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

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 ν1(a1g), ν2(eg) and ν5(f2g) of the hexaaquaindium(III) ion, [In(OH2)63+] (Oh symmetry), respectively. The infrared active mode at 472 cm−1 has been assigned to ν3(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 ν1(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+.