Liquid ammonia ionization

With the thermospray interface (Figure 4.38(a)), the mobile phase, usually containing an ammonium ethanoate buffer, is passed through a heated probe (350-400°C) into an evacuated source chamber where it forms a supersonically expanding mist of electrically charged droplets. The liquid evaporates to leave charged solid particles which then release molecular ions such as MH+ and, VI by an ammonia chemical ionization (Cl) process. The analyte ions are skimmed off into the mass spectrometer whilst the vaporized solvent is pumped away. An electron beam is also employed to enhance the production of ions by Cl. 

Hi) Liquid ammonia Liquid NH3 has a strong tendency to accept protons. In this medium even a weak acid, e.g., CH3COOH ionizes to a considerable extent and thus, behaves as a strong acid. 

The development of the theory of solvent systems was begun by Franklin in 1905. Reasoning from formal analogy to the hydrogen ion-hydroxyl ion theory he defined acids and bases in liquid ammonia. According to his theory, if water ionizes into hydronium and hydroxyl ions, liquid ammonia must ionize into ammonium and amide ions ... 

Aromatic steroids are virtually insoluble in liquid ammonia and a cosolvent must be added to solubilize them or reduction will not occur. Ether, ethylene glycol dimethyl ether, dioxane and tetrahydrofuran have been used and, of these, tetrahydrofuran is the preferred solvent. Although dioxane is often a better solvent for steroids at room temperature, it freezes at 12° and its solvent effectiveness in ammonia is diminished. Tetrahydrofuran is infinitely miscible with liquid ammonia, but the addition of lithium to a 1 1 mixture causes the separation of two liquid phases, one blue and one colorless, together with the separation of a lithium-ammonia bronze phase. Thus tetrahydrofuran and lithium depress the solubilities of each other in ammonia. A tetrahydrofuran-ammonia mixture containing much over 50 % of tetrahydrofuran does not become blue when lithium is added. In general, a 1 1 ratio of ammonia to organic solvents represents a reasonable compromise between maximum solubility of steroid and dissolution of the metal with ionization. 

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 ... 

Liquid ammonia is the best-known and most widely studied non-aqueous ionizing solvent. Its most conspicuous property is its ability to... 

Liquids that form conducting solutions are called ionizing solvents. A few other compounds (ammonia, NH3i sulfur dioxide, S02, sulfuric acid, H2SO<, etc.) are ionizing solvents but water is by far the most important. We will discuss water exclusively but the same ideas apply to the other solvents in which ions form. 

In these terms, Shatenshtein s partial rate factors (37) for the H—D exchange of monosubstituted benzenes in liquid ammonia are not anomalous (2m). The analysis of this rate data is included in Table XXVIII. Even though the data are not precise enough to meet the desired standard of precision of fits, the sequences > X and P/> p p - P/ are clearly evident in the results. Consequently, this data set (which is of the or type) may be taken as providing supporting evidence with the pyridinium ionization (which is of the gr type) for unique X blends for the positions o- and p- to ionization from the ring position as compared with the X blends for side-chain ionization. 

Mass Spectrometry. Mass spectrometry holds great promise for low-level toxin detection. Previous studies employed electron impact (El), desorption chemical ionization (DCI), fast atom bombardment (FAB), and cesium ion liquid secondary ion mass spectrometry (LSIMS) to generate positive or negative ion mass spectra (15-17, 21-23). Firm detection limits have yet to be reported for the brevetoxins. Preliminary results from our laboratory demonstrated that levels as low as 500 ng PbTx-2 or PbTx-3 were detected by using ammonia DCI and scans of 500-1000 amu (unpublished data). We expect significant improvement by manipulation of the DCI conditions and selected monitoring of the molecular ion or the ammonia adduction. 

Both the a- and a -hydrogens of dibenzyl sulfone were shown to be ionized by means of two equivalents of potassium amide in liquid ammonia. ... 

We selected liquid ammonia because of its pronounced solubilizing characteristics and powerful ionizing properties. At -33°C and atmospheric pressure, the pKa-value for auto-ionization of liquid ammonia [2NH3 = NH2 + NH4 ] is 34 and since the equivalent value for water is only 14, many substances (with pKa-values between 14 and 34) which are neutral in water should be capable of splitting off protons in liquid ammonia. Acidic... 

