DOI: 10.1002/cphc.201000723 Vaporisation of a Dicationic Ionic Liquid Revisited Joana Vitorino, [b, c] Jo¼o P. Leal, [b, c] Peter Licence, [d] Kevin R. J. Lovelock, [d] Peter N. Gooden, [d] Manuel E. Minas da Piedade, [b] Karina Shimizu, [a] Luís P. N. Rebelo, [e] and JosØ N. Canongia Lopes* [a, e] 1. Introduction The publication in 2006 in Nature of the paper “The Distillation and Volatility of Ionic Liquids“ [1] has had a significant impact on the ionic liquid literature. Until that time it was widely as- sumed that ionic liquids were nonvolatile and that the unusual combination of physico-chemical properties exhibited by them was dominated by this characteristic. The realisation that some classes of aprotic ionic liquids have a detectable vapour pres- sure and could be evaporated at moderately elevated temper- atures under vacuum (T > 473 K, p < 0.1 Pa), stimulated tremen- dous interest in both direct distillation and indeed the study of the “ionic liquid vapour phase”. [2, 3] Temperature-programmed desorption (TPD), coupled with line-of-sight mass spectrometry (LOS-MS), has been shown to be a versatile and robust method for studying the nature of the vapour phase of ther- mally desorbed ionic liquids. [4] However the nature and indeed limited mass range of the detection system currently used for LOS-MS studies, a quadrupole mass spectrometer with an upper mass range of 511 amu, did not facilitate the investiga- tion of species of high mass range, or of in situ reactions of isolable ionic species with either reagent gases or other neu- trals introduced into the analyser. Recently, the group of Jones and Licence [5] highlighted the flexibility of TPD by successfully characterising the thermally desorbed vapour phase of the dicationic ionic liquid 1,3-bis(3- methylimidazolium-1-yl) propane bis(trifluoromethanesulfonyl)- imide, [C 3 (mim) 2 ][NTf 2 ] 2 (Scheme 1). They showed that the di- cationic ionic liquid could be successfully evaporated, with a molar enthalpy of vaporisation (D vap H m ) at 298 K of 190  15 kJ mol 1 . Although the LOS-MS detector employed in that study had a high mass limit of only 511 amu, thereby preclud- ing the opportunity to observe the parent ion {[C 3 (mim) 2 ] [NTf 2 ] 2 } + C (m/z 766), the detection of {[C 3 (mim) 2 ][NTf 2 ]} + (m/z 486) upon electron impact ionization (8–70 eV) suggested that the vapour was composed of higher-mass clusters of ions, for example, triplets formed by one dication and two anions. Moreover, the fact that no ions were apparent in the spectra when the electron beam energy was reduced below 8 eV con- firmed that the vapour consisted exclusively of neutral species, that is, as neutral ion triplets (NITs). A mechanistic scheme for the production of the most prominent ions in the spectra, namely {[C 3 (mim) 2 ][NTf 2 ]} + (m/z 486), [CH 2 =CHCH 2 mim] + (m/z 123) and [C 3 (mim) 2 ] 2 + (m/z 206/2 = 103), was also dis- cussed. The present work supports the LOS-MS results and extends their scope, based on a Fourier-transform ion cyclotron reso- nance mass spectrometry (FT-ICR-MS) investigation of the vapour originating from the reduced pressure evaporation of [C 3 (mim) 2 ][NTf 2 ] 2 (l). In this case the NIT character of the gas The vaporization of a dicationic ionic liquid at moderate tem- peratures and under reduced pressures —recently studied by line-of-sight mass spectrometry—was further analyzed using an ion-cyclotron resonance mass spectroscopy technique that allows the monitoring of the different species present in the gas phase through the implementation of controlled ion–mol- ecule reactions. The results support the view that the vapour phase of an aprotic dicationic ionic liquid is composed of neu- tral ion triplets (one dication attached to two anions). Molecu- lar dynamics simulations were also performed in order to ex- plain the magnitude of the vaporization enthalpies of dication- ic ionic liquids vis-à-vis their monocationic counterparts. Scheme 1. [C 3 (mim) 2 ][NTf 2 ] 2 dicationic ionic liquid. [a] Dr. K. Shimizu, Dr. J. N. Canongia Lopes Centro de Química Estrutural/IST Av. Rovisco Pais 1049 001 Lisboa (Portugal) Fax: (+ 351) 218 464 455 E-mail : jnlopes@ist.utl.pt Homepage: http://cqe.ist.utl.pt [b] J. Vitorino, Dr. J. P. Leal, Dr. M. E. Minas da Piedade Departamento de Química e Bioquímica/FCUL 1649-016 Lisboa (Portugal) [c] J. Vitorino, Dr. J. P. Leal Unidade de CiÞncias Químicas e RadiofarmacÞuticas/ITN 2686-953, SacavØm (Portugal) [d] Dr. P. Licence, Dr. K. R. J. Lovelock, Dr. P. N. Gooden School of Chemistry, University of Nottingham Nottingham, NG7 2RD (UK) [e] Prof. L. P. N. Rebelo, Dr. J. N. Canongia Lopes Instituto de Tecnologia Química e Biológica/UNL Av. Repfflblica, Apartado 127, 2780 901 Oeiras (Portugal) Homepage: http://www.itqb.unl.pt ChemPhysChem 2010, 11, 3673 – 3677  2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3673