Liquid Ammonia Solutions. III. The Nature of Solutions of the Alkali and Alkaline Earth Iodides

Abstract

The spectra of liquid ammonia solutions of the alkali and alkaline earth iodides, except Ca12, exhibit an absorption band at about 2500 A, the position and intensity of which is temperature dependent; the position of the band is also dependent upon the concentration of added inert salt. Analysis of the data indicates that the band arises from a chargetransfer-to-solvent transition. The absence of this band in solutions of CaIz suggests that a molecular complex is present in this system. IntroductionThe position of the first ultraviolet absorption band for the iodide ion is solvent and temperature dependent; this band has been described as arising by a charge-transfer-to-solvent (ctts) mechanism.' We present here the results of a detailed study of the ultraviolet absorption band reported2 for iodide ions in liquid ammonia which support this model.

Liquid ammonia solutions. IV. Spectral characterization of the amide ion

“…The suggestion that amide ions and aggregates containing this ion have the same spec- tral characteristics is consistent with earlier observations on liquid ammonia solutions of weak acids9 and the alkali or alkaline earth iodides. 10 Spectral data for solutions of potassium amide in liquid ammonia indicate that the absorption band arises from a process that involves a charge transfer to solvent (ctts), as has been observed for iodide ions in liquid ammonia10 and in various other solvents.11 Briefly, this process has been described as the transfer of an electron from a solvated species to an orbital which is defined by the oriented solvent molecules in the first solvation layer. A consideration of the detailed description of this model11*5'0 indicates that a change in the nature of the solvent cavity will bring about the following changes in spectral characteristics: (a) in a given solvent an increase in temperature should cause an increase in the effective radius of the first solvation shell and the energy of the transition will decrease, and (b) the molar extinction coefficient at the band maximum should also decrease with increasing temperature.…”

“…A value of 2.8 A has been calculated for the radius of the cavity formed by ammonia molecules about an iodide ion using the same type of model. 10 The temperature variation of the radius of the cavity can be obtained by differentiating eq 3 with respect to temperature (eq 8). The value of dE,max/d7', obtained from Figure 1, is -47.7 cal/deg for the amide ion in liquid ammonia, which gives a variation of r0 with temperature (eq 8) of 0.07 A/50°; similar results have been obtained for the iodide-ammonia system, the corresponding values being 60 cal/deg and 0.10 A/50°, respectively.10 A more detailed ctts model1 ld for the transition observed for NH2_ could not be applied because certain important parameters, e.g., the heat of solvation of NH2_ in ammonia, were not available nor could they be calculated.…”

Theory and applications of charge-transfer-to-solvent spectra

“…Generally, where E, , , is independent of cation, then so is dE,,,/dT. Some slight dependence of dEm,,ldT on cation is observed for alkali iodides in ammonia, 45 but the differences are small. By far the greater change is observed when alkali metal cation is replaced by barium or strontium, when dE,,,/dT is halved.…”

“…A similar trend is observed in the effect of added salt on the OH stretch in methanol.266 A similar model also accounts for the effect of added salt on the spectra of iodide in ammonia. 45 Although division of salt efects into separate effects of anions and cations is necessarily arbitrary, some interesting trends emerge for spectra in aqueous solution.…”

The chemistry of liquid ammonia

“…Although the existence of ion-pairs in liquid ammonia solutions of electrolytes can be inferred from conductivity data, the results of spectroscopic investigations indicate that the formation of ion pairs does not affect the spectra of the species involved. An insight into the nature of solvated ionic species in liquid ammonia is obtained from a spectroscopic study of solutions containing the iodide ion (9). The alkali and alkaline earth iodides exhibit an absorption band (Fig.…”