Protonic species and their structures

Detailed information on the proton hydrate series in the solid state is available, and the reader is referred to articles reviewing structural studies and vibrational spectroscopic studies of proton hydrates , but also general overviews the most recent being the comprehensive review of 1986.  

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Protonic species and their structures 2.2 Proton hydrates... 

Such limitation does not detract from the mass-spectrometric approach, which has provided valuable information on the stability of the protonated arenes, and their dissociation patterns. Moreover, it seems appropriate to mention here that, as early as 1957, Field and Franklin established a correlation between the orienting influence of substituents and the appearance potential of the corresponding ionic species, and that their proposed structure of the arenium ions observed in chemical... 

As far as crystalline structures are concerned, X-ray and/or neutron diffraction methods and vibrational spectroscopy are complementary. The former can determine with accuracy the structure of the rigid framework but have some difficulties in locating protonic species, particularly if they are (statically or dynamically) disordered and if their concentration is low. The latter are well-suited for the identification of protonic entities and their types of association, the investigation of some structural details such as different crystallographic sites, (non)equivalent molecules, protonation sites, A-H and A... B distances and the nature and degree of structural disorder. 

The demetalating abilities of buffer species depend on both their structures and their acidities. Thus, while pyridine-2-carboxylic (picolinic) acid catalyzes the demetalation even of the rather inert lm, its 3- and 4-isomers (nicotinic and isonicotinic acids) are inactive. The difference is rationalized to result from the ability that only coordinated picolinic acid has to deliver a proton to an amidato nitrogen in an intramolecular manner. The reaction order in picolinic acid equals one for la and two for lm. For lm, inactive pyridine and nicotinic acid speed up the demetalation in the presence of picolinic acid (Fig. 8). 

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