ABSTRACT
In the present work, four branched methylated, 1,2-dimethyl-3-isopropyl-imidazolium (i-[C3Dmim+]) and protonated,1-methyl-3-isopropyl-imidazolium (i-[C3mim+])-based ionic liquids (ILs) with varying anion (Br-, BF4-, PF6-, and NTf2-) were synthesized and investigated by NMR, infrared (IR) and Raman spectroscopy. Based on infrared and Raman spectroscopy, complete vibrational assignments have been performed. The IR and Raman analysis revealed that the vibrational spectra are virtually unaffected upon methylation, while significant frequency changes were observed by changing anion. Furthermore, to determine the electronic structure, energetic stability, and vibrational properties of these i-[C3Dmim]Y, i-[C3mim]Y (Y = Br, BF4, PF6, and NTf2) ion pairs, quantum chemical calculations including the dispersion correction method are performed both on single ions and on ionic couples. The calculated electron density was analyzed to expose non-covalent intra- and interionic interactions by the quantum theory of atoms in molecules (AIM) and interpreted in terms of both anion dependence and type of interaction. Computational results suggest that for all ionic couples the most energetically stable configuration is obtained with the anions located close to the C2 position of the imidazolium cation. However, in the case of i-[C3mim]NTf2 and i-[C3Dmim]BF4, similar energies were obtained in configurations 2 and 3 where the anion is located above the imidazolium ring. For i-[C3mim]Br a stronger hydrogen bond is predicted than for other studied ILs. Calculations indicate that a red shift of the CH stretching bands should occur due to hydrogen bonding; indeed, such displacement of bands is experimentally observed.
ABSTRACT
Several recent studies of hydroxyl-functionalized ionic liquids (ILs) have shown that cation-cation interactions can be dominating these materials at the molecular level when the anion involved is weakly interacting. The hydrogen bonds between the like ions led to the formation of interesting chain-like, ring-like, or distinct dimeric (i. e. two ion pairs) supermolecular clusters. In the present work, vibrational spectroscopy (ATR-IR and Raman) and density functional theory (DFT) calculations of the hydroxyl-functionalized imidazolium ionic liquid C2 OHmimCl indicate that anion-cation hydrogen bonding interactions are dominating, leading to the formation of distinct dimeric ion pair clusters. In this arrangement, the Cl- anions function as a bridge between the cations by establishing bifurcated hydrogen bonds with the OH group of one cation and the C(2)-H of another cation. Cation-cation interactions, on the other hand, do not play a significant role in the observed clusters.
ABSTRACT
Raman spectroscopy was used to distinguish the differences in the molecular organization of the alpha, beta' and beta polymorphs, as well as the liquid state, of tristearin with focus placed on the C=O, C-H and C-C Raman-active stretching regions. The ester carbonyl stretching region permitted polymorphic discrimination due to significant differences in the number of modes, their relative frequencies and their full-widths at half-maximum. In the liquid state, the absence of obvious signatures in this region indicated that many local micro-environments likely exist about the ester carbonyl of molten tristearin. The ratio between the symmetrical and asymmetrical C-H stretching modes was linearly correlated with the enthalpy of fusion for each polymorph. The C-C stretching modes, which provided insight into the trans/gauche content, were polymorph independent, but changed significantly upon transition into the liquid state (p < 0.05). Overall, Raman spectroscopy allowed for the quick discrimination of tristearin polymorphs from a conformational and thermodynamic perspective.
