Imines proton affinities

In case of immonium ion fragmentations, the large difference of proton affinities, APA, between imine and alkene clearly favors the formation of immonium ion plus neutral alkene, whereas imine loss is restricted to highly energetic precursors. 

The general strategy for generating radical adducts to heterocyclic molecules relies on selective protonation of a suitable neutral precursor to prepare a cation of a well-defined structure. The gas-phase acid is chosen so as to attack only the most basic site in the molecule, or alternatively, non-selective protonation can be used to prepare a mixture of ions. For example, protonation of imidazole with NH4+ occurs selectively on the imine nitrogen atom (N-l), which has the highest proton affinity and is the only position that can be protonated by an exothermic reaction (Scheme 22) [239]. 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

The molecular origin of the 20 fold increase in mutagenicity of K-region arene imines over that of the corresponding arene oxides has been rationalised by the increased proton affinity of the former [146],... 

Proton affinities of imines and heats of formation of immonium ions have been calculated for the gas phase by ab initio methods.di-Imines are more basic than their trans-isomtrs, reflecting the unusually high (15-17kJmor ) energy difference between the cis- and fran -isomers, which decreases significantly in the immonium ions. An additivity scheme for group contributions in immonium ions is proposed. 

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