Supramolecular assemblies of quaternary ammonium cations and halide anions in the gas phase: ESMS-FTICR data and computer modelling

Supramolecular aggregates of tetraalkylammonium halides (R4NX) are formed by electrospray out of acetonitrile solution. Mass spectrometry reveals 88 charged aggregates for R= Me, Et, Bu; X= Br, I, ranging up to [(Bu4N)39Br42]3- in size. With the objective of improving calculations of intermolecular energies for supramolecular aggregates of ions, calibrated semi-empirical potentials for inter-ion interactions have been developed and applied to these aggregates. The accuracy of the calculated energies is supported by the measured collisional dissociation energy of (Et4N+)4 (I-)5. Energy optimisations indicate that the probable structures have the halide ions dispersed in a matrix of cations, which, for Bu4N+, can be mutually attractive. The aggregates are structurally fluid, with multiple structures separated by 4-8 kJmol(-1). The energy calculations are entirely consistent with the observed formation of large aggregates, and of multiply charged anions. It is estimated that the cohesive energies of supramolecular assemblies of ions such as these reach about 40 kJmol(-1) per constituent ion.