Alkaline earth cations, ionic radii

A2-18C6, and of 1,10-T2-18C6. The thermodynamic reasons for this can be understood better by referring to Figure 15.17 where we graph ArG°, ArH°, and TAtS° for the formation of the Sr2+, Ag+, and Hg2+ complexes with the three macrocycles.11 The thermodynamic differences ArZ° for the three cations are plotted on the same scale so that comparisons can easily be made. The alkaline earth cation Sr2+ was chosen for comparison since it has nearly the same ionic radius as Hg2+ and Ag+. 

The observation of very pronounced inhibition of the sodium dodecyl sulfate-catalyzed hydrolysis of methyl orthobenzoate by inorganic cations is also consistent with the proposed mechanism for the micelle catalyzed reaction (Romsted et al., 1967 Dunlap and Cordes, 1968). For alkali-metal cations, the inhibition was found to increase with increasing ion size, i.e. ionic radius, but for alkaline-earth cations the inhibitory effectiveness was observed to be relatively independent of the ion. For... 

The above 2+ fit parameters can also be used to derive Dpi, from Dg, the nearest (in terms of ionic radius) alkaline earth cation. For the Icenhower and London (1996) experiments we again obtain a linear correlation with Aor ... 

Two alkaline-earth cations, Mg " and Ca ", were studied at the Al-3 ring. The alkaline-earth cations show trends like alkali cations with similar ionic radius the location of Mg is similar to Lr and that of Ca similar to Na [128]. Mg is close to the plane of T-atoms of the six-ring Al-3, with 203 pm as the shortest Mg-0 distance. The position of Ca " at the six-ring Al-3 is at 30 pm above the plane of T-atoms of the ring, with 229 pm as shortest Ca-0 distance. Such a location of the cation allows simultaneous coordination of more than one probe molecule (CO or N2). IR spectra suggest that even three CO molecules can be coordinated to a single Ca cation [146], likely due to its stronger electrostatic field compared to Na. ... 

Figure 7.3 plots the ratio of crystal radius versus charge for selected ions. Oxyanions—sulfate, selenate, phosphate, arsenate, borate, molybdate, carbonate, and silicate—are represented by their central cations S6+, Se6+, P5+, As5+, B3+, Mo4+, C4+, and Si4+. The ions fall into three behavioral groups. Ions of high ionic potential, the alkali and alkaline earth cations and the halide anions, large univalent and divalent ions, are highly water soluble, easily weatherable, and leach readily from soils to the sea over geologic time. 

However, within a series of monovalent cations with increasing ionic radius, such as Li+,K+,Cs , the permeability decreases in this order for nonpolar gases. The same is true for alkaline earth cations in the series Mg +,Mg +,Ca. Therefore, it is concluded that both the size of the cation plays a role in increasing the void volume, and the change in polarity is another factor for the performance [84]. In brominated PPE, the permeability of gases increases with the degree of bromination [85]. 

The alkali and alkaline earth cations are hydrated in solution and their rates of water exchange are extremely fast, almost approaching the diffusion controlled limit. The rate determining step in the exchange process is the dissociation of a solvent molecule to leave a vacancy in the first coordination sphere, Fig. 1.14. The smaller ions with the most intense electrostatic field (proportional to charge/ionic radius) and the higher heats... 

Partial master curves of 10 g.dL"l solutions of a,o)-alkaline earth dicarboxylato PBD in xylene at 297 K are reported in Figure 10, and result from a good frequency-temperature superposition of the experimental data.l7 Only the G" master curve of the solution of Be-based HTP is ill-defined due to the poor accuracy in the determination of the very small values of G". The shift factors support an apparent Arrhenius-type of dependence (Figure 11), from which the activation energy of the observed secondary ionic relaxation process was calculated and found to decrease as the radius of the alkaline earth cations increases (Figure 12). One also observes that the relaxation spectrum calculated by the first order approximation of Ninomiya and Ferry S is displaced along the time scale in relation with the cation size (Figure 13). The dynamic behavior of the 10 g.dL solution is obviously... 

By studying a series of complexes, it is possible to observe the differences in structural type that occur with change of cation radius. Table 6 shows the ionic radii for the alkali and alkaline earth metal cations, together with the average ligand cavity radii for simple polyethers.33 From this information it can be seen that the predicted optimal fit situation for 1 1 complexes would arise for Li+ and 12-crown-4 (74) for Na+ and 15-crown-5 (75) and for K+ and Ba2+ and 18-crown-6 (76). For 24-crown-8 (77) all of the cations have smaller radii than that of the ligating cavity. 

When Z is a simple aquacation, two types of complex are formed depending upon the ionic radius of Z. For alkali, alkaline earth and most transition metal cations the product contains Z"+ in quasi-octahedral coordination. Equilibrium constants for reaction (6) have been determined for Li+, Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Fe2+, Co2+, Ni, Cu2+ and Zn2+.93 For the transition metals, log K lies between 3 and 9, and is sensitive both to Z and to the lacunary polyanion involved. Larger cations, Sr24, Ba2+, and tri- and tetra-valent lanthanides and actinides are also able to bind two lacunary ligands in a manner similar to that illustrated in Figure 18. Although the stepwise formation of 1 1 and 2 1 complexes of the... 

The reactions of chlorobenzene and benzaldehyde with ammonia over metal Y zeolites have been studied by a pulse technique. For aniline formation from the reaction of chlorobenzene and ammonia, the transition metal forms of Y zeolites show good activity, but alkali and alkaline earth metal forms do not. For CuY, the main products are aniline and benzene. The order of catalytic activity of the metal ions isCu> Ni > Zn> Cr> Co > Cd > Mn > Mg, Ca, Na 0. This order has no relation to the order of electrostatic potential or ionic radius, but is closely related to the order of electronegativity or ammine complex formation constant of metal cations. For benzonitrile formation from benzaldehyde and ammonia, every cation form of Y zeolite shows high activity. 

Fig. 13. Plot of 2 versus radius, r (in pm)/ionic charge, for the complexation of (29) with a series of alkaline earth and lanthanide metal cations.

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. ... 

Spectra have also been reported for alkaline earth complexes of bipyridyl, terpyridyl, and substituted bipyridyls and phenanthrolines (106,107) of the type M +(L )2. For Be, Mg, Ca, and Sr the spectra show the presence of a ground, orvery lowexcited, triplet state consistent with a divalent metal cation in a tetrahedral environment with one electron on each chelate ligand. The similarity in splittings for Be and Mg is thought to show that interligand interactions prevent the achievement of the small Be + ionic radius. Treatment of bipyridyl with zinc amalgam does... 

Fig. 3.2 Plots of —z((AsGi) against ionic radius r for the alkali metal cations ( ), alkaline earth metal cations ( ), halide anions (a) and the sulhde anion (t). The parameters 5s and/dd are determined from the intercept and slope of these plots, respectively, according to equation (3.5.14). The plot for cations has been shifted vertically by 1 mol MJ for the sake of clarity.

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