Temperature and pressure dependence of excitation spectra as a probe of the solution structure and equilibrium thermodynamics of a Eu(III) complex containing a modified dota ligand

The temperature and pressure dependence of the ⁷F₀ → ⁵D₀ and ⁷F₁ → ⁵D₀ excitation spectra for aqueous solutions of Eu(III) with racemic-tetra(α-carboxyethyl)DOTA have been measured. Individual excitation and emission peaks have been assigned to square antiprism (SAP) and twisted square antiprism (TSAP) isomers of the complex, and the variation of relative peak areas as a function of temperature and pressure have been used to estimate the enthalpy and volume change associated with the isomer equilibrium. The effects of temperature on the initial state populations have been treated explicitly, although for the data presented, these effects are within the uncertainty of the measurement. The results show that the SAP isomer of Eu(ceDOTA) is the more compact, more ordered, and lower enthalpy structure, consistent with previous NMR experiments on the differences in the observed rate of water exchange for the isomeric species.