Alkyl Chain Length Dependence of the Dynamics and Structure in the Ionic Regions of Room-Temperature Ionic Liquids

The dynamics of four 1-alkyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide room-temperature ionic liquids (RTILs) with carbon chain lengths of 2, 4, 6, and 10 were studied by measuring the orientational and spectral diffusion dynamics of the vibrational probe SeCN^–. Vibrational absorption spectra, two-dimensional infrared (2D IR), and polarization-selective pump–probe (PSPP) experiments were performed on the CN stretch. In addition, optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments were performed on the bulk liquids. The PSPP experiments yielded triexponential anisotropy decays, which were analyzed with the wobbling-in-a-cone model. The slowest decay, the complete orientational randomization, slows with increasing chain length in a hydrodynamic trend consistent with the increasing viscosity. The shortest time scale wobbling motions are insensitive to chain length, while the intermediate time scale wobbling slows mildly as the chain length increases. The 2D IR spectra measured in parallel (⟨XXXX⟩) and perpendicular (⟨XXYY⟩) polarization configurations gave different decays, showing that reorientation-induced spectral diffusion (RISD) contributes to the dynamics. The spectral diffusion caused by the RTIL structural fluctuations was obtained by removing the RISD contributions. The faster structural fluctuations are relatively insensitive to chain length. The slowest structural fluctuations slow substantially when going from Emim (2 carbon chain) to Bmim (4 carbon chain) and slow further, but more gradually, as the chain length is increased. It was shown previously that K⁺ causes local ion clustering in the Emim RTIL. The K⁺ effect increases with increasing chain length. The OHD-OKE measured complete structural randomization times slow substantially with increasing chain length and are much slower than the dynamics experienced by the SeCN^– located in the ionic regions of the RTILs

Dynamics of Dihydrogen Bonding in Aqueous Solutions of Sodium Borohydride

Dihydrogen bonding occurs between protonic and hydridic hydrogens which are bound to the corresponding electron withdrawing or donating groups. This type of interaction can lead to novel reactivity and dynamic behavior. This paper examines the dynamics experienced by both borohydride and its dihydrogen-bound water solvent using 2D-IR vibrational echo and IR pump–probe spectroscopies, as well as FT-IR linear absorption experiments. Experiments are conducted on the triply degenerate B–H stretching mode and the O–D stretch of dilute HOD in the water solvent. While the B–H stretch absorption is well separated from the broad absorption band of the OD of HOD in the bulk of the water solution, the absorption of the ODs hydrogen bonded to BHs overlaps substantially with the absorption of ODs in the bulk H₂O solution. A subtraction technique is used to separate out the anion-associated OD dynamics from that of the bulk solution. It is found that both the water and borohydride undergo similar spectral diffusion dynamics, and these are very similar to those of HOD in bulk water. Because the B–H stretch is triply degenerate, the IR pump–probe anisotropy decays very rapidly, but the decay is not caused by the physical reorientation of the BH₄^– anions. Spectral diffusion occurs on a time scale longer than the anisotropy decay, demonstrating that spectral diffusion is not yet complete even when the transition dipole has completely randomized. To prevent chemical decomposition of the BH₄^–, 1 M NaOH was added to stabilize the system. 2D-IR experiments on the OD stretch of HOD in the NaOH/water liquid (no borohydride) show that the NaOH has a negligible effect on the bulk water dynamics

Influence of Water on Carbon Dioxide and Room Temperature Ionic Liquid Dynamics: Supported Ionic Liquid Membrane vs the Bulk Liquid

The influence of water on the dynamics of a room temperature ionic liquid (RTIL), 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (EmimNTf₂), and CO₂ in the RTIL was studied in the bulk liquid and a supported ionic liquid membrane (SILM) using two-dimensional infrared (IR) and IR polarization selective pump–probe spectroscopies. In the water-saturated bulk EmimNTf₂, the complete orientational randomization and structural spectral diffusion (SSD) of CO₂ became faster than in the dry EmimNTf₂. In the poly­(ether sulfone) SILM, only the longer time components of the SSD became faster in the water-saturated RTIL; the complete orientational randomization remained similar to the dry RTIL in the SILM. The implication is that the presence of water in EmimNTf₂ contained in the SILM facilitates the fluctuation of globally modified RTIL structure in the pores, but the local RTIL environments are relatively unaffected

