Lewis acid-base model resonance

The model of Krygowski and Fawcett [Kr 75], developed for the description of the solvent effect and taking into account exclusively the Lewis acid-base properties of the solvent, also appears suitable for the description of the solvent dependence of the chemical shift. For example, this model reflects well the results of the Na resonance studies by Erlich et al referred to earlier [Er 70, Er 71, Gr 73]. In addition to the interaction between the sodium ion and the solvent, it also points to the dependence of the chemical shift on the concentration as a result of ion-pair formation. However, the authors themselves [Fa 76] reported that the model was unsuitable for the description of other NMR data reflecting the solvent effect. 

A profitable way to quantitate the Lewis acid sites is to study the adsorbed molecules directly. For instance. Fig. 11 shows the C NMR results for butyl-amine adsorbates on 7-alumina [62]. In the adsorbate, there are two distinct resonances each for the a- and the P-carbon atoms of butylamines. Based on the model complex spectra included in Fig. 11, these peaks can be assigned to molecules bound to Bronsted and Lewis acid sites, respectively. In a similar fashion, C [63] and especially N CPMAS NMR studies of pyridine adsorbates on 7-alumina are able to differentiate among physisorbed molecules (64... 

---