Effect of temperature on the complexation of NpO2+ with benzoic acid: Spectrophotometric and calorimetric studies

Abstract

a b s t r a c tThe equilibrium constants of the 1:1 NpO 2 + /benzoate complex were determined by spectrophotometric titrations at variable temperatures (T = 283 to 343 K) and the ionic strength of 1.05 mol Á kg À1 . The enthalpy of complexation at T = 298 K was determined by microcalorimetric titrations. Similar to other monocarboxylates, benzoate forms a weak complex with NpO 2 + and the complexation is strengthened as the temperature is increased. The complexation is endothermic and is entropy-driven. The enhancement of the complexation at elevated temperatures is primarily attributed to the increasingly larger entropy gain when the water molecules are released from the highly-ordered solvation spheres of NpO 2 + and benzoate to the bulk solvent where the degree of disorder is higher at higher temperatures.The spectroscopic features of the Np(V)/benzoate system, including the effect of temperature on the absorption bands, are discussed in terms of ligand field splitting and a thermal expansion mechanism.

Effect of temperature on the thermodynamic and spectroscopic properties of Np(v) complexes with picolinate

“…In that system, smaller blue shis were observed for the Np(V)/benzoate complex (0.29 cm À1 K À1 ) than the free aqua NpO 2 + ion (0.32 cm À1 K À1 ) because the structural coordinates of the former is expected to be less subjective to thermal expansion than that of the latter. 19 The prediction by mechanism ( 2) is again validated by the results on the Np(V)/picolinate complexation from the present study. As shown in Fig.…”

“…This correlation was shown for the complexation of Np(V) with a series of monocarboxylic acids including benzoic acid and acetic acid. 19 In the present study, the spectra red shis for the Np(V)/ picolinate complexes are larger than those for the Np(V) complexes with benzoic acid or acetic acid, because picolinate is a stronger bidentate ligand with a larger Nephelauxetic effect.…”

“…This trend can be explained by the mechanism of thermal expansion [22][23][24] of the complex coordinates in the solution. As discussed previously, 19 the dependence of the positions of optical absorption bands of complexes on temperature is a general phenomenon that could be induced by electronic coupling to the dynamics of the complex coordinates. Primarily, there are two mechanisms contributing to the observed shis: (1) the mechanism of electron-phonon coupling; and (2) the mechanism of thermal expansion of the complex coordinates in the solution.…”

“…This trend is known for the complexation of Np(V) with many ligands and can be discussed in terms of the perturbation of electronic energy levels upon complexation as previously described in the literature. 19 The absorption bands of Np(V) in the near-IR region originate from the intra-conguration 5f 2 transitions. The positions of the absorption bands depend on the energy levels of the 5f 2 states that are strongly inuenced by the ion-ligand complexation and the ligand-eld splitting.…”

Complexation of Np(V) with the Dicarboxylates, Malonate, and Succinate: Complex Stoichiometry, Thermodynamic Data, and Structural Information

“…54,56 Also changes of the hydration in the first and second solvation sphere or changes of the complex geometries due to increasing temperatures might contribute to this shift. 57,58 The temperature-induced shift of the absorption band of the Np(V) ion to shorter wavelengths is contrary to the bathochromic shift of the spectra, which results from the complexation of Np(V) with malonate and succinate. Due to this temperature dependency of the absorption bands, each series of spectra must be treated separately, and single component spectra must be determined for all the studied experimental conditions.…”