Cation size influence

Exceptionally stable and electrochemically well-behaved SAMs bearing dithia-crown-annelated TTF derivatives were prepared using bipodal binding of the receptor molecules to Au electrodes [73, 74]. The CVs in Fig. 9 are of much better quality compared to those in Fig. 8, indicating that formation of TTF" " crown after the first oxidation does not repel the metal ion from the complex. The interplay between macrocycle size and cation size influences the shift of the redox waves upon complexation. The redox potentials of the SAM of la are not sensitive to the presence of li" " and K+ in solution, while Na+ causes a small shift of 10 mV in the first wave. The SAM of lb shows high sensitivity to Na+ (55-60 mV, both waves) and a moderate sensitivity to K+ (20-30 mV), but li+ does not influence the signal. 

The fourth lifetime components, r4, have values in the range of 2.7 - 5.0 ns (shown in Table 1 and 2), which, using equation (1), gives 0.34 - 0.47 nm for radii of the holes that are listed in Table 3 and 4. These lifetimes have also lower intensities than r3, suggesting the presence of the larger voids already in the initial phase of zeolite formation, but in a smaller extent. Here the correlation to the influence of cation sizes cannot be established. 

It was shown that the rate of olefin production was dependent on the size of the metal cation (Li+, Na+, K+, Rb+, and Cs+) of the base. The increase in olefin production with an increase in cation size was explained as an increase in cation solvation by solvent with the larger cations, making t-butoxide a stronger base and thus increasing the rate of proton abstraction. Possibly these differences in cation solvation influence the reactivity of the carbanion intermediate for the conversion of benzothiophene and account for increased reactivity with the larger cation, potassium. 

A large effective cation size should suppress the cation-siloxane coordination, favor the Iree ion pair 3 in reaction 10, and enhance charge separation. The anticipated effects would be values of n approaching 1 in the rate equation 9, greatly enhanced rates of polymerization, and suppressed formation of cyclosiloxanes. Evidence that these effects are achieved is indicated by the effects seen with R4N countercations (35), the lithium cryp-tates (25, 27), and the crown ether-potassium silanolate complexes (39, 40). Additional evidence for the influence of the countercation on the equilibria is seen in deviations of the amounts of oligomer produced in equilibrated poly(dimethylsiloxane) from the normal distribution caused by specific interactions between the potassium silanolate chain ends (37, 38). More de-... 

To probe the potential influence of cation size differences on the distribution, Na[ Cs SEDOR experiments have also been conducted on mixed sodium cesium borate glasses containing 30 mol% alkali [57]. Being a nucleus (1=7/2) with a moderately small nuclear electric quadrupolar moment, the Cs nucleus features similar spectroscopic characteristics as Li. Usually sizeable first-order quadrupolar splittings reduce the inversion efficiency of the n-... 

Cation was found to be the key parameter influencing both the nanotube growth rate and length [49], With increasing cation size, the interfacial oxide layer was getting thinner. Ionic transport was facilitated and the nanotube growth was enhanced. The thinnest nanotubes ever reported was 5 nm, which was obtained in an electrolyte containing 0.5 M tetrabutylammonium fluoride in formamide with 5% water (Fig. 8). 

It was of interest to examine the influence of the cation size on the mode of bonding, and therefore the isostructural strontium and barium uranyl dimalon-ate trihydrates were studied [42]. As in the case of the ammonium salt, one malonate ligand is bidentate to a uranium atom giving rise to a six-membered ring. 

Cation size and charge are known to influence the selectivity of zeolite synthesis (1). For instance, faujasite synthesis is quite specific to sodium systems, whereas zeolite L is formed in the presence of potassium. The effects of cation composition on the distribution of silicate species has been examined by only a few authors. Ray and Plaisted (13.), using trimethylsilation/... 

Since pH is known to influence strongly the distribution of silicate anions in alkaline solutions (7.9), one might expect the observed changes in silicate anion distribution with cation size to be a result of changes in pH. Reference to Table 1 shows, however, that for a fixed silicate ratio, the pH of the solutions is relatively independent of the base composition and certainly does not increase with cation size, as might be expected from a consideration of hydroxide dissociation constants (171. This suggests that the influence of cation composition is a consequence of direct cation-anion interactions, rather than an effect of pH. 

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