A Raman Spectroscopic Study of Aqueous La(CH3CO2)3 Solutions and La(CH3CO2)3•1.5 H2O(cr)

Aqueous solutions of La(CH3CO2)(3), NaCH3CO2 and La(ClO4)(3) were studied using Raman spectroscopy. In dilute NaCH3CO2 solution, acetate is fully hydrated and forms only minor amounts of ion pairs. The characteristic Raman bands are discussed and assigned. In fairly dilute La(ClO4)(3) solutions, the La3+(aq) ion occurs as the nonahydrate. The separation of the carboxylate bands, nu(as) - nu(s) (Delta-value), in NaCH3CO2(cr) compared to La(CH3CO2)(3)center dot 1.5H(2)O(cr) correlates with the bonding type of acetate which is "ionic" in the former but bidentate chelating/tridentate chelating in the latter. Other acetate bands such as the deformation mode of the CO2 moiety, delta CO2, and the two rocking vibrations (rho), as well as the C-C stretch show marked differences in their band positions in NaCH3CO2(cr) compared to the ones in La(CH3CO2)(3)center dot 1.5H(2)O(aq). In a ternary solution of La(CH3CO2)(3)/LaCl3 with a molar ratio La3+(aq): (aq) = 3.87: 1.00), the bands of the bound acetate on La3+ were characterized and compared to those of fully hydrated acetate, . In this solution, almost all acetate is ligated to La3+ in a bidentate fashion and two complex species could be identified (molar ratios La3+: = 1:1 and 1:2, respectively). In La(CH3CO2)(3) solutions in H2O and D2O strong acetato complexes are formed and the bands of the bound acetate were characterized and compared with the ones of the fully hydrated acetate modes. A dilution series down to 0.0037 mol center dot L-1 in La(CH3CO2)(3)(aq) and to 0.0150 mol center dot L-1 in La(CH3CO2)(3)(D2O) showed that two acetate complexes are formed in these solutions. Again, it was shown that in these solutions the bound acetates on La3+ exist as bidentate ligands. DFT frequencies of the acetate on clusters {La(OH2)(7)O2CCH3)}(2+) and {La(OH2)(5)(O2CCH3)(2)}(+) compared well with the measured values. By determining the ligation number, , it can be established that in dilute solutions, below 0.04 mol center dot L-1, a complex with a 1:1 stoichiometry (La3+: ) exists in equilibrium with "free" acetate while in more concentrated solutions a 1:2 complex also forms. La3+(aq) hydrolysis is slight and very small equilibrium concentrations of CH3COOH were detected (C-C stretch at 893 cm(-1)). From quantitative Raman measurements, K (1) was determined to be 160 +/- 10 at 22 A degrees C.