The ammoniated electron

In the century since its discovery, much has been learned about the physical and chemical properties of the ammoniated electron and of solvated electrons in general. Although research on the structure of reaction products is well advanced, much of the work on chemical reactivity and kinetics is only qualitative in nature. Quite the opposite is true of research on the hydrated electron. Relatively little is known about the structure of products, but by utilizing the spectrum of the hydrated electron, the reaction rate constants of several hundred reactions are now known. This conference has been organized and arranged in order to combine the superior knowledge of the physical properties and chemical reactions of solvated electrons with the extensive research on chemical kinetics of the hydrated electron. 

Physical chemical studies of dilute alkali metal-ammonia solutions indicate the principal solution species as the ammoniated metal cation M+, the ammoniated electron e , the "monomer M, the "dimer" M2 and the "metal anion" M. Most data suggest that M, M2, and M are simple electrostatic assemblies of ammoniated cations and ammoniated electrons The reaction, e + NH3 - lf 2 H2 + NH2 is reversible, and the directly measured equilibrium constant agrees fairly well with that estimated from other thermodynamic data. Kinetic data for the reaction of ethanol with sodium and for various metal-ammonia-alcohol reductions of aromatic compounds suggest that steady-state concentrations of ammonium ion are established. Ethanol-sodium reaction data allow estimation of an upper limit for the rate constant of e + NH4+ 7, H2 + NH3. 

The ammoniated electron, e, has been described by Ogg (35) and various other authors as an electron-in-a-cavity. In this model, the electron is stabilized in a large cavity by polarization and orientation of... 

The dimer species, M2, was described by Huster (21) as a simple dissolved diatomic molecule. Such a species would be very unstable with respect to the ammoniated species, M + and e and may be ruled out by thermochemical data. The expanded metal dimer model of Becker, Lindquist, and Alder, in which two ammoniated metal ions are held together by a pair of electrons in a molecular orbital located principally between the two ions, is just as difficult to reconcile with optical, volumetric, and NMR data as the expanded metal monomer. In order to account for the similar absorption spectra of e, M, M2 (and any other species such as M or M4 that might exist at moderate concentrations of metal), Gold, Jolly, and Pitzer (16) assumed that species such as M and M2 consist of ionic aggregates in which the ammoniated electrons remain essentially unchanged from their state at infinite dilution. 

Several models have been proposed for the structure of the ammoniated electron. One which has shown qualitative agreement with experimental... 

It is interesting that the greater stability of the ammoniated electron makes its observation easier in continuous radiolysis than that of the hydrated electron. The ESR spectrum of dilute solutions shows a single, narrow line. The g factor is 2.0012 which is close to the free electron value and indicates only a weak interaction between electron and solvent [84]. 

A detailed study of the reaction of water with solutions of the alkali metals in ammonia has been made [94]. Interpretation of the data is complicated by the large number of possible species present but it appears that the reaction of the ammoniated electron with water... 

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... 

A powerful aprotic solvent capable of dissolving alkali and alkaline earth metals is hexamethylphosphoramide. The characteristic blue paramagnetic solution is observed [101]. It had been demonstrated that the ammoniated electron could be produced in the electrolysis of liquid ammonia [102] and recently it has been demonstrated that the solvated electron is produced in the electrolysis of hexeimethylphosphoramide [103, 104]. It is, however, doubtful whether eaq can be produced in this manner [102]. 

Several reports on reactions in liquid ammonia have been published, including the proceedings of the Fifth International Conference on excess electrons and metal-ammonia solutions,proton exchange at cobalt(III) ammine complexes, and the reaction of the ammoniated electron with thiosulfate... 

JONAH - The partial molal volume of the electron is an experimental parameter which can be measured and shows a great difference between the hydrated electron and the ammoniated electron. Have you calculated or are you trying to calculate its value ... 

Europium and ytterbium metal dissolve in liquid ammonia to give blue solutions containing the ammoniated electron. They are the only lanthanide metals to do so samarium does not undergo this reaction (Thompson et al, 1966 Warf, 1970). 

The alkali metals are soluble in liquid ammonia, and certain amines, to give solutions which are blue when dilute. The solutions ate paramagnetic and conduct electricity, the carrier being the solvated electron. In dilute solutions the metal is dissociated into metal ions and ammoniated electrons. The metal ions are solvated in the same way that they would be in a solution of a metal salt in ammonia, and so comparison can be made with, for example, [Na(NH3)4]+I-, the IR and Raman spectra of which indicate a tetrahedral coodination sphere for the metal.39... 

Blandamer et al. (4) have recently made a half-hearted effort to resurrect the e2 2 species. They state that if a species such as M2 or M contains two ammoniated electrons with sufficient overlap of the electronic wave functions to cause the species to exist in a singlet state, then only by a coincidence could the absorption spectrum be similar to that of the far-separated ammoniated electrons. They suggest that a comparable coincidence could just as well occur in the case of the e2 2 species. Some clarification of the ionic aggregate model is therefore needed. It should be recognized that the optical absorption peak does shift slightly... 

