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

Supramolecular aggregates of tetraalkylammonium halides (R4NX) are formed b y electrospray out of acetonitrile solution. Mass spectrometry reveals 88 c harged aggregates for R = Me, Et, Bu; X = Br, I, ranging up to [(Bu4N)(39)B r-42](3-) in size. With the objective of improving calculations of intermol ecular energies for supramolecular aggregates of ions, calibrated semi-empi rical 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). Ener gy optimisations indicate that the probable structures have the halide ions dispersed in a matrix of cations, which, for Bu4N+, can be mutually attrac tive. The aggregates are structurally fluid, with multiple structures separ ated by 4-8 kJ mol(-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 o f ions such as these reach about 40 kJ mol(-1) per constituent ion.