Quantum chemical model calculations using density functional theory (DFT) were used to assign iron-catecholate and iron-oxalate vibrations and to get quantitative predictions of the isotopic shifts. Full geometry optimizations and vibrational analyses were performed for catechol, Fe(catecholate)(2-), Fe(4-methylcatecholate)(2-), [Fe(oxalate)(3)](3-), and Fe(oxalate)(2-). The advantages of Fe(0) versus Fe(III) models rue discussed. For selected systems O-16/18, Fe-54/57, and H-1/2 isotopic substitution shifts are reported. They were successfully matched to experimental patterns from recent resonance Raman studies of tyrosine hydroxylase and allowed more precise assignments of the observed bands. The nature of the catecholate C=O and Fe-O vibrations were clarified, and the existence of a chelate vibration mode was confirmed. A band predicted at similar to 320 cm(-1) was assigned to a new vFe-O vibration with a large Fe-54/57 isotope effect, and no significant Fe-54/57 shifts were observed for the other Fe-O vibrations. We note that with the commonly used diatomic harmonic oscillator model one can only make a rough estimation of these shifts. DFT model calculations are suggested as a more precise tool when interpreting isotopic substitution shifts in vibrational spectra.