Carbon Dioxide in a Supported Ionic Liquid Membrane: Structural and Rotational Dynamics Measured with 2D IR and Pump–Probe Experiments

Supported ionic liquid membranes (SILMs) are porous membranes impregnated with ionic liquids (ILs) and used as advanced carbon capture materials. Here, two-dimensional infrared (2D IR) and IR polarization selective pump–probe (PSPP) spectroscopies were used to investigate CO₂ reorientation and spectral diffusion dynamics in SILMs. The SILM contained 1-ethyl-3-methylimidazolium bis­(trifluoromethylsulfonly)­imide in the poly­(ether sulfone) membrane with average pore size of ∼350 nm. Two ensembles of CO₂ were observed in the SILM, one in the IL phase in the membrane pores and the other in the supporting membrane polymer. CO₂ in the polymer displayed a red-shifted IR absorption spectrum and a shorter vibrational lifetime of the asymmetric stretch mode compared to the IL phase. Despite the relatively large pore sizes, the complete orientational randomization of CO₂ and structural fluctuations of the IL (spectral diffusion) in the pores are slower than in the bulk IL by ∼2-fold. The implication is that the IL structural change induced by the polymer interface can propagate out from the interface more than a hundred nanometers, influencing the dynamics. The dynamics in the polymer are even slower. This study demonstrates that there are significant differences in the dynamics of ILs in SILMs on a molecular level compared to the bulk IL, and the study of dynamics in SILMs can provide important information for the design of SILMs for CO₂ capture

Conformational Dynamics and Stability of HP35 Studied with 2D IR Vibrational Echoes

Two-dimensional infrared (2D IR) vibrational echo spectroscopy was used to measure the fast dynamics of two variants of chicken villin headpiece 35 (HP35). The CN of cyanophenylalanine residues inserted in the hydrophobic core were used as a vibrational probe. Experiments were performed on both singly (HP35-P) and doubly CN-labeled peptide (HP35-P₂) within the wild-type sequence, as well as on HP-35 containing a singly labeled cyanophenylalanine and two norleucine mutations (HP35-P NleNle). There is a remarkable similarity between the dynamics measured in singly and doubly CN-labeled HP35, demonstrating that the presence of an additional CN vibrational probe does not significantly alter the dynamics of the small peptide. The substitution of two lysine residues by norleucines markedly improves the stability of HP35 by replacing charged with nonpolar residues, stabilizing the hydrophobic core. The results of the 2D IR experiments reveal that the dynamics of HP35-P are significantly faster than those of HP35-P NleNle. These observations suggest that the slower structural fluctuations in the hydrophobic core, indicating a more tightly structured core, may be an important contributing factor to HP35-P NleNle’s increased stability

Structural Influences on the Fast Dynamics of Alkylsiloxane Monolayers on SiO₂ Surfaces Measured with 2D IR Spectroscopy

There is widespread interest in alkyl chain surface monolayers and their applications. In many applications, alkyl monolayers are functionalized with active headgroups. Here we report the impact of major structural variations on the fast dynamics of alkylsiloxane monolayers. The monolayers were deposited with controlled structures on flat amorphous silica surfaces, and the terminal sites were functionalized with a metal carbonyl headgroup. The CO symmetric stretching mode of the headgroup served as a vibrational probe for detecting the fast structural dynamics of the monolayers using two-dimensional infrared vibrational echo spectroscopy (2D IR) to measure spectral diffusion, which is made quantitative by determining the frequency–frequency correlation function (FFCF) from the time-dependent data. Two methods of functionalizing the surface, independent attachment via a single Si–O bond formed with alkylmonochlorosilane precursors and network attachment via siloxane networks (−Si–O–Si–O−) formed with alkyltrichlorosilane precursors, were compared for several chain lengths. The two types of monolayers produced chain dynamics and structures that were independent of the manner of attachment. For densely packed monolayers, the FFCF decayed mildly slower when the alkyl chain length was decreased from C11 (chain with 11 methylenes) to C4. However, when the chain length was further reduced by one more methylene to C3, substantially slower dynamics were observed. When the chain density was reduced below 50% of fully packed monolayers, the single-component nature of the dynamics changed to a fast component plus an extremely slow component, possibly because of the collapse and entanglement of loosely packed alkyl chains

Water of Hydration Dynamics in Minerals Gypsum and Bassanite: Ultrafast 2D IR Spectroscopy of