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(a_1g), ν₂(e_g) and ν₅(f_2g) of the hexaaquaindium(III) ion, [In(OH₂)₆³⁺] (O_h symmetry), respectively. The infrared active mode at 472 cm⁻¹ has been assigned to ν₃(f_1u). 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⁺.