Ionic radius ammonium

The above speculation [21] may be extended to include the related quaternary ammonium compounds such as xylocholine (XXXIX). It is probable that the volumes of the guanidinium ion and the trimethylammonium group are similar. The ionic radius of the guanidinium ion (IX) is about 3A the ionic radius of the tetramethylammonium ion has been estimated [300] to be 3-4A, although rather smaller values have also been proposed [301-303]. Crystallographic analyses of muscarine iodide [304], choline chloride [305] and acetylcholine bromide [306] have revealed that the carbon to nitrogen distance is about l-SA, and that a hydrogen bond (C-H-0 distance 2-87-3 07A) exists in the crystals of these compounds. 

Professor J. D. Dunitz has suggested to me that potassium ion may have replaced ammonium ion, which was ubiquitous in prebiotic times. When oxygen concentration in the atmosphere rose and ammonium ion disappeared it could have been replaced in emerging living systems by potassium ion, which in the meantime had become available and has nearly the same ionic radius as ammonium. 

J6 The ammonium km is about the same size (r+ = 151 pm) as the potassium ion ir. 152 pm) and this is a usef ul fact to remember when explaining the resemblance in properties between these two tuns. For example, (he solubilities of ammonium salts arc similar to those of potassium sails. Explain the relation between ionic radius and soloWiiy. On the other hand, all of the potassium halides crystallize in the NaClstrocture with C.N. = 6 (see Chapter 4). but none of the ammonium halides does so. The coordination numbers of the ammonium halides are either four or eight- Suggest an explanation. 

The above reported work is supplemented by the fact127) that in the presence of water the so-called effective ionic radius (as defined by considering the hydration sphere) of the ammonium is according to Conway25) 2.5 A. This is comparable to that of cesium or rubidium. With regard to sterical considerations, it is par-... 

As the coordination number of an ion is thus increased from 6 to 8, it appears that the ionic radius also suffers a slight increase, presumably because the repulsion forces, exerted by the electron clouds of neighboring ions, increase. For the rubidium halides and the ammonium halides which can assume either structure (depending upon pressure and temperature), the interionic distances in the structures having coordination number 8 are about 3 percent greater than the distances in structures with coordination number 6. This increase is close to the value that would be... 

The dissociation constants of trityl and benzhydryl salts are KD 10 4 mol/L in CH2C12 at 20° C, which corresponds to 50% dissociation at 2-10-4 mol/L total concentration of carbocationic species (cf. Table 7) [34]. The dissociation constants are several orders of magnitude higher than those in analogous anionic systems, which are typically KD 10-7 mol/L [12]. As discussed in Section IV.C.l, this may be ascribed to the large size of counterions in cationic systems (e.g., ionic radius of SbCL- = 3.0 A) compared with those in anionic systems (e.g., ionic radius of Li+ 0.68 A), and to the stronger solvation of cations versus anions. However, the dissociation constants estimated by the common ion effect in cationic polymerizations of styrene with perchlorate and triflate anions are similar to those in anionic systems (Kd 10-7 mol/L) [16,17]. This may be because styryl cations are secondary rather than tertiary ions. For example, the dissociation constants of secondary ammonium ions are 100 times smaller than those of quaternary ammonium ions [39]. 

A preferential uptake of the Ba " ion (due to having an ionic radius close to that of the ammonium ion) among the alkaline earth metals also confirms this hypothesis. At the same time, the affinity for other divalent elements is at least one order of magnitude higher than that for alkali or alkaline earth metal cations. The Kd values for most transition metals and lead are in the range from 20000 to more than 100000. However, the absolute values of their uptake in 0.1 M metal nitrate solutions are relatively low. The lEC values are higher than 1.0 meq g" for only two ions, Cu and Hg. ... 

Typically, monovalent cations such as alkali metals are separated using a dilute mineral acid as the eluent. Examples for the separation of alkali metals are displayed in Figs. 3-129, 3-130 (Section 3.4.1.1), and 3-132 (Section 3.4.1.2). These figures reveal that the retention of the alkali metals increases with increasing ionic radius. Compared to conventional instrumental analysis methods, the advantage of ion chromatography is the simultaneousness of the method. Without any doubt, the key ion in this chromatogram is ammonium which elutes between sodium and potassium, Its sensitive detection by other methods is very difficult. 

