Browsing by Author "Shinsuke Shigeto"

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Complementing Crystallography with Ultralow-Frequency Raman Spectroscopy: Structural Insights into Nitrile-Functionalized Ionic Liquids
(Wiley-VCH Verlag, 2016) Hong-Kai Chen; Nitin Srivastava; Satyen Saha; Shinsuke Shigeto
Functionalized ionic liquids are a subclass of ionic liquids that are tailored for a specific application. Structural characterization in both solid and liquid phases is central to understanding their physical properties. Here, we used ultralow-frequency Raman spectroscopy, which can measure Raman spectra down to approximately 5cm-1, to study the structures and physical properties of 1-(4-cyanobenzyl)-3-methylimidazolium salts with five different anions. A comparison of the observed low-frequency Raman spectral patterns enabled us to predict the crystal symmetry of one of the synthesized salts for which single-crystal X-ray diffraction data were unobtainable. Real-time tracking of the low-frequency Raman spectral changes during melting revealed peak shifts indicative of different degrees of microscopic heterogeneity in the ionic liquids. The results show that our method provides a facile means that is complementary to X-ray crystallography, for obtaining structural information of ionic liquids. Low frequency, high potential: Fast ultralow-frequency Raman spectroscopy is used to study the structures, intermolecular interactions, and melting processes of a new series of nitrile-functionalized ionic liquids. This method not only provides information on crystal symmetry that is complementary to crystallography, but it also reveals the distinct nature of microscopic heterogeneity in the ionic liquids studied. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Evidence of C–F-P and aromatic π–F-P weak interactions in imidazolium ionic liquids and its consequences
(Elsevier B.V., 2018) Sumit Kumar Panja; Nitin Srivastava; Jyoti Srivastava; Namburi Eswara Prasad; Hemanth Noothalapati; Shinsuke Shigeto; Satyen Saha
A simple change from alkyl group to alkene in side chain of imidazolium cation with same anion resulted in a drastic impact on physical properties (e.g., melting point) from bmimPF6 IL to cmimPF6 IL. The underlying reasons have been elucidated by structural and interaction studies with the help of DSC, SCXRD, vibrational and multi-nuclear NMR spectroscopic techniques. Experiments reveal existence of new weak interactions involving the carbon and π cloud of the imidazolium aromatic ring with fluoride of PF6 anion (i.e., C2–F-P and π–F-P) in cmimPF6 but are absent in structurally similar prototype IL, bmimPF6. Though weak, these interactions helped to form ladder type supramolecular arrangement, resulting in quite high melting point for cmimPF6 IL compared to bmimPF6 IL. These findings emphasize that an IL system can behave uniquely because of the existence of uncommon weak interactions. © 2017 Elsevier B.V.
Importance of weak interactions and conformational equilibrium in N-butyl-N-methylpiperidinium bis(trifluromethanesulfonyl) imide room temperature ionic liquids: Vibrational and theoretical studies
(Elsevier, 2014) Madhulata Shukla; Hemanth Noothalapati; Shinsuke Shigeto; Satyen Saha
Piperidinium cation-based room temperature ionic liquids (RTILs) constitute an important class of ILs because of their unique electrochemical properties as well as non-aromatic nature of the cation. However, detailed structural studies are yet to be done. In this paper, we discuss the molecular structure and vibrational spectra of N-butyl-N-methylpiperidinium bis(trifluromethanesulfonyl) imide, (PIP14NTf2; where, PIP14 is N-butyl-N-methylpiperidinium and NTf2 is bis(trifluromethanesulfonyl) imide), obtained with a combined approach of infrared (IR) and Raman spectroscopies in the liquid state and density functional theory (DFT) and Hartree-Fock (H-F) based theoretical calculations. DFT calculations, which are found to produce the most stable geometry compared to other two methods (MP2 and H-F), reproduce the experimental IR and Raman spectra reasonably well. Our findings reveal structural properties that profoundly influence intermolecular interactions and melting point. There exists a large variation in the melting point of the ILs studied. While the bromide salt of the piperidinium derivative (PIP14Br) is solid with very high melting point (241°C), the corresponding NTf2 salt is low viscous liquid at room temperature (mp: -25°C). bmimBr (bmim = 1-butyl-1-methylimidazolium) exhibits a substantially lower melting point of 79°C than PIP14Br, suggesting that more number of strong classical hydrogen bonding interactions in the latter is primarily responsible for the much higher melting point. In addition, involvement of the alkyl group in PIP14 in H-bonding interaction provides additional rigidity in n-butyl chain which is otherwise absent in bmimBr. Interaction energy for PIP14Br is found to be higher than PIP14NTf2, showing a positive correlation between interaction energy and melting point. A blue shift in C-H stretching wavenumber as evident from IR and Raman spectra of PIP14Br IL is a clear indication of the stronger hydrogen bonding as compared to PIP14NTf2 IL. Furthermore, we experimentally observe the existence of cisoid-transoid conformational equilibrium of NTf2- anion in the Raman spectrum of PIP14NTf2 for the first time and determined that transoid NTf2- anion to be more stable than the corresponding cisoid conformer by 1.04 kcal/mol using DFT. Examination of various conformational possibilities of the cation shows that the butyl group preferentially exists in gauche conformation. © 2014 Elsevier B.V. All rights reserved.
Significance of weak interactions in imidazolium picrate ionic liquids: Spectroscopic and theoretical studies for molecular level understanding
(Royal Society of Chemistry, 2015) Sumit Kumar Panja; Nidhi Dwivedi; Hemanth Noothalapati; Shinsuke Shigeto; A.K. Sikder; Abhijit Saha; Sailaja S. Sunkari; Satyen Saha
The effects of interionic hydrogen bonding and π-π stacking interactions on the physical properties of a new series of picrate anion based ionic liquids (ILs) have been investigated experimentally and theoretically. The existence of aromatic (C2-H⋯O) and aliphatic (C7-H⋯O-N22 and C6-H⋯O-N20) hydrogen bonding and π-π stacking interactions in these ILs has been observed using various spectroscopic techniques. The aromatic and aliphatic C-H⋯O hydrogen bonding interactions are found to have a crucial role in binding the imidazolium cation and picrate anion together. However, the π-π stacking interactions between two successive layers are found to play a decisive role in tight packing in ILs leading to differences in physical properties. The drastic difference in the melting points of the methyl and propyl derivatives (mmimPic and pmimPic respectively) have been found to be primarily due to the difference in the strength and varieties of π-π stacking interactions. While in mmimPic, several different types of π-π stacking interactions between the aromatic rings (such as picrate-picrate, picrate-imidazole and imidazolium-imidazolium cation rings) are observed, only one type of π-π stacking interaction (picrate-picrate rings) is found to exist in the pmimPic IL. NMR spectroscopic studies reveal that the interaction of these ILs with solvent molecules is different and depends on the dielectric constant of the solvent. While an ion solvation model explains the solvation in high dielectric solvents, an ion-pair solvation model is found to be more appropriate for low dielectric constant solvents. The enhanced stability of these investigated picrate ILs compared with that of inorganic picrate salts under high doses of γ radiation clearly indicates the importance of weak interionic interactions in ILs, and also opens up the possibility of the application of picrate ILs as prospective diluents in nuclear separation for advanced fuel cycling process. © the Owner Societies 2015.