic species, which is in turn higher than that of the bidentate species. The reverse is the case for the COO - symmetric stretch. As a result, the frequency separations (Δν a - s ) between the COO - antisymmetric and symmetric stretches for the unidentate, bidentate, and ionic species are in the following order: Δν a - s (unidentate) > Δν a - s (ionic) > Δν a - s (bidentate). It is demonstrated that such a correlation between the vibrational frequencies of the COO - group and the types of its coordination to a divalent metal cation is related to changes in the CO bond lengths and the OCO angle. The results of the present study clarify the physical basis of the empirical structure−frequency correlation, which has been used in the analysis of the infrared spectra of Ca 2+ -binding proteins.