Mixed acceptor number

Figure 2.11, on the other hand, shows the acceptor numbers of mixtures of water and aprotic solvents. Because water is protic and selectively interacts with Et3P = 0 (strong Lewis base), many of the relations curve upward. However, with HMPA, the relation curves downward, because HMPA is a strong base and easily interacts with water to weaken the interaction between water and Et3P = O. The acidity and basicity of mixed solvents are influenced not only by the acidity and basicity of the constituent solvents but also by the mutual interactions between the molecules of constituent solvents. At present, however, this cannot be treated theoretically. 

Ej.(30) becoming a mixed function of n and a. Later, a number of twin probes were tried, perhaps the most important being those from which the DN ( solvent donicity ) and AN ( acceptor number ) scales of Gutmann [40] were generated. These involve electron donation by solvent to probe, for example, to SbClj, and electron donation by probe, for example, EtjPO to the protonic solvent, respectively. Quite good correlations exist between and DN [5], and between a and AN [1],... 

A number of other donor—acceptor molecular soHds such as TMPD -TCNQ or TMPD chloraniF crystalline as mixed stacks of alternating D and A molecules. These compounds typically have much higher resistivities than the segregated salts because the alternating -DADA- sequence leaves no continuous channel for conduction. 

Since spillover phenomena have been most directly sensed through the use of IR in OH-OD exchange [10] (in addition, in the case of reactions of solids, to phase modification), we used this technique to correlate with the catalytic results. One of the expected results of the action of Hjp is the enhancement of the number of Bronsted sites. FTIR analysis of adsorbed pyridine was then used to determine the relative amounts of the various kinds of acidic sites present. Isotopic exchange (OH-OD) experiments, followed by FTIR measurements, were used to obtain direct evidence of the spillover phenomena. This technique has already been successfully used for this purpose in other systems like Pt mixed or supported on silica, alumina or zeolites [10]. Conner et al. [11] and Roland et al. [12], employed FTIR to follow the deuterium spillover in systems where the source and the acceptor of Hjp were physically distinct phases, separated by a distance of several millimeters. In both cases, a gradient of deuterium concentration as a function of the distance to the source was observed and the zone where deuterium was detected extended with time. If spillover phenomena had not been involved, a gradientless exchange should have been observed. 

More recent developments in the field of the Pirkle-type CSPs are the mixed r-donor/ r-acceptor phases such as the Whelk-Of and the Whelk-02 phases.The Whelk-Of is useful for the separation of underiva-tized enantiomers from a number of families, including amides, epoxides, esters, ureas, carbamates, ethers, aziridines, phosphonates, aldehydes, ketones, carboxylic acids, alcohols and non-steroidal anti-inflammatory drugs.It has been used for the separation of warfarin, aryl-amides,aryl-epoxides and aryl-sulphoxides. The phase has broader applicability than the original Pirkle phases. The broad versatility observed on this phase compares with the polysaccharide-derived CSPs... 

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