Add time:07/22/2019         Source:sciencedirect.com

We have devised a new approach that extends the kinetic method to systems where the competitive dissociation (the cornerstone of the kinetic method) will occur not at one binding center but at two spatially different places. This approach was tested on dissociation of complexes of two symmetric doubly protonated diamines with seven different crown ethers resulting in the relative binding order of alkylammonium cation (half of the protonated diamine dication) to the crown ethers. For the experiments with 1,10-diaminodecane, leading to the n-alkylammonium binding ladder, the order was dicyclohexano-18-crown-6 > dibenzo-21-crown-7 > 18-crown-6 > dibenzo-24-crown-8 > dibenzo-18-crown-6 > 15-crown-5 > 12-crown-4. When N,N′-dimethyl-1,8-octanediamine was used, the experiments gave rise to the relative binding of N-methyl-n-alkylammonium cation, which was dibenzo-24-crown-8 > dicyclohexano-18-crown-6 > dibenzo-21-crown-7 > dibenzo-18-crown-6 > 18-crown-6 > 15-crown-5 > 12-crown-4. The observed binding trends were explained in terms of size of the alkylammonium cation, size of the cavity of the crown, flexibility of the crown substituents, and overall crown polarizability. Our data qualitatively correlate well to the literature gas-phase values for these systems as well as to the solution data (where available).

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