The triple ion [(CH3)4N+ . PF6- . (CH3)4N+] generated by electrospray ionization of
tetramethylammonium hexafluorophosphate, is characterized by collision-induced dissociation and
infrared spectroscopy of the mass-selected gaseous ion in the range from 1000 to 2000 cm-1. Quantum
chemical calculations are used to elucidate the structures and energetics of the triple ion and its
subunits and for the assignment of the infrared frequencies. The infrared bands observed in the gasphase
match quite closely with those of the bulk substance, suggesting that for this particular kind of
ion associate the structural differences between the gas phase and the condensed state are almost
negligible.