NH3-molecule

The first few rotation-inversion levels of NH3 are presented in Fig. 17. They are specified by the quantum numbers J, the total rotational angular momentum, 

Furthermore, the NH3 molecules consist of two nuclear spin modifications of total spin 3/2 (parallel) and 1/2 (antiparallel). The Fermi statistics of the three hydrogen nuclei divide the NH3 rotational energy levels into an ortho-and para-species, respectively, depending on whether AT is a multiple of 3 or not. Transitions between the two modifications are strongly forbidden. As a further consequence, for K = 0 only alternating levels occur. 

In Section B we have discussed how the basic quantities of line emission and absorption, the excitation temperature Tex and optical depth r can be determined from observations. Energies required for rotational excitation are generally low enough ( 200 cm-1) so that the rotational levels are expected to be populated even at the very low kinetic temperatures of the interstellar molecular clouds. On the other hand, with a few exceptions such as H20 and NH3, one may assume that only the lowest energy levels of interstellar molecules are populated. Under these conditions the observable fractional column density Nx may not deviate appreciably from the total column density N of a molecule, which can be computed by means of Eq. (17) on the assumption of LTE. 

However, the assumption of LTE for molecules in interstellar space is highly doubtful. In fact— as will be shown in the following Section — there are a number of observational results which clearly indicate deviations from LTE emission and absorption in interstellar space and, as soon as more interstellar molecular transitions will be observed, LTE distributions may well turn out to be the exception rather than the rule. 

The population of the molecular energy levels in interstellar space is determined by microwave and infrared radiation and by collisions with various collision partners (including dust grains) having kinetic temperatures between 5 °K and 100 °K. 

---

The interpretation of these remarkable properties has excited considerable interest whilst there is still some uncertainty as to detail, it is now generally agreed that in dilute solution the alkali metals ionize to give a cation M+ and a quasi-free electron which is distributed over a cavity in the solvent of radius 300-340 pm formed by displacement of 2-3 NH3 molecules. This species has a broad absorption band extending into the infrared with a maximum at 1500nm and it is the short wavelength tail of this band which gives rise to the deep-blue colour of the solutions. The cavity model also interprets the fact that dissolution occurs with considerable expansion of volume so that the solutions have densities that are appreciably lower than that of liquid ammonia itself. The variation of properties with concentration can best be explained in terms of three equilibria between five solute species M, M2, M+, M and e ... 

These hydrates are not ionically dissociated but contain chains of H2O molecules cross-linked by NH3 molecules into a three-dimensional H-bonded network. 

The question arises whether an external electric field will have any large influence on the direction of these proton transfers. In the NH3 molecule all three protons are situated in one hemisphere of the electronic cloud, and so give to the molecule a dipole moment. In the (NH4)+ ion, on the other hand, it is generally accepted that the four protons are placed symmetrically at the corners of a tetrahedron. Accordingly, the (NH4)+ ion will have no dipole moment. 

In Fig. 37 two areas have been shaded. The area in the upper left corner, where protons in occupied levels are unstable, we have already discussed. In the lower right-hand corner the shaded area is one where vacant proton levels cannot remain vacant to any great extent. In aqueous solution any solute particle that has a vacant proton level lower than that of the hydroxyl ion will capture a proton from the solvent molecule, since the occupied level of the latter has the same energy as the vacant level of a hydroxyl ion. Consequently any proton level that would lie in this shaded area will be vacant only on the rare occasions when the thermal agitation has raised the proton to the vacant level of a hydroxyl ion. On the other hand, there are plenty of occupied proton levels that lie below the occupied level of the H2O molecule. For example, the occupied level of the NH3 molecule in aqueous solution lies a long way below that of H20. 

Substituted Ammonium Ions. Like NH4C1 the substance NH3-(CH3JCI, where a CH3 group has been substituted for one hydrogen, forms a crystalline solid and so do the substances NH2(CH3)2C1 and NH(CH3)3C1. When one of these substances is dissolved in water, it is completely dissociated into Cl- ions and molecular positive ions corresponding to (NH4)+. Suppose now that such a solution contains an NH3 molecule, and consider the following proton transfer... 

With these principles in mind, consider the NH3 molecule ... 

Notice the similarity between the equation just written and that for the NH3 molecule cited above—... 

When ammonia is added to an aqueous solution of a copper(II) salt, a deep, almost opaque, blue color develops (Figure 15.1). This color is due to the formation of the Cu(NH3)42+ ion, in which four NH3 molecules are bonded to a central Cu2+ ion. The formation of this species can be represented by the equation... 

The nitrogen atom of each NH3 molecule contributes a pair of unshared electrons to form a covalent bond with the Cu2+ ion. This bond and others like it, where both electrons are contributed by the same atom, are referred to as coordinate covalent bonds. 

