Monoalkylammonium

The n values were high for all of the ionic liquids investigated (0.97-1.28) when compared to molecular solvents. The n values result from measuring the ability of the solvent to induce a dipole in a variety of solute species, and they will incorporate the Coulombic interactions from the ions as well as dipole-dipole and polarizability effects. This explains the consistently high values for all of the salts in the studies. The values for quaternary ammonium salts are lower than those for the monoalkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. The values for the imidazolium salts are lower still, probably reflecting the delocalization of the charge in the cation. 

The difference in the hydrogen bond acidities and basicities was far more marked. The a value is largely determined by the availability of hydrogen bond donor sites on the cation. Values range from 0.8-0.9 for the monoalkylammonium salts, and are slightly lower (0.3-0.8) for the imidazolium salts. In the absence of a... 

Table 3.5-1 lists the E values for the alkylammonium thiocyanates and nitrates and the substituted imidazolium salts. It can be seen that the values are dominated by the nature of the cation. Eor instance, values for monoalkylammonium nitrates and thiocyanates are ca. 0.95-1.01, whereas the two tetraalkylammonium salts have values of ca. 0.42-0.46. The substituted imidazolium salts lie between these two extremes, with those with a proton at the 2-position of the ring having higher values than those with this position methylated. This is entirely consistent with the expected hydrogen bond donor properties of these cations. 

Although the measurement of the refractive index of a liquid is relatively straightforward, few have been recorded for ionic liquids to date. Monoalkylammonium... 

As to alkylammonium IPRs, it is clear that the retention enhancement of oppositely charged analytes increases with increasing numbers of alkyl nitrogen substituents monoalkylammonium < dialkylammonium < trialkylammonium < tetraalkylammo-nium. Again, the longer the alkyl chain, the more effective the IPR [12],... 

The two families of macrocyclic phosphine oxide disulfoxides (100 X = (CH2)2, 1,2-xylyl, 1,3-xylyl) and (101 X = (CH2)3 s, 1,2-xylyl, 1,3-xylyl) have been prepared stereoselectively for comparisons of three-point binding abilities with monoalkylammonium ions <92TL2107, 93PAC461, 93JA7900). Compound (101 X = 1,2-xylyl) also binds halide ions <94JA4069>. 

The values for quaternary ammonium salts are lower than those for the mono-alkylammonium salts. This probably arises from the ability of the charge center on the cation to approach the solute more closely for the monoalkylammonium salts. 

A variation of the receptor polarization effect is the binding strategy shown by schematic Complex 9. In this case the binding of an ion to one face of an appropriately designed receptor polarizes (and perhaps preorganizes) the receptor so that it can bind the counterion to the opposite face with increased affinity. Two examples that use this strategy are the macrocyclic phosphine oxide 10, which can simultaneously bind a monoalkylammonium cation and Cf J " and the cyclopeptide 11, which simultaneously binds an alkyltrimethylammonium cation and tosylate anion " (Chart 2). 

The linebroadening of chloride, bromide and iodide NMR signals effected by substituted ammonium ions was described in the preceding subsection. In addition to this phenomenon, which could be ascribed to an altered halide ion quadrupole relaxation rate close to nonpolar groups, another type of halide ion relaxation enhancement is obtained in the presence of monoalkylammonium ions with long alkyl chains. 

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