Liquid ammonia pulse radiolysis

The electrons ejected from molecules by the passage of ionizing radiation through condensed media can be solvated very soon after the primary ionizing event and the solvated electron, e q, so formed can undergo chemical reactions with solute and solvent molecules. The main evidence for the existence of solvated electrons in the liquid phase has been obtained by the use of pulse radiolysis in conjunction with optical spectroscopy (Hart and Boag, 1962). Very recently the e.s.r. spectrum of the solvated electron has been obtained by a similar method (Avery et ah, 1968). The solvated electron is not located on one solvent molecule but is associated with an assembly of molecules which form a potential well around the electron by virtue of dipolar and polarization forces. There is a close similarity between this system and the blue solutions obtained by dissolving alkali metals in liquid ammonia. 

Fletcher JW, Seddon WA. (1975) Alkali metal species in liquid amines, ammonia and ethers. Formation by pulse radiolysis. J Phys Chem 79 3055-3064. 

Baxendale et al. observed, by pulse radiolysis, that Ag° as well as Agj, produced by the scavenging of hydrated electrons e and H radicals, were easily oxidized by oxygen back to Ag". This observation demonstrated that the silver atom and the first oligomers deviate from the known noble character of bulk silver. In the case of Cu" irradiated in liquid ammonia, no stable copper clusters were formed. However, molecular hydrogen was instead produced, as a consequence of copper corrosion in its nascent state.Therefore it was... 

The solvated electron in liquid ammonia was discovered in 1864 by Weyl, and was identified in 1908 by Kraus as an electron, e am, in a cavity surrounded by ammonia molecules. It is prepared when an alkali metal, for example sodium, is dissolved in ammonia, to form a stable blue color under these conditions the electron is present in equilibrium with the metal atom and cation [34]. By contrast, e am produced by pulse radiolysis of liquid ammonia is unstable due to its reactions with other radiolysis products for example, in pure ammonia at —45 °C, its lifetime is ca. 7 ps [35], The major decay reactions are thought to involve the oxidizing radicals NH2 and NH [36], because addition of potassium ethoxide stabilizes e am this is explicable, since ethoxide ion is expected to scavenge these radicals [36a]. 

The solvated electron in ammonia has a strong absorption band in the infrared region, and /Imax shifts from 1850 nm at 23 °C to 1410 nm at —75°C this is attributed to the effect of temperatiu-e on the orientation of the ammonia molecules in the first solvation shell [37], On the other hand, G(e am) = 0-32 pmol J remains constant over the same temperature range [37]. It is relatively straightforward, therefore, to study one-electron redox reactions in liquid ammonia by pulse radiolysis, but relatively few investigations have been made. 

Pulse radiolysis of alkali-metal amides in ND3 gives a primary product suggested to be (eam)-, which decays in microseconds to give an equilibrium mixture of (eam) and a metal-electron species.42 Studies have been made of the behaviour of the silver electrode43 and of the Cu Cu Cu0 system44 in acid solutions of liquid ammonia. 

See reviews a) Edwards P.R, The electronic properties of metal solutions in liquid ammonia and related solvents, Adv. Inorg. Chem. Radiochem., i982,25, 135-185. b) Boag J.W., Pulse radiolysis a historical account of the discovery of the optical absorption spectrum of the hydrated electron, in Early developments in radiation chemistry", KrohJ. (Ed.), Royal Society ofChemistry, Cambridge, 1989, 7-20. 

Vast material on the properties of solvated electrons in liquid ammonia has been accumulated by now . i23-i27> Ammonia was the first solvent for which it was shown that the properties of solvated electrons obtained by different methods (by dissolving alkali metals, by pulse radiolysis, and by cathodic generation) were identical (see Fig. 4). 

While solvated electrons and their ion-pairs seem to be the most abundant species in liquid ammonia and hexamethylphosphoric-triamide, the negative alkali ions and their ion-pairs predominate in ethereal solvents. In fact, pulse radiolysis of tetrahydrofuran solutions of alkali tetraphenylborides revealed that the e, Met+ pairs, observed immediately after a pulse, rapidly decay, being converted into the corresponding Met" ions645. 

The same typical absorption spectrum has been observed in both the continuous [82] and pulse [83] radiolysis of liquid ammonia, viz. 

Dissociative electron capture has been observed in the pulse radiolysis of solutions of methyl chloride, benzyl chloride and iodobenzene in liquid ammonia [91], e.g. 

The decay of Bn h the pulse radiolysis of liquid ammonia is second order with a rate coefficient [96] of (1.1 0.2) x 10 ° Imole sec". By contrast, the disappearance of Bn h 3 in alkali metal-ammonia solutions at concentrations <10 is first order and very slow. The ammoniated electron formed in the radiolysis probably reacts with other intermediates produced, e.g. -NHj or NH4, and does not decay via the reaction... 

Pulse radiolysis of some scavenger solutions in water, intermediates spectra, and kinetics of their decay in liquid ammonia are investigated. Rate constant and activation energy are calculated for the latter. The dependence of the disappearance of intermediates on concentration is analyzed. It is shown that rate constant of reactions of pseudo-first order is not proportional to acceptors concentration. One of the possible reasons is that first order reaction was not taken into consideration. On this basis, rate constants of reactions with acceptors and these of monomolecular decay are calculated. It is revealed the decay of intermediates in 10 5-10 3M perchloric acid solutions does not depend upon HsO+ ion concentration. This fact is contrary to the present day theories about the nature of intermediates. 

Silica cells, which gave the possibility of working up to 25 °C., were used in experiments with liquid ammonia and ammonium solutions. A thermostated jacket for investigating shortlived intermediates by pulse radiolysis with fast spectrophotometric recording is shown in Figure 1. 

It is known that the first idea of solvated electrons appeared to explain the properties of alkaline metal solutions in liquid ammonia. In this connection the pulse radiolysis of liquid ammonia is of special interest because it is possible to compare properties of intermediates formed by different methods. 

The spectral chracteristics of intermediates formed by irradiation of liquid ammonia probably do not differ greatly from those in metal-ammonium solutions (4) (Figure 3). In Figure 3, one sees the optical spectra of an intermediate which appears in the pulse radiolysis of liquid... 

Bnijsman E., H. F. M. Mass and W. A. H. Asman (1986) Anthropogenic ammonia emissions in Enrope. Atmospheric Environment 21, 1009-1022 Bnrford, I R. and I M. Bremner (1972) Is phosphate reduced to phosphine in waterlogged sods Soil Biology and Biochemistry 4, 489-495 Buxton, G. V. (1982) Basic radiation chemistry of liquid water. In The study of fast processes and transient species by electron pulse radiolysis (Eds. J. H. Baxendale and F. Busi), D. Reidel Publ. Comp., pp. 241-266... 

In liquid ammonia the NH2 radicals are produced by y radiolysis [12] or pulse radiolysis with high-energy electrons [14]. In the photolytic system NH3(l) + hv (>.max = 254 nm) and in the radiolytic system, solvated electrons are produced [15] which react with NH2 radicals. The NHJ ion, however, has not yet been observed directly [16]. 

---