Vibrational spectroscopy and dynamics of small anions in ionic liquid solutions

Abstract

Fourier-transform infrared (FTIR) and time-resolved IR spectroscopies have been used to study vibrational band positions, vibrational energy relaxation (VER) rates, and reorientation times of anions in several ionic liquid (IL) solutions. The ILs primarily investigated are based on the 1-butyl-2,3-dimethylimidazolium ([BM(2)IM]) cation with thiocyanate (NCS-), dicyanamide (N(CN)2-), and tetrafluoroborate (BF4-) anions. Spectroscopic studies are carried out near 2000 cm-1 for the C[Triple Bond]N stretching bands of NCS- and N(CN)2- as the IL anion as well as for NCS-, N(CN)2-, and azide (N3-) anions dissolved in [BM2IM][BF4]. The VER studies of N(CN)2- are reported for the first time. VER of N3-, NCS-, and N(CN)2- is measured in normal solvents, such as N-methylformamide, to compare with the IL solutions. The spectral shifts and VER rates of the anions in IL solution are quite similar to those in polar aprotic, conventional organic solvents, i.e., dimethylsulfoxide, and significantly different than those in methanol, in which there is hydrogen bonding. Similar studies were also carried out for the anions in another IL, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), in which the C2 hydrogen is present. The results for the anions are similar to those in the [BM2IM] containing ILs, in which the C2 hydrogen is methyl substituted. This suggests that substituting this hydrogen has, at most, a minor effect on the degree of hydrogen bonding in the anion-IL solvation interaction based on the infrared spectra and dynamics.

Translational and Rotational Motions for TFSA-Based Ionic Liquids Studied by NMR Spectroscopy

“…33,34 Experimental studies on the rotational motion of small molecules dissolved in ILs have also been reported. [35][36][37][38][39] We calculated  2 in Eq. 7 by using the measured viscosity data and assuming V of the van der Waals volumes of the cations in Table 3.…”

Temperature Dependence of the Dielectric Properties and Dynamics of Ionic Liquids

“…Much progress has been made in recent years on the measurement and interpretation of their thermodynamic and transport properties. [4,5] The dynamic characteristics of RTILs have also received attention, including studies using optical heterodyne-detected Raman-induced Kerreffect spectroscopy (OHD-RIKES), [6][7][8][9][10] time-resolved infrared, [11] NMR relaxation, [12][13][14][15] and emission [16][17][18] spectroscopies, quasielastic neutron scattering, [20] terahertz spectroscopy, [21][22][23] and dielectric relaxation spectroscopy (DRS). [24][25][26][27][28][29][30][31][32][33][34] Detailed knowledge of the dielectric behaviour of RTILs is particularly important since such properties are critical for understanding their solvation dynamics.…”

Vibrational Relaxation of Cyanate or Thiocyanate Bound to Ferric Heme Proteins Studied by Femtosecond Infrared Spectroscopy

“…In typical T 1 measurement experiments, the time-dependent absorption change at the peak of the bleach or absorption is probed. 23,[32][33][34][35][36] When the absorption band to be probed is distinctly separate from the other absorption that can be influenced by the IR pump pulse, the decay kinetics at the peak absorption or bleach can be a good representation of the T 1 of the corresponding vibrational mode. The time constants for the decays of the peak absorptions in the transients of Mb heavily overlap with that of the free anion, the decays at the peak absorptions cannot be used to determine their T 1 values.…”