A simple geometrical explanation for the occurrence of specific large aggregated ions in some protic ionic liquids.
Thermochemistry calculations on positively charged aggregates of protic ionic liquids (PILs) support the aggregation pattern observed in electrospray ionization mass spectra (ESI-MS) (Kennedy, D. F.; Drummond, C. J. J. Phys. Chem. B 2009, 113, 5690-5693). We can show that for ethyl ammonium nitrate (EAN) C8A7(+) is the far most stable aggregate for enthalpic and entropic reasons as well. The stability can be understood by the formation of a compact hydrogen-bond network. The strength of the H-bond network in C8A7(+) makes this aggregate the most stable independent from the length of the cation alkyl chain length and the chosen solvent. Such strongly bound aggregates may justify the classification of PILs as "poor ionic liquids".
Ludwig
Ralf
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