David W. James

Ion-ion-solvent interactions in solution. I. Solutions of LiClO4 in acetone

Vibrational spectra and 7Li, 13C and 35C1 n.m.r. spectra have been obtained for solutions of LiC104 in acetone for salt concentrations from 0 05 to 6 m. Infrared spectra give qualitative indications of ion association. Analysis of the Raman band due to C-C stretching in acetone yields solvation numbers for the Li+ ion of the order of 3. Component band analysis of the CIO4- symmetric stretching vibrational band and the various n.m.r. spectra lead to the identification of solvent- separated ion pairs, contact ion pairs and ion aggregates, in addition to free solvated ions. The dependence on salt concentration of all four species has been determined. The association quotient for the association equilibrium.

Vibrational Spectra of Single Crystals of Group I Nitrates

The Raman and infrared spectra of single crystals of the nitrates of the alkali metals and silver have been studied at room temperature. The internal vibrations of the nitrate ion and the external vibrations of the crystal lattice are assigned on the basis of a factor‐group analysis of the crystal. For the rhombohedral crystals the frequencies were little changed from those expected for a free ion with the exception that infrared and Raman frequencies were distinct. For crystals of lower symmetry the vibrations of the free ion give rise to many components which can, however, be adequately described on the basis of the factor‐group analysis. Strong combination bands between internal and external vibrations are observed and assigned in the infrared spectra.

Structure of Molten Nitrates. III. Vibrational Spectra of LiNO3, NaNO3, and AgNO3

The Raman spectra of the molten nitrates of lithium,sodium, and silver change as the temperature is raised. Close to the melting point well‐developed low‐frequency modes can be distinguished close to the exciting line while at higher temperature these have become somewhat broader but still quite distinct. There are also changes in the magnitude of the splitting of degenerate modes and in the energy of the other vibrations. The spectra have been interpreted on the basis of a quasicrystalline model in which cubic packing is assumed. Spectral changes from the solid, volume change on fusion, entropy changes, and molten‐state radial distribution functions are explained on the basis of the model.

Ion–ion–solvent interactions in solution. Part 4.—Raman spectra of aqueous solutions of some nitrates with monovalent cations

The profile of the band due to the symmetric stretching of the nitrate ion in the Raman spectra of solutions of NH4NO3, Me4N·NO3, LiNO3, KNO3 and RbNO3 has been component analysed. For the cations NH+4 and Me4N+ only one component was present. The bandwidth indicates that the ammonium ion causes enhanced dephasing while the tetramethylammonium ion produces essentially no perturbation. Solutions of LiNO3 gave evidence of four concentration-dependent species identified as aquated ions, solvent-separated ion pairs, contact ion pairs and ion aggregates. Solutions of KNO3 and RbNO3 gave evidence of three species identified as aquated ions, ion pairs and ion aggregates. It was not possible to distinguish between contact ion pairs and solvent-separated ion pairs. Association quotients calculated for LiNO3 solutions were K1(1 mol dm–3)= 1.34 dm3 mol–1 and K2(6 mol dm–3)= 1.5, for KNO3 solutions K(1 mol dm–3)= 2.0 dm3 mol–1 and for RbNO3 solutions K(1 mol dm–3)= 1.0 dm3 mol–1.

Structural studies of ammonium nitrate 1. Phases III, IV and V

Abstract The laser Raman spectra of NH 4 NO 3 and ND 4 NO 3 have been measured between 210 and 320 K. It is shown that the phase transition V → IV is probably a λ transition which occurs gradually between 210 and 256 K with an abrupt change at 256 K. The λ transition is due to rotational disorder of ammonium ions as shown by the localised disorder mode at 172 cm −1 . The spectrum of phase IV shows clear evidence of T and L components of the nitrate ion asymmetric stretch. This is inconsistent with the assigned space group Pmmn. An explanation based on a thermally inducted IV → III transition is proposed.

