The Relation Between the Bond Angle and Carbon-Oxygen Stretching Frequencies of Carboxylato Complexes

The symmetric (νs) and antisymmetric (νas) OCO stretching modes of carboxylate containing compounds encode structural information that can be difficult to decipher due to the sensitivity of these spectral features to small shifts in charge distribution and structure, as well as the anharmonicities of these two vibrational modes. In this work, we discuss the relation between the frequency of these modes and the geometry of the carboxylate group, showing that the splitting between νs and νas (νs-a) can be predicted based only on the OCO bond angle obtained from quantum chemical calculations with reasonable accuracy (± 58 cm-1, R2 = 0.992). The relationship is shown to hold for infrared spectra of carboxylato groups in a variety of molecules measured in vacuo. The physical origins of this model are discussed in the framework of normal mode analysis.