Strategy It s best to approach a problem of this type systematically. You might start (1) by putting two NH3 molecules trans to one another and see how many different structures can be obtained by placing the third NH3 molecule at different positions. Then (2) start over with two NH3 molecules cis to one another, and follow the same procedure. 

Starting with two NH3 molecules trans to one another (Fig. 15.6a, p. 416), it clearly doesn t matter where you put the third NH3 molecule. All the available vacancies are equivalent in the sense that they are cis to the two NH3 molecules already in place. Choosing one of these positions arbitrarily for NH3 gives the first isomer (Figure 15.6b) the Cl- ions occupy the three remaining positions. 

The NH3 molecule acts as a Brensted-Lowry base in water, accepting a proton from a water molecule ... 

The grouping of ammonium salts in a separate section serves to emphasize the similarities of behaviour which are apparent in reactions yielding the volatile NH3 molecule, following removal of a proton from the NH4 cation. This property is not unique indeed, many cations are volatile and numerous salts leave no residue on completion of decomposition. Few kinetic investigations have, however, been reported for other compounds, in contrast to the extensive and detailed rate measurements which have been published for solid phase decompositions of many ammonium salts. Comparisons with the metal salts containing the same anion are sometimes productive, so that no single method of classification is altogether satisfactory. 

Hajek et al. [173] have reported a detailed kinetic study of the solid phase decomposition of the ammonium salts of terephthalic and iso-phthalic acids in an inert-gas fluidized bed (373—473 K). Simultaneous release of both NH3 molecules occurred in the diammonium salts, without dehydration or amide formation. Reactant crystallites maintained their external shape and size during decomposition, the rate obeying the contracting volume equation [eqn. (7), n = 3]. For reaction at 423 K of material having particle sizes 0.25—0.40 mm, the rate coefficients for decompositions of diammonium terephthalate, monoammonium tere-phthalate and diammonium isophthalate were in the ratio 7.4 1.0 134 and values of E (in the same sequence) were 87,108 and 99 kJ mole-1. 

Sei e-Tfst 3.3A (a) Give the VSEPR formula of an NH3 molecule. Predict (b) its electron arrangement and (c) its shape. 

This solution has pH > 7. When a few drops of a solution of strong base are added, the incoming OH ions remove protons from NH4+ ions to make NH3 and H20 molecules. When instead a few drops of a strong acid are added, the incoming protons attach to NH3 molecules to make NH4+ ions and hence are removed from the solution. In each case, the pH is left almost unchanged. 

The alkali metals also release their valence electrons when they dissolve in liquid ammonia, but the outcome is different. Instead of reducing the ammonia, the electrons occupy cavities formed by groups of NH3 molecules and give ink-blue metal-ammonia solutions (Fig. 14.14). These solutions of solvated electrons (and cations of the metal) are often used to reduce organic compounds. As the metal concentration is increased, the blue gives way to a metallic bronze, and the solutions begin to conduct electricity like liquid metals. 

Ammonia is a pungent, toxic gas that condenses to a colorless liquid at — 33°C. The liquid resembles water in its physical properties, including its ability to act as a solvent for a wide range of substances. Because the dipole moment of the NH3 molecule (1.47 D) is lower than that of the H20 molecule (1.85 D), salts with strong ionic character, such as KCI, cannot dissolve in ammonia. Salts with polarizable anions tend to be more soluble in ammonia than are salts with greater ionic character. For example, iodides are more soluble than chlorides in ammonia. Liquid ammonia undergoes much less autoprotolysis than water ... 

Ammonia is very soluble in water because the NH3 molecules can form hydrogen bonds to H20 molecules. Ammonia is a weak Bronsted base in water it is also a reasonably strong Lewis base, particularly toward d-block elements. For example, it reacts with Cu2+(aq) ions to give a deep-blue complex (Fig. 15.4) ... 

There are three NH3 molecules (ammine) and three H20 molecules (aqua). 

Just as there are weak acids, there are also weak bases. A weak base does not readily accept protons from water molecules but does quantitatively accept protons from hydronium ions. Ammonia is the most common weak base. Ammonia exists predominantly as NH3 molecules in aqueous solution, but it undergoes quantitative proton transfer with hydronium ions to generate ammonium ions ... 

The lighter NH3 molecules diffuse more rapidly than the heavier HCl molecules, so the white band of salt forms closer to the HCl end of the tube, as can be seen in Figure 5-15. 

Concentrated aqueous ammonia is known commercially as ammonium hydroxide (NH4 OH) (because H2 O molecules can transfer protons to NH3 molecules to... 

On the other hand, NH3 can provide both nitrogen and hydrogen atoms. Each NH3 molecule also contains two extra hydrogen atoms that can be burned to produce water ... 

---