Gas-phase dissociation of ionic liquid aggregates studied by electrospray ionisation mass spectrometry and energy-variable collision induced dissociation

摘要

Positive singly charged ionic liquid aggregates [(C(n)mim)(m+1)(BF4)(m)](+) (mim = 3-methylimidazolium; n = 2, 4, 8 and 10) and [(C(n)mim)(m+1)(A)(m)](+) (A = Cl-, BF4-, PF6-, CF3SO3- and (CF3SO2)(2)N-) were investigated by electrospray ionisation mass spectrometry and energy-variable collision induced dissociation. The electrospray ionisation mass spectra (ESI-MS) showed the formation of an aggregate with extra stability for m = 4 for all the ionic liquids with the exception Of [C(4)mim][CF3SO3]. ESI-MS-MS and breakdown curves of aggregate ions showed that their dissociation occurred by loss of neutral species ([C(n)mim][A])(a) with a >= 1. Variable-energy collision induced dissociation of each aggregate from m = 1 to m = 8 for all the ionic liquids studied enabled the determination of E-cm,E-1/2 values, whose variation with m showed that the monomers were always kinetically much more stable than the larger aggregates, independently of the nature of cation and anion. The centre-of-mass energy values correlate well with literature data on ionic volumes and interaction and hydrogen bond energies.