Ana M. Fernandes

Evaluation of Cation−Anion Interaction Strength in Ionic Liquids

Electrospray ionization mass spectrometry with variable collision induced dissociation of the isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) ions of imidazolium-, pyridinium-, pyrrolidinium-, and piperidinium-based ionic liquids (ILs) combined with a large set of anions, such as chloride, tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, and bis[(trifluoromethyl)sulfonyl]imide, was used to carry out a systematic and comprehensive study on the ionic liquids relative interaction energies. The results are interpreted in terms of main influences derived from the structural characteristics of both anion and cation. On the basis of quantum chemical calculations, the effect of the anion upon the dissociation energies of the ionic liquid pair, and isolated [(cation)(2)anion](+) and/or [(anion)(2)cation](-) aggregates, were estimated and are in good agreement with the experimental data. Both experimental and computational results indicate an energetic differentiation between the cation and the anion to the ionic pair. Moreover, it was found that the quantum chemical calculations can describe the trend obtained for the electrostatic cation-anion attraction potential. The impact of the cation-anion interaction strengths in the surface tension of ionic liquids is further discussed. The surface tensions dependence on the cation alkyl chain length, and on the anion nature, follows an analogous pattern to that of the relative cation-anion interaction energies determined by mass spectrometry.

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

Positive singly charged ionic liquid aggregates [(C(n)mim)(m+1)(BF(4))(m)](+) (mim = 3-methylimidazolium; n = 2, 4, 8 and 10) and [(C(4)mim)(m+1)(A)(m)](+) (A = Cl(-), BF(4) (-), PF(6) (-), CF(3)SO(3) (-) and (CF(3)SO(2))(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][CF(3)SO(3)]. 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 >or= 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, 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.

Chemical ionization of amino and hydroxy group containing arylalkyl compounds with ions in a nitromethane plasma

Abstract The gas-phase ion/molecule reactions of organic molecules containing several functional groups, including amino, hydroxy and carboxy groups, have been investigated under nitromethane chemical ionization conditions. Three main reaction channels are observed in the ion source: (a) proton transfer, (b) electron transfer and (c) hydride abstraction. The product ion [M+NO]+ is also formed, but in a very low abundance. Initial electrophilic attack of the nitrosonium ion on the aromatic ring is postulated to explain the elimination of HNO from the [M+NO]+ adduct ions, observed for all substrates studied. Elimination of water is a characteristic fragmentation pathway for all substrates possessing a benzylic hydroxy group. Fragment ions resulting from cleavage of the molecular ions of the amines, formed by charge transfer, react with the neutral molecules forming two types of adduct ions: [M+immonium]+ and [M+C7H7]+, which have been characterized through the study of their unimolecular decompositions. The latter provide strong evidence for the existence of two types of structures: a covalent and an ion/molecule complex, that is a non-covalent structure.

Toward an Understanding of the Mechanisms behind the Formation of Liquid–liquid Systems formed by Two Ionic Liquids

Biphasic systems composed of aprotic ionic liquids (ILs) allow the development of new separation processes constituted only by nonvolatile solvents. Six pairs of cholinium- and phosphonium-based ILs were found to be able to form biphasic systems, and the respective liquid-liquid phase diagrams were determined. Although IL anions do not seem to control the phase separation phenomenon, they play an important role in defining the size of the biphasic region. A linear dependence of the ILs mutual solubilities with the IL anions volume was found, supporting the relevance of the entropy of mixing. With the exception of the system formed by ILs with a common anion, the remaining liquid-liquid systems display a significant ion exchange extent between the phases, which inversely depends on the ILs cohesive energy. It is finally shown that four-phase systems with a remarkable performance in the separation of mixtures of dyes can be prepared.

Evidence for the formation of acyclic ions from the radical cations and cyclic ions from the protonated molecules of α,ω-diamines upon loss of ammonia

The structural characterization of the ions generated by the electron ionization-induced loss of ammonia from the molecular ions of α,ω-diamines, using ion/molecule reactions in combination with collision-induced dissociation (CID) studies, is described. The results of the experiments of ion/molecule reactions using dimethyl disulfide exclude the distonic radical cation structure for those long-lived ions proposed earlier by other authors for ions generated within a few microseconds following ionization. The unimolecular and CID characteristics of the ions [M---NH3]√+ of 1,4-diaminobutane and 1,5-diaminopentane and of their fragment ion m/z=56, are discussed in terms of the structures CH3CH2CH=CHNH2√+ and CH3CH2CH2CH=CHNH2√+ for the ions [M---NH3]√+, respectively. The structure of the closed shell ions resulting from loss of ammonia from the protonated α,ω-diamines was also probed through the CID spectra of model ions prepared by chemical ionization with methane in the chemical ionization source of the mass spectrometer.