The interaction of carbon trioxide and carbonate on Pt(111) has been studied through the ab initio cluster model approach within the Hartree-Fock and hybrid density functional theory based methods. The equilibrium geometry of adsorbed CO3 and CO32- are found to be very similar, suggesting that, once on the surface, both molecules lead to the same adsorbed species but having different adsorption energies. This behavior is rationalized in terms of cost-benefit arguments. The analysis of the vibrational frequencies leads to several important conclusions. The most important one is that vibrational frequencies of the adsorbed species are not always comparable to those of carbonate complexes. However, it is shown that the vibrational frequency for the highest A(1) normal internal mode can be transferred from the complex to the surface. The theoretical analysis is next used to interpret the "in situ" infrared spectra of adsorbed carbonate in an electrochemical environment.