For highly polar media, the yield of the solvated electron can serve as a lower limit to the ionization yield. This method needs short-time measurement and may work for liquid water and ammonia. Farhataziz et al. (1974) determined the G value—that is, the 100-eV yield—of solvated electrons in liquid NH3 to be about 3.1 at -50 ns. This corresponds to a W value of 32 eV, compared with the gas-phase value of 26.5 eV. The difference may be attributed to neutralization during the intervening time. In liquid water, it has been found that G(eh) increases at short times and has a limiting value of 4.8 (Jonah et al., 1976 Sumiyoshi et al, 1985). This corresponds to W,. = 20.8 eV compared with Wgas = 30 eV (Combecher, 1980). Considering that the yield of eh can only be a lower limit of the ionization yield, suggestions have... 

The conductivity of liquid ammonia is sufficiently high to indicate a very slight degree of autoionization. In order for ions to be produced, something must be transferred from one molecule to another, and in solvents such as water or ammonia it is proton transfer that occurs. Accordingly, the ionization of liquid ammonia can be shown as... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

Liquid ammonia is a base, so reactions with acids generally proceed to a greater degree than do the analogous reactions in water. For example, acetic acid is a weak acid in water, but it ionizes completely in liquid ammonia. Even though ammonia is a base, it is possible for protons to be removed, but only when it reacts with exceedingly strong bases such as N3, O2, or 11. Some of the important types of reactions that occur in liquid ammonia will now be illustrated. 

Because of the base strength of liquid ammonia, acids that are weak in water ionize completely in liquid ammonia. 

Solutions of alkali metals in liquid ammonia have been studied by many techniques. These include electrical conductivity, magnetic susceptibility, nuclear magnetic resonance (NMR), volume expansion, spectroscopy (visible and infrared), and other techniques. The data obtained indicate that the metals dissolve with ionization and that the metal ion and electron are solvated. Several simultaneous equilibria have been postulated to explain the unique properties of the solutions. These are generally represented as follows ... 

These data were measured at about 1 wt % NH in the liquid phase instead of 0.1 wt % NH, used for the NaOH measurements. For this reason they do not extrapolate to the same volatility ratio at zero concentration of electrolyte. When correction is made for ionization and solvent effects of ammonia, then the two intercepts agree. 

It is therefore apparent that dissociation constants may only be compared in the same solvent. Ammonia is a stronger donor than water, but liquid ammonia has a much lower dielectric constant than the latter. The acidity constant of hydrochloric acid in liquid ammonia is much lower than in water, in which it is completely ionized and completely dissociated, whereas the complete ionization in liquid ammonia is not followed by extensive ionic dissociation due to its low dielectric constant. On the other hand, the acidity constant of acetic acid is somewhat higher in liquid ammonia than in water since in the latter if Ion is much lower than in liquid ammonia, in which complete ionization is achieved. 

If then, one considers these two compounds as ionizing solvents, it should be possible to prepare derivatives containing either the cation or anion of the solvent which, in the parent solvent, would have properties analogous to those of either an acid or a base. Here one is guided by the analogy with solvents such as liquid ammonia, in which anuno-nium salts behave as acids and metal amides as bases. This expectation was, in fact, realised in full. 

It was also observed, in 1973, that the fast reduction of Cu ions by solvated electrons in liquid ammonia did not yield the metal and that, instead, molecular hydrogen was evolved [11]. These results were explained by assigning to the quasi-atomic state of the nascent metal, specific thermodynamical properties distinct from those of the bulk metal, which is stable under the same conditions. This concept implied that, as soon as formed, atoms and small clusters of a metal, even a noble metal, may exhibit much stronger reducing properties than the bulk metal, and may be spontaneously corroded by the solvent with simultaneous hydrogen evolution. It also implied that for a given metal the thermodynamics depended on the particle nuclearity (number of atoms reduced per particle), and it therefore provided a rationalized interpretation of other previous data [7,9,10]. Furthermore, experiments on the photoionization of silver atoms in solution demonstrated that their ionization potential was much lower than that of the bulk metal [12]. Moreover, it was shown that the redox potential of isolated silver atoms in water must... 

The results of some of the many aminations of pyridine and its derivatives that have been carried out appear in Table 14. Yields are quoted where possible but these should not be used for quantitative comparisons as reaction and work up conditions vary widely. 2-Alkylpyridines aminate at the vacant a-position, except when the substituent is very large. 2-f-Butylpyridine does not undergo the Chichibabin reaction, probably because the bulky 2-f-butyl group prevents adsorption on to the sodamide surface. In contrast, 2-phenylpyridine undergoes amination in very good yield. Aminations of 2- and 4-methyl-pyridines do not involve attack on the anhydrobases in aprotic solvents, but some ionization does take place in liquid ammonia. 4-Benzylpyridine forms a carbanion (148) which is only aminated with difficulty by a second mole of sodamide (equation 103). 

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