A spectroscopic study of ion association and solvation in aqueous solutions of some halate salts

Abstract

The ion association and solvation behaviour of the aqueous solutions of NaClO3 , LiClO3 , Cd(ClO3)2 , Zn(ClO3)2 , NaBrOa and NaIO3 was investigated using Raman, infrared and NMR spectroscopy. Within each system, a range of solutions of different concentrations (up to saturation) was prepared, and the vibrational and NMR spectra of each solution acquired. Resolution of strongly overlapping bands in the vibrational spectra was achieved using Fourier self-deconvolution and component band analysis procedures (this was not necessary for the NMR spectra). In dilute solution the vibrational spectra for all the chlorate salts were very similar, and it is suggested that all ions are fully solvated at low concentration. Splitting of the doubly degenerate v3 band of ClO3- in these spectra (into v3a and v3b) is attributed to removal of the C3 axis of the ClO3- anion (which belongs to the C3V point group) by hydration. The hydration interaction is not strong enough to cause splitting of the doubly degenerate v3 band. Solvent shared ion pairing takes place for all chlorate salts. This is evidenced by the appearance of a second pair of split v3 components in the infrared, denoted v3a’ and v3b' (in increasing order of frequency). v3b’ is poorly resolved but evident on the high frequency side of v3b, and v3a’ is deduced to be at slightly lower frequency than v1. Although v3a’ is not directly observable (being very strongly overlapped with v1), its presence is indicated by its influence on the shift in position of v1 in the infrared. This influence is virtually absent in the Raman spectrum (from which the true shift in v1 is taken) because of the considerably lower intensity of v3 compared to v1. For LiClO3 and NaClO3 the increase in the v1 frequency indicates that the association of the cation with Cl03- is non-directional, whereas the low frequency shift of v1 for Cd(ClO3)2 and Zn(ClO3)2 is indicative of a more directional interaction which results in the weakening of the Cl-O2 bond. Both solvation and association of the CIO3- anion is primarily though the oxygen atoms. For LiClO3 and Cd(ClO3)2 a third pair of split v3 components (v3a” and v3b”) arises in the infrared (although v3a” is not directly observable) and these are attributed to contact ion pairs. These are not seen in the spectra of NaClO3or Zn(ClO3)2, indicating that only solvent shared ion pairing takes place. For LiClO3 there is also the appearance of a band at ~ 900 cm-1 in the infrared which (as it is not observed in the Raman) is assigned to an antisymmetric stretching mode of a centro-symmetric ion aggregate in which two chlorate anions are coordinated (through the oxygen atoms) to a Li+ cation. This band will have Au symmetry, and the corresponding Ag band in the Raman must be coincident with the v1 band of ClO3-. At high concentrations, some splitting of v4 in the Raman is seen for all salts, although it is considerably less for Zn(ClO3)2 and NaClO3 than in Cd(ClO3)2 and LiClO3. This splitting is attributed to further loss of the C3 axis of ClO3- due to ion pairing, mainly of the contact type (although there will be some small splitting from solvent shared ion pairing). For NaClO3 and Zn(ClO3)2 the v4 profile acquires little asymmetry from the splitting indicating that only solvent shared ion pairing is taking place. For LiClO3 and Cd(ClO3)2 the splitting is very much more evident and supports the presence of contact ion pairing at high concentrations. From the increase in frequency and narrowing of the OH stretching profile of water in the infrared spectra, it can be seen that all the salts produce a structure breaking effect on water, most of this being attributable to the ClO3- ion. These data, as well as the 17O and 1H NMR data, indicate that Zn2+ is more strongly solvated than Cd2+, and Li+ is more strongly solvated than Na+. Both vibrational and NMR data indicate that the cation-water and cation-solvent interactions are stronger for the divalent cations, attributable to the greater charge of these cations and the partial covalence in these interactions.The changes in the vibrational data for the aqueous solutions of NaBrO3 and NaIO3 were very small, and indicated that most of the ions remained fully solvated, although some Solvent shared ion pairing was indicated (less so for NaIO3). Both the vibrational and NMR data were consistent with the IO3- ion being more strongly solvated than BrO3-, which in turn is more strongly solvated than ClO3-.By comparison of the results reported in this Thesis for the chlorate salts, with similar ion association and solvation studies reported in the literature for perchlorate and nitrate salts, it appears that the tendency for ion association to occur increases in the order ClO4-, ClO3-, NO3-.