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Dynamics in a Room Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy Steven A Yamada, Heather E Bailey, Amr Tamimi, Chunya Li, and Michael D. Fayer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b12011 • Publication Date (Web): 18 Jan 2017 Downloaded from http://pubs.acs.org on January 19, 2017
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Journal of the American Chemical Society
Dynamics in a Room Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy. Steven A. Yamada, Heather E. Bailey, Amr Tamimi,‡ Chunya Li,+ and Michael D. Fayer* Department of Chemistry Stanford University, Stanford, CA 94305, USA *Phone: (650) 723-4446; Email:
[email protected] +
Permanent address: College of Chemistry and Materials Science South Central University for Nationalities, Wuhan, Hubei, PRC Abstract
The dynamics of the room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2) were investigated with two dimensional infrared (2D IR) vibrational echo spectroscopy and polarization selective pump-probe (PSPP) experiments. The CN stretch frequency of a modified Bmim+ cation (2-SeCN-Bmim+), in which a SeCN moiety was substituted onto the C-2 position of the imidazolium ring, was used as a vibrational probe. A major result of the 2D IR experiments is the observation of a long timescale structural spectral diffusion component of 600 ps in addition to short and intermediate timescales similar to those measured for selenocyanate anion (SeCN‒) dissolved in BmimNTf2. In contrast to 2-SeCN-Bmim+, SeCN‒ samples its inhomogeneous linewidth nearly an order of magnitude faster than the complete structural randomization time of neat BmimNTf2 liquid (870 ± 20 ps) measured with optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments. The orientational correlation function obtained from PSPP experiments on 2-SeCN-Bmim+ exhibits two periods of restricted angular diffusion (wobbling-in-a-cone) followed by complete orientational randomization on a timescale of 900 ± 20 ps, significantly slower than observed for SeCN‒ but identical within experimental error to the complete structural randomization time of BmimNTf2. The experiments indicate that 2-SeCN-Bmim+ is sensitive to local motions of the ionic region that influence the spectral diffusion and reorientation of small, anionic and neutral 1
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molecules as well as significantly slower, longer-range fluctuations that are responsible for complete randomization of the liquid structure.
‡
Current address: Department of Chemistry and Biochemistry, University of Oregon Eugene, OR 97403, USA
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I. Introduction Despite their ubiquity and usefulness in chemistry, common solvents such as organic liquids and water often fail to safely and efficiently support processes of technological importance, including carbon dioxide capture and storage, dissolution and processing of biological macromolecules, and proton/ion transport in membrane fuel cells and batteries.1 Significant research effort is being redirected toward the discovery and characterization of novel solvent systems that meet these particular challenges. Room temperature ionic liquids, a subgroup of salts that are liquid at temperatures
