Pyrazolium- versus Imidazolium-Based Ionic Liquids: Structure, Dynamics and Physicochemical Properties

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 [bm₃pz], 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 <i>D</i> 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 <i>D</i>_cation > <i>D</i>_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

Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes

Four different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene (<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene (<b>4</b>), were prepared and their enantiomers, which are stable at r.t., were separated. Vibrational circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD) and circularly polarized luminescence (CPL) spectra were recorded. Each type of experimental spectrum was compared with the corresponding theoretical spectrum, determined via Density Functional Theory (DFT). Following the recent papers by Nakai et al., this comparison allowed to identify some features related to the helicity and some other features typical of the substituent groups on the helical backbone. The Raman spectrum of compound <b>1</b> is also examined from this point of view