title: Pyrazolium- versus imidazolium-based ionic liquids: structure, dynamics and physicochemical properties.
abstract: Ionic liquids (ILs) composed of two different pyrazolium cations with dicyanamide and bis(trifluoromethanesulfonyl)imide anions have been synthesized and characterized by NMR, Kamlet-Taft solvatochromic parameters, conductivity and rheological measurements, as well as ab initio calculations. Density functional calculations for the two pyrazolium cations, 1-butyl-2-methylpyrazolium [bmpz] and 1-butyl-2,3,5-trimethylpyrazolium [bm(3)pz], provide a full picture of their conformational states. Homo- and heteronuclear NOE show aggregation motives sensitive to steric hindrance and the anions' nature. Self-diffusion coefficients D for the anion and the cation have been measured by pulsed field gradient spin-echo NMR (PGSE-NMR). The ionic diffusivity is influenced by their chemical structure and steric hindrance, giving the order D(cation) > D(anion) for all of the examined compounds. The measured ion diffusion coefficients, viscosities, and ionic conductivity follow the Vogel-Fulcher-Tammann (VFT) equation for the temperature dependencies, and the best-fit parameters have been determined. Solvatochromic parameters indicate an increased ion association upon going from bis(trifluoromethanesulfonyl)imide to dicyanamide-based pyrazolium salts, as well as specific hydrogen bond donor capability of H atoms on the pyrazolium ring. All of these physical properties are compared to those of an analogous series of imidazolium-based ILs.
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via del Risorgimento 35, 56126 Pisa, Italy. cinziac@farm.unipi.it
authors.forename: Cinzia
authors.initials: C
authors.lastname: Chiappe
authors.forename: Angelo
authors.initials: A
authors.lastname: Sanzone
authors.forename: Daniele
authors.initials: D
authors.lastname: Mendola
authors.forename: Franca
authors.initials: F
authors.lastname: Castiglione
authors.forename: Antonino
authors.initials: A
authors.lastname: Famulari
authors.forename: Guido
authors.initials: G
authors.lastname: Raos
authors.forename: Andrea
authors.initials: A
authors.lastname: Mele
mesh_terms: D000838:Anions / Q000737:chemistry; D002412:Cations / Q000737:chemistry; D004553:Electric Conductivity; D006860:Hydrogen Bonding; D007093:Imidazoles / Q000737:chemistry*; D052578:Ionic Liquids / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D011720:Pyrazoles / Q000737:chemistry*; D014783:Viscosity
chemical_list: D000838:Anions; D002412:Cations; D007093:Imidazoles; D052578:Ionic Liquids; D011720:Pyrazoles; C031280:pyrazole; C029899:imidazole