Ammonia radiolysis

The ammonia radiolysis products are hydrogen, nitrogen and hydrazine in 3uelds of 6, 2, and 0.5 molecules, respectively, per 100 eV. The total yield of the ammonia decomposition is 4 [14]. 

The data available (see [230 Section XI.37]) show that this decomposition involves primary NHg and NHg ions, formed by the interaction of electrons with NH3 molecules, and secondary NH ions, generated by the interaction of NHg and NHg with ammonia, as well as neutral particles (H, NH, NHg, N2H3) produced by dissociative recombination together with NH4 ions and by recombination of the latter with electrons. The above mentioned end products are formed in the interaction between neutral atoms and radicals. 

The yield of ammonia radiolysis products depends on temperature, pressure, dose rate and, under flow conditions, on the contact time. The effect of temperature on nitrogen and hydrogen yields and the total yield of ammonia conversion have been studied earlier [208]. The extent of the decomposition of ammonia increases with temperature (Table 12). 

The kinetic functions of ammonia radiolysis have been calculated with account for the radical mechanism [118] over a wide range of variations in dose rate I, temperature T, pressure p, and certain other parameters. The dependence of G on T, p, and I is very complicated due to the competing of various radical processes. 

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Table 12. Temperature dependence of the ammonia radiolysis products ...

If we assume no solvation energy for atomic hydrogen in ammonia (and thus probably overestimate the heat) we calculate AH° = 15 kcal./mole for the latter reaction. It is hoped that the rate constant for the NH4 + + e reaction soon will be evaluated by the same techniques of pulse radiolysis (18) or photolysis (36) which have been used to study the H30+ + e reaction. 

Jhe discovery by radiation chemists of solvated electrons in a variety of solvents (5, 16, 20, 22, 23) has renewed interest in stable solutions of solvated electrons produced by dissolving active metals in ammonia, amines, ethers, etc. The use of pulsed radiolysis has permitted workers to study the kinetics of fast reactions of solvated electrons with rate constants up to the diffusion-controlled limit (21). The study of slow reactions frequently is made difficult because the necessarily low concentrations of electrons magnify the problems caused by impurities, while higher concentrations frequently introduce complicating second-order processes (9). The upper time limit in such studies is set by the reaction with the solvent itself. 

The C(2)--OH-adduct is likely to eliminate ammonia, and from the low ammonia yield it seems not to exceed 1.5% (von Sonntag and Schuchmann 2001). With dGuo, some H-abstraction occurs also at the sugar moiety. Among others, this is indicated by an isomerization of dGuo (in low yields, see Table 10.13). Nearly all sugar-derived radicals must have reducing properties, and, based on the pulse radiolysis data mentioned above, the upper limit of their yield must be much less than the total of 17%. 

The metals, and to a lesser extent Ca, Sr, Ba, Eu, and Yb, are soluble in liquid ammonia and certain other solvents, giving solutions that are blue when dilute. These solutions conduct electricity electrolytically and measurements of transport numbers suggest that the main current carrier, which has an extraordinarily high mobility, is the solvated electron. Solvated electrons are also formed in aqueous or other polar media by photolysis, radiolysis with ionizing radiations such as X rays, electrolysis, and probably some chemical reactions. The high reactivity of the electron and its short lifetime (in 0.75 M HC104, 6 x 10"11 s in neutral water, tm ca. 10-4 s) make detection of such low concentrations difficult. Electrons can also be trapped in ionic lattices or in frozen water or alcohol when irradiated and again blue colors are observed. In very pure liquid ammonia, the lifetime of the... 

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. 

Triazene HN=N-NH2 is formed in the pulse radiolysis of aqueous solutions of hydrazine. In the radiolysis, hydrazyl radicals N2H3- are formed that dimerize to tetrazane, which decomposes under elimination of ammonia to form triazene. ... 

We have already mentioned that Dorfman and collaborators have developed a versatile technique to observe ort-lived carbenium ions in solution generated by dissociative pulse radiolysis. This novel approach to the characterisation of transient species has also allowed this schod to measure the rate constants of many electrophilic reactions between carbenium ions (the benzylium ion in particular) and various nucleophiles. In the first paper of the series Jones and Dorfman reported the rate constants of the benzylium ion reaction with methanol, ethanol, the bromide and the iodide ions in ethylene chloride at 24 C. Values of about 5 x 10 sec were obtained for the halide ions and of around 10 sec for the alcohols. Later studies confirmed that the reaction of halide ions vrith benzylium, diphenyl-methylium and triphenylmethylium ions is at the limit of diffusion control. Reaction rate constants of these three carbenium ions with amines and alcdiols were also reported in the same paper. More recently, these studies have been extended to include cyclopropylphenylmetiiylium ion as electrophile, ammonia as nucleophile and methylene chloride and trichloroethane as solvents These results are extremely... 

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). 

The next example of comparatively simplicity, this time nonaqueous, is the crystalline alanine. There are several products of irradiation of that solid crystalline amino acid. In this state it occurs as zwitterion as NMR shows, i.e. the amine group is protonated -N+H3. Single ionization spurs, of a low energy, cause deamination which leads to detachment of ammonia and formation of a free radical. Pulse radiolysis of single crystals of L-alanine shows, that the alanine derived radical CH3-C H-C02-, which shows the spectrum with maximum at 348 nm [9], stabilizes during 5 milliseconds [10], It is usually observed not spectroscopically but by the EPR method [11] it shows extreme stability, being applied as reference dosimeter. 

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. 

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