Authors Rudolph, W.W. ; Fischer, D. ; Tomney, M.R. ; Pye, C.C.
Title Indium(III) hydration in aqueous solutions of perchlorate, nitrate and sulfate. Raman and infrared spectroscopic studies and ab-initio molecular orbital calculations of indium(III)-water clusters
Date 21.12.2004
Abstract Raman and infrared spectra of aqueous In3+-perchlorate, -nitrate and -sulfate solutions were measured as a function of concentration and temperature. Raman spectra of In3+ perchlorate solutions reveal a strongly polarized mode of medium to strong intensity at 487 cm–1 and two broad, depolarized modes at 420 cm–1 and 306 cm–1 of much lesser intensity. These modes have been assigned to 1(a1g), 2(eg) and 5(f2g) of the hexaaquaindium(III) ion, [In(OH2)63+](Oh symmetry), respectively. The infrared active mode at 472 cm–1 has been assigned to 3(f1u). The Raman spectra suggest that [In(OH2)63+] is stable in acidified perchlorate solutions, with no inner-sphere complex formation or hydroxo species formed over the concentration range measured. In concentrated In(NO3)3 solutions, In3+ can exist in form of both an inner-sphere complex, [In(OH2)5ONO2]2+ and an outer-sphere complex [In(OH2)63+·NO3–]. Upon dilution the inner-sphere complex dissociates and the amount of the outer-sphere complex increases. In dilute solutions the cation, [In(OH2)63+], exists together with free nitrate. In indium sulfate solutions, a stable In3+ sulfato complex could be detected using Raman spectroscopy and 115-In NMR. Sulfato complex formation is favoured with increase in temperature and thus is entropically driven. At temperatures above 100 °C a basic In3+ sulfate, In(OH)SO4 is precipitated and characterised by wet chemical analysis and X-ray diffraction. Ab initio geometry optimizations and frequency calculations of [In(OH2)n3+] clusters (n= 1–6) were carried out at the Hartree–Fock and second order Møller–Plesset levels of theory, using various basis sets up to 6-31+G*. The global minimum structure of the aqua In3+ species was reported. The unscaled vibrational frequencies of the [In(OH2)63+] cluster do not correspond well with experimental values because of the missing second hydration sphere. The theoretical binding enthalpy for [In(OH2)63+] accounts for ca. 60% of the experimental single ion hydration enthalpy for In3+. Calculations are reported for the [In(OH2)183+] cluster (In[6 + 12]) with two full hydration spheres (T symmetry), for which the calculated 1(InO6) mode occurs at 483 cm–1(HF/6-31G*), which is in good agreement with the experimental value at 487 cm–1, as are the other frequencies. The theoretical binding enthalpy for [In(OH2)183+] was calculated and underestimates by about 15% the experimental single ion hydration enthalpy of In3+.
Journal Physical Chemistry, Chemical Physics 6 (2004) 5145-5155

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