Solvation and ion association in solutions containing oxyanions

Ion solvation and association behaviour has been studied for solutions of lithium perchlorate and lithium nitrate in N,N-dimethylformamide (DMF) methanol and ethanol. The techniques of Raman spectroscopy and multinuclear n.m.r. have been employed, and in some studies the cation has been caged in the cryptand Kryptofix 221. Solvation in DMF was examined through quantitative analysis of the OCN bending vibrational band of the DMF in the Raman spectra. A range of univalent and divalent cations was employed and solvation numbers and some information on solvation geometries were obtained. In alcohol solutions determination of solvation numbers was not possible but the involvement of Li+ and NO–3 produced observable spectroscopic changes, while the ClO–4 anion appeared to have only a weak solvation. Evidence for at least two associated species was obtained for both salts in DMF and ethanol and the concentration dependence of the species was determined. In methanol association was much less with the behaviour approaching that observed in aqueous solution.

Ion–ion–solvent interactions in solution. Part 3.—Aqueous solutions of sodium nitrate

The profile of the band due to the symmetric stretching vibration in the Raman spectrum of aqueous solutions of NaNO3 has been studied as a function of concentration. Analysis based on the Fourier transform indicates that there are at least three principal components in the band. Analysis gives the positions of the three components as 1047.6, 1050.0 and 1052.0 cm–1, with a fourth component appearing at 1070.0 cm–1 in the most concentrated solutions. On the basis of concentration dependence and band shape these components have been assigned to the free aquated nitrate ion (1047.6), the solvent-separated ion pair (1050.0), the contact ion pair (1052.0) and ion aggregates (1070.0). Association quotients K1 and K2 for the two equilibria Na+(aq)+ NO–3(aq) [graphic omitted] Na+· H2O · NO–3(aq) [graphic omitted] Na+· NO–3·(aq) are K1= 2.7 dm3 mol–1(0.5 mol dm–3) and K2= 3.3 (6 mol dm–3).

Ion–ion–solvent interactions in solution. Part 6.—Aqueous solutions of metal nitrates having cations with incomplete valence shells

The Raman spectra of aqueous solutions of AgNO3, TlNO3, Zn(NO3)2, Cd(NO3)2, La(NO3)3 and Th(NO3)4 are analysed as a function of concentration. For all solutes, bands due to aquated-nitrate ions, solvent-separated ion pairs (outer-sphere complexes) and contact ion pairs (inner-sphere complexes) are reported. The nature of the inner-sphere complex leads to a distinctly different band from contact ion pairs for simple monovalent cations. This difference is interpreted in terms of an electron-exchange mechanism. The inner-sphere complexes are discernable at all concentrations studied. The association quotients compare favourably with values from other measurements for solutions of Zn(NO3)2 and Th(NO3)4.

Relaxation processes in aqueous solution: a Raman spectral study of hydrogen bonded interactions

Vibrational relaxation and rotational reorientation times for the nitrate ion in aqueous solutions of group I and group II metal nitrates have been measured at room temperature using polarised Raman spectra. The vibrational relaxation occurs by energy transfer to hydrogen-bound water molecules. In dilute solution all vibrational relaxation times approach a common value of 1.4 ps. At higher concentration there is a marked cation dependence with relaxation being more rapid in the presence of highly polarizing cations. It is shown that hydrogen bonding to only one water molecule is necessary to describe the relaxation process. The rotational reorientation times of the nitrate ion reflect constraints imposed by both hydrogen bonding to water and coulombic interaction with the cationic species. The low concentration limited values of τOR are different for different electrolytes. Also in contrast to vibrational relaxation (which shows linear dependence on cation concentration) the variation of rotational reorientation with concentration is non-linear with the curvature reflecting different contributions to the constraining field.

Structure of molten nitrates. Part 4.—Relative intensity study in Raman spectra

The intensities of the symmetrical stretching frequency of the nitrate ion in melts of univalent metal nitrates are reported relative to the intensity in NaNO3. Whereas the intensity for the alkali metals shows a gradual decrease with increasing values for z/r in the alkali metal nitrates, the values for AgNO3 and TlNO3 are much greater and appear unrelated to the z/r of the cation. The results are discussed in terms of covalent interactions the most important of which appears to be the donation of π electrons from the metal atom to the nitrate ion. The relative intensities of the symmetrical stretching frequencies in LiClO4 and NaClO3 are also included.