Subscriber access provided by UNIV OF DURHAM
B: Liquids; Chemical and Dynamical Processes in Solution
Broadband Terahertz Spectroscopy of Imidazolium-Based Ionic Liquids Sen Mou, Andrea Rubano, and Domenico Paparo J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.7b10886 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
Broadband Terahertz Spectroscopy of Imidazolium-Based Ionic Liquids Sen Mou †, Andrea Rubano†,‡, and Domenico Paparo ‡,* † Dipartimento di Fisica “Ettore Pancini”, Università di Napoli “Federico II”, Complesso Universitario di Monte Sant’Angelo, via Cintia, 80126 Napoli, Italy.
‡ ISASI – Inst. of Applied Sciences and Intelligent Systems, Consiglio Nazionale delle Ricerche, via Campi Flegrei 34, 80078 Pozzuoli, Italy.
ABSTRACT: Ionic liquids are liquid salts at ambient temperature composed of organic cations and organic/inorganic anions. Outstanding physical and chemical properties of ionic liquids lead to increasing application in scientific and industrial field. Ionic liquids have been already investigated by different spectroscopic techniques, including terahertz (THz) time-domain spectroscopy. The usual THz frequency range extends up to 2-3 THz, a relatively narrow band, which can only show the intermolecular vibrational modes. Here we report about broadband THz spectroscopy of ILs up to 13 THz. Bandwidth of intermolecular absorption band presents unexpected behavior and strong sharp intramolecular absorptions are shown. In addition, we found violation of the
1 ACS Paragon Plus Environment
The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 32
approximation of harmonic oscillator used to predict the peak shift of intermolecular absorption band.
INTRODUCTION Ionic liquids (ILs) are a class of salts composed by organic cations and organic/inorganic anions which remain in liquid state at room temperature.1-2 ILs have outstanding physical and chemical properties such as low volatility, low combustibility, high thermal and chemical stability and enhanced solvation ability. Furthermore, these properties can be tailored by selecting various constituent cations and anions and/or by mixing different neat ILs.3-4 As more than a trillion of ILs5 have been reported, it is possible to synthesize various ILs for different purposes and applications. In these compounds, and especially in their mixtures, the precise knowledge of their vibrational modes is a key for predicting the specific behavior of the product. Vibrational modes can be generally divided into intermolecular vibration (usually at low frequencies, 0-5 THz) and intramolecular vibrations (usually at higher frequencies 5-50 THz). To characterize ILs vibrational modes, optical spectroscopic techniques are generally used. The most common vibrational spectroscopic techniques include Raman spectroscopy,6-8 infrared (IR) spectroscopy,9-10 femtosecond Raman-induced Kerr effect spectroscopy,11-18 dielectric
spectroscopy,19-22
far-infrared
spectroscopy23
and
terahertz
(THz)
spectroscopy.23-29 THz spectroscopy can directly access the refractive index and the absorption coefficient without making use of Kramers-Kroenig relations as done with common farinfrared spectroscopic techniques. In this manuscript we address the problem of 2 ACS Paragon Plus Environment
Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
The Journal of Physical Chemistry
spectrally separate intra- and inter- molecular vibration modes by measuring the absorption coefficients and refractive indices of four different imidazolium-based ILs with broadband THz time-domain spectroscopy (THz-TDS) based on a high energy laserinduced plasma THz source. The existing literatures9-10, 26 on IR spectroscopy of imidazolium-based ILs have shown that the absorption peaks in the region from 2800 to 3000 cm-1 (corresponding to 84-90 THz) are ascribed to asymmetric and symmetric C-H vibrations in the alkyl groups attached the imidazolium ring while the peaks in the 3000 - 3200 cm-1 (corresponding to 90-96 THz) range are ascribed to C-H vibrations of imidazolium ring. Moreover, the crystal structures of this compound in the solid phase have attracted attention too. Monoclinic and orthorhombic crystal polymorphs of [C4C1im]Cl have been investigated by X-ray powder patterns and Raman spectroscopy.30 Subsequently the crystal structure of monoclinic crystal of [C4C1im]Cl was determined independently by Saha et al.31 and Holbrey et al.32 It was found that in monoclinic crystal of [C4C1im]Cl the anions arrange as zigzag chains.33 The elucidation of crystal structure is helpful to understand ILs in liquid state as the crystal lattice is still retained upon melting.34 Nanostructural organization of ILs was also investigated with Raman-induced Kerr effect spectroscopy. Xiao et al.35 studied mixtures of ILs with femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES). They found that optical Kerr effect spectra of binary mixture of 1,3-pentylmethylimidazolium bis(trifluoromethanesulfonyl)imide ([C5mim][NTf2]) and 1,3-pentylmethylimidazolium bromide ([C5mim]Br) in the low-frequency range (