The perturbation of the Si-H vibration of ammoniated trichlorosilylated silica (figures 12.14 and 12.16) consists at first sight of at least 3 distinct bands, indicating that at least 3 different species with an Si-H band exist on the surface. The strong electron donating effect of the amine functions will cause a low wavenumber shift of the Si-H band. 

Although agreement among workers in the field is not complete, it now appears that the predominant species in concentrated solution is the solvated Na2 molecule with the two 3a electrons delocalized over a number of surrounding solvent molecules, but still paired. The blue species is probably an ammoniated electron—that is, the electride ion. ... 

Ion-dipole complexes ( n), from an ion combined with dipole molecules such as water and ammonia. As far as the type of bonding is concerned, we meet in this case also both the electrostatic bonding, as in the hydrates [Mg(H20)6]2+, and the electron pair bond in the ammoniates, for example, such as that of trivalent cobalt [Co(NH3)6]3+. 

For example, the one-electron models incorrectly predict (even at a qualitative level) the Knight shifts in and NMR spectra of ammoniated electron, and solvated electrons in amines (Sec. 4.1). The same problem arises in the explanation of magnetic (hyperfine) parameters obtained from ESEEM spectra of trapped (hydrated) electrons in low-temperature alkaline ices. The recent resonance Raman spectra of also appear to be incompatible with the one-... 

The metal-NHs reductions of carbonyl groups are exceedingly fast reactions for the reaction of acetone with an ammoniated electron the rate is 9 x 10 M" s". Although many, particularly older, published experimental procedures for the metal-NHs reduction of ketones employ prolonged reaction times with excess metal, these conditions are unnecessarily harsh. The reactions of carbonyl compounds with metals in NH3 are effectively instantaneous and by using short reaction times it appears that reduction of terminal alkenes and disubstituted alkynes can be avoided.In addition to the functional groups mentioned above, alcohols, amines and ethers, other than epoxides, are usually stable to reductions of aldehydes and ketones by dissolving metals. " ... 

The binding of ammonia to the cluster induces a change in electronic structure relative to that of the bare cluster. This is probed by reacting ammoniated clusters with hydrogen and comparing the reaction rate constants as a function of cluster size with the naked iron clusters. The absolute reaction rate constants toward H 2 for the fully ammoniated clusters are about an order of magnitude smaller than those for the bare clusters. The minima in reactivity observed for bare iron clusters are shifted to smaller cluster size for the ammoniated species for example, Fe,3 is reactive with H2, but upon... 

The diffusion coefficient for the hydrated electron has been calculated from conductivity measurements [32] to be (4.9 0.25) x 10 cm sec". The equivalent conductance is much higher than that of all other ions except OH" and but considerably lower than the ammoniated... 

Althou solutions of sodium in liquid ammonia have been used extensively as a reducing agent in organic chemistry [88, 89], the results obtained have, in the main, been qualitative. In view of the relative stability of such solutions, it is a little surprising that more quantitative data are not available. The scarcity of data is probably due to the difficulties of interpretation since, as has been discussed earlier, species other than the simple ammoniated electron are produced. 

Many of the available computations on radicals are strictly applicable only to the gas phase they do not account for any medium effects on the molecules being studied. However, in many cases, medium effects cannot be ignored. The solvated electron, for instance, is all medium effect. The principal frameworks for incorporating the molecular environment into quantum chemistry either place the molecule of interest within a small cluster of substrate molecules and compute the entire cluster quantum mechanically, or describe the central molecule quantum mechanically but add to the Hamiltonian a potential that provides a semiclassical description of the effects of the environment. The 1975 study by Newton (28) of the hydrated and ammoniated electron is the classic example of merging these two frameworks Hartree-Fock wavefunctions were used to describe the solvated electron together with all the electrons of the first solvent shell, while more distant solvent molecules were represented by a dielectric continuum. The intervening quarter century has seen considerable refinement in both quantum chemical techniques and dielectric continuum methods relative to Newton s seminal work, but many of his basic conclusions... 

Zintl also found that the most convenient way to produce the solutions of the anionic clusters in liquid ammonia is to extract alkali metal/post-transition element alloys in the solvent. However, detailed solid-state characterization of the clusters is very difficult using this technique, since poorly crystalline and often pyrophoric solids are obtained once the solvent is evaporated. These troublesome solids are alkali-metal ammoniates of cluster ions, " of which only [Li(NH3)4]3-[Li2(NH3)2Sb5] -2NH3 seem to have been completely structurally characterized, Furthermore, the ammoniates most often slowly revert back to the alloy upon further loss of ammonia. The last step involves transfer of electrons from the strongly reducing cluster anion back to the alkali-metal ion and thus represents a major synthetic obstacle. 

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