Two-dimensional ultrafast vibrational spectroscopy of azides in ionic liquids reveals solute-specific solvation

Abstract

The stereochemistry and the reaction rates of bimolecular nucleophilic substitution reactions involving azides in ionic liquids are governed by solute-solvent interactions. Two-dimensional ultrafast vibrational spectroscopy (2D-IR) shows that the picosecond dynamics of inorganic azides are substantially slower than organic azides in a series of homologous imidazolium ionic liquids. In water, both organic and inorganic azides spectrally diffuse with a ∼2 ps time constant. In the aprotic solvent tetrahydrofuran, both kinds of azides spectrally diffuse on a timescale >5 ps. In ionic liquids, like 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), organic azides spectrally diffuse with a 2-4 ps time constant, and inorganic azides spectrally diffuse with a >40 ps time constant. Such a striking difference suggests that neutral (organic) and charged (inorganic) azides are incorporated in the ionic liquids with different solvation structures.

Dynamics of Dicyanamide in Ionic Liquids is Dominated by Local Interactions

“…The sensitivity of the probe dynamics to the probe molecule and its very immediate environment is also in line with what has been concluded from experiments using different azides as vibrational probes. 29 The small, yet detectable difference between the variation of τ rot of [DCA] − for different anions of the RTIL in this study in turn reflects the anion dependence of the RTIL structure and the (mostly repulsive) interaction of [DCA] − with the RTIL’s anions. These differences together with the dynamics of the cations, which we do not probe with our experiments, are the likely origin of the observed deviation of the RTIL dynamics of binary mixtures of [emim][DCA] and [emim][BF 4 ] as determined from dielectric relaxation spectroscopy.…”

“…To study the solvation of molecules and ions with sizes comparable to those of the ionic liquids’ ions, ultrafast infrared spectroscopies have become popular in recent years. 8,16,23−30 Among these studies, small anions with prominent vibrational signatures 31 like SCN – , 8,26−28,30 SeCN – , 32 and N 3 – 25−27,29 have emerged as powerful vibrational probes. Using such anionic probe molecules, the vibrational relaxations of the C≡N and N≡N stretching vibrations have been used to obtain information on their coupling to the ionic liquid environment; 25,27 their rotational dynamics has been used to infer information on hydrodynamic friction; 8,32 and the temporal evolution of the vibrational frequencies (frequency–frequency correlations) has made structural (solvation cage) fluctuations in ionic liquids experimentally accessible.…”

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H‐Bonding Liquid Observed by Transient 2D‐IR Spectroscopy

“…Spectral diffusion is now only observed for the residual ESA1, whereas ESA3 remains strongly inhomogeneous up to a t 2 time of at least 5ps( Figure 3, first row), our detection limit corresponding to three vibrational lifetimes.T he CLS of ESA3 remains close to unity at all accessible time delays (blue line in Figure 4). [16] Asubstantial slowing down of the translational dynamics of water has also been observed in the solvation shell of different solutes. [15] Similarly slow spectral diffusion has only been reported for ionic liquids.…”

Transient Glass Formation around a Quadrupolar Photoexcited Dye in a Strongly H‐Bonding Liquid Observed by Transient 2D‐IR Spectroscopy

“…[15] Similarly slow spectral diffusion has only been reported for ionic liquids. [16] Asubstantial slowing down of the translational dynamics of water has also been observed in the solvation shell of different solutes. [17] Here,t he glasslike behavior is transient:i ti sn ot observed at early times (8 ps) after electronic excitation of ADA,b ut only when the tight complex is formed.…”

“…This absence of spectral diffusion indicates that the environment of the tight complex is static on the timescale of typical fluctuations of this solvent and resembles that of a glass . Similarly slow spectral diffusion has only been reported for ionic liquids . A substantial slowing down of the translational dynamics of water has also been observed in the solvation shell of different solutes .…”