Ammonia dimer, dissociation

The ammonia dimer dissociation spectrum was found to consist of two bands... 

From a consideration of these results and of the optical absorption data, the following unified model may be proposed. When the metal atom is dissolved in ammonia, it dissociates and produces the metal ion and electron. Some of the electrons get trapped in a polaron state and some in clusters around the metal ion. In dilute solutions it is supposed that most of the electrons are present as polarons which accounts for the success of the polaron theory in explaining the value of H in dilute solutions. As the concentration increases, dimer clusters are produced by the association of monomer clusters. The two-electron polaron is probably unstable. As the concentration increases toward saturation, the monomers and dimers form a lattice-like arrangement and the unpaired electrons get delocalized as in a metal. In the next section it will be seen that this model explains satisfactorily the optical, electrical, and other properties listed in Section I. 

A totally different view was proposed by Becker, Lindquist, and Alder (2) who considered the electron to be trapped at a solvated cation site in which the solvated metal cation has its valence electron localized on the protons of the ammonia molecules of the innermost solvation layer. This species, with its valence electron in a sort of enlarged Rydberg orbital, may then dissociate or undergo dimerization. 

Gallium(III) bromide is a hygroscopic, white solid which sublimes readily and melts at 122.5° to a covalent, dimeric liquid. The solid is ionic and its electrical conductivity at the melting point is twenty-three times that of the liquid.5 The vapor pressure of the liquid at T°K is given by the equation log p(mm.) = 8.554 — 3129/T and the heat of dissociation of the dimer in the gas phase is 18.5 kcal./mol.3 At 125° the liquid has the following properties 5,6 density, 3.1076 dynamic viscosity, 2.780 c.p. surface tension, 34.8 dynes/cm. and specific conductivity, 7.2 X 10-7 ohm-1 cm.-1 Gallium(III) bromide readily hydrolyzes in water and forms addition compounds with ligands such as ammonia, pyridine, and phosphorus oxychloride. 

The dissociation to dinitrogen and dihydrogen, the dimerization to ammonium azide, and the cleavage to ammonia and dinitrogen are only of minor importance. 

Nothing is known of the structure of the NO ion which may be present in sodium nitroxylate, Na2N02, formed by the reaction between NaN02 and Na metal in liquid ammonia as a bright-yellow solid. The paramagnetic susceptibility of this salt is far too small for (Na 02(NO2) ", and is interpreted as due to about 20 per cent dissociation of the dimeric N2O4 ion.)... 

Clearly, the analogy between the Fast SCR and the Enhanced SCR chemistries has deeper roots than just a combination of stoichiometries. It actually originates from the key mechanistic role played by nitrates adspecies in both reactions. Such nitrates are either formed via NO2 dimerization, disproportionation, and hetero-lytic chemisorption in the Fast SCR chemistry (see Sect. 9.5.3 and Table 9.1), or are formed directly by nitric acid adsorption when feeding aqueous solutions of NH4NO3 or nitric acid in the case of Enhanced SCR. It is well known that ammonium nitrate participates in the dissociation equilibrium (9.16) with nitric acid and ammonia. 

The most favorable dimer of frans-triazene was predicted to form by simultaneous approach of the hydrogen of N, to N of the second molecule dissociation of the dimer can lead to the tautomeric triazenes [12]. The tautomerization of triazenes by a [1.3]-H shift probably has a high energy barrier [9]. The thermodynamically favored protonation site of trans-tnazene is N , and Np is the one least favored [12, 14]. Kinetic reasons suggest a protonation at N,, because the resulting cation can be considered a complex of an ammonia molecule and a diazonium ion, products which also result from the protonation of organic triazenes [14]. 

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