title: Dynamics in a Room-Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy.
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 time scale structural spectral diffusion component of 600 ps in addition to short and intermediate time scales similar to those measured for selenocyanate anion (SeCN-) dissolved in BmimNTf2. In contrast to 2-SeCN-Bmim+, SeCN-samples its inhomogeneous line width nearly an order of magnitude faster than the complete structural randomization time of neat BmimNTf2liquid (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 time scale 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 molecules as well as significantly slower, longer-range fluctuations that are responsible for complete randomization of the liquid structure.
authors.affiliation: Department of Chemistry Stanford University , Stanford, California 94305, United States.
authors.forename: Steven A
authors.initials: SA
authors.lastname: Yamada
authors.affiliation: Department of Chemistry Stanford University , Stanford, California 94305, United States.
authors.forename: Heather E
authors.initials: HE
authors.lastname: Bailey
authors.affiliation: Department of Chemistry Stanford University , Stanford, California 94305, United States.
authors.forename: Amr
authors.identifier: 0000-0001-6873-4471
authors.initials: A
authors.lastname: Tamimi
authors.affiliation: Department of Chemistry Stanford University , Stanford, California 94305, United States.
authors.forename: Chunya
authors.initials: C
authors.lastname: Li
authors.affiliation: Department of Chemistry Stanford University , Stanford, California 94305, United States.
authors.forename: Michael D
authors.identifier: 0000-0002-0021-1815
authors.initials: MD
authors.lastname: Fayer