Dynamics in a Room-Temperature Ionic Liquid from the Cation Perspective: 2D IR Vibrational Echo Spectroscopy.

The dynamics of the room-temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf) 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 BmimNTf. 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 BmimNTf 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 time scale of 900 ± 20 ps, significantly slower than observed for SeCN but identical within experimental error to the complete structural randomization time of BmimNTf. 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.