It is clear that, if available, monocations such as potassium and sodium will be incorporated into a quadruplex. The potassium and ammonium ions are too large to be coordinated by a single G-quartet in a coplanar fashion. As a result, coordination of these ions occurs between two G-quartets planes. Each quadruplex involving n quartets will then accommodate n — 1) of these specific ions. For example, quantitative determination of ammonium peak intensity revealed that three NH4 ions are placed between four quartets. In contrast, the smaller Na" ion (ionic radius of 1.18 A) allows for in-plane coordination. Multiple Na" ions are therefore not restricted to the spacing between G-quartets, and can move further away form each other to reduce electrostatic repulsions. In any case, empty sites between quartets are probably very rare. In fact, although vacant coordination sites are likely to exist (as ions move between sites, see below), their lifetime must be very short as demonstrated, for example, by Federiconi et who determined a site occupancy of 0.97 K " ions per tetramer in GMP quadruplexes prepared in 0.5 M KCl. 

Monovalent ions (K, Na ) tend to bind weakly and are particularly difficult to probe. Thus, mimics have been an important tool for probing monovalent ion binding sites. Ammonium ion has an ionic radius similar to K , and is a useful NMR probe as its protons can give rise to NOEs to nucleic acid protons. In addition, the nitrogen of NH4+ may be detected directly. An example is the detection of NH4 + binding to the DNA quadruple [d(G4T4G4)]2 use of N... 

Another alkaline catalyst applied for the preparation of resoles is ammonia It is well known that ammonia can dissociate in aqueous solution to form ammonium hydroxide which can also function as a catalyst. Furthermore, the anunonium ion has a comparable ionic radius to the potassium ion. However, ammonia also catalyses the formation of Mannich-bases, Schiffsch-bases, and other structures that are represented as by-products in the mixture of resoles formed. These by-products contribute to a coloring ofthe... 

As the ammonium ion is usually classed within the series of alkali metal ions, with an ionic radius equal to that of Rb, so it is useful in crystal chemistry to ponder upon the similarity between and an alkaline... 

The properties of the ammonium ion can in many respects be considered to be similar to those of the alkali ions. The ammonium ion has an effective ionic radius which is close to that of the rubidium ion. In line with this the halide ion relaxation rates in aqueous solutions of NH Cl [248], NH Br [227 248] and NH I [227] increase very slowly with concentration, i.e. the same behaviour as noted above for the... 

TABLE 4.1 Snnmiary of the Mean Distance of Closest Approach Values (a/10" m) for Ammonium Salts in Aqueous Solutions, Estimated fiom Experimental Data, Ionic Radius and other Theoretical Approaches... 

Specific Ligand Design.—The specificity of crown and related ligands for certain metal and ammonium cations is well known, and is rationalized, at least partially, by a best fit relationship between ionic radius and macrocycle cavity size. A... 

It should not be inferred that the crystal structures described so far apply to only binary compounds. Either the cation or anion may be a polyatomic species. For example, many ammonium compounds have crystal structures that are identical to those of the corresponding rubidium or potassium compounds because the radius NH4+ ion (148 pm) is similar to that of K+ (133 pm) or Rb+ (148 pm). Both NO j and CO, have ionic radii (189 and 185 pm, respectively) that are very close to that of Cl- (181 pm), so many nitrates and carbonates have structures identical to the corresponding chloride compounds. Keep in mind that the structures shown so far are general types that are not necessarily restricted to binary compounds or the compounds from which they are named. 

As we end this section, let us reconsider ionic radii briefly. Many ionic compounds contain complex or polyatomic ions. Clearly, it is going to be extremely difficult to measure the radii of ions such as ammonium, NH4, or carbonate, COs, for instance. However, Yatsimirskii has devised a method which determines a value of the radius of a polyatomic ion by applying the Kapustinskii equation to lattice energies determined from thermochemical cycles. Such values are called thermochemical radii, and Table 1.17 lists some values. 

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