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

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 nu(1)(a(1g)), nu(2)(e(g)) and nu(5)(f(2g)) of the hexaaquaindium(III) ion, [In(OH2)(6)(3+)] (O-h symmetry), respectively. The infrared active mode at 472 cm(-1) has been assigned to nu(3)(f(1u)). The Raman spectra suggest that [In(OH2)(6)(3+)] 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)(5)ONO2](2+) and an outer-sphere complex[In(OH2)(6)(3+).NO3-]. Upon dilution the inner-sphere complex dissociates and the amount of the outer-sphere complex increases. In dilute solutions the cation, [In(OH2)(6)(3+)], 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 100degreesC 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)(n)(3+)] clusters (n=1-6) were carried out at the Hartree-Fock and second order Moller-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)(6)(3+)] cluster do not correspond well with experimental values because of the missing second hydration sphere. The theoretical binding enthalpy for [In(OH2)(6)(3+)] accounts for ca. 60% of the experimental single ion hydration enthalpy for In3+. Calculations are reported for the [In(OH2)(18)(3+)] cluster (In[6+12]) with two full hydration spheres (T symmetry), for which the calculated nu(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)(18)(3+)] was calculated and underestimates by about 15% the experimental single ion hydration enthalpy of In3+.