INSIDE IONIC LIQUIDS BY NMR METHODS

Ionic liquids (ILs) are salts generally made of an organic, bulky cation - such as N- alkyl,N-methylimidazoilium, N,N-dialkylpyrrolidinium, tetralkylammonium etc. - and a variety of anions, the most popular being PF6-, BF4- and bis(trifluoromethanesulfonyl)imide (TFSI). Such structural features are largely responsible of the inability of the ions to organize in a crystal lattice, thus giving isotropic, homogeneous liquids in a wide range of temperatures. It was demonstrated that many classes of ILs do exhibit local heterogeneity with formation of polar and non-polar domains in the nanometer length-scale. These aspects are still object of fundamental investigation with a variety of physical methods, such as neutron and X-ray scattering, vibrational spectroscopies, NMR, and represent one of the most fascinating characteristics of these systems. The NMR studies of the local structure of ILs mainly rely upon two approaches: diffusivity and intermolecular, homo- and heteronuclear NOE. This communication has the main goal of introducing two novel developments in NMR of ILs. The first is the use of 129Xe NMR spectroscopy as a new component of the toolkit for the investigation of the structural features of ILs. 129Xe has been successfully used as "spin spy" for molecular solvents, polymers and porous materials, but only recently for uncovering structural features of ILs at atomic level. Some paradigmatic examples on the most common imidazolium based ILs in terms of available void volume inside the liquid will be presented and discussed. The second concerns a critical re-thinking of the interpretation of intermolecular NOE in ILs. Quantitative {1H-19F} and {1H-7Li}NOE data on LiTFSI doped pyrrolidinum based ILs will be presented and discussed by considering both long- and short range interactions, in line with the recent theoretical models proposed by Gabl, Steinhauser and Weinga?rtner.