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 interact ing with a metal ion (Na+, Mg2+, and Ca2+) in the unidentate, bidentat e, bridging, and pseudobridging forms are studied by nb initio molecul ar orbital calculations. Effects of a water molecule coordinating to e ither the acetate ion or the metal ion are also examined. The calculat ions are carried out by using the self-consistent reaction field metho d at the Hartree-Fock level with the 6-31+G* basis set. For the speci es 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 a re significantly different in the unidentate form. The frequency of th e COO- antisymmetric stretch of the unidentate species is higher than that of the ionic species, which is in turn higher than that of the bi dentate species. The reverse is the case for the COO- symmetric stretc h. As a result, the frequency separations (Delta nu(a-s)) between the COO- antisymmetric and symmetric stretches for the unidentate, bidenta te, and ionic species are in the following order: Delta nu(a-s), (unid entate) > Delta nu(a-s) (ionic) > Delta nu(a-s) (bidentate). It is dem onstrated that such a correlation between the vibrational frequencies of the COO- group and the types of its coordination to a divalent meta l cation is related to changes in the CO bond lengths and the OCO angl e. 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 Ca2+-binding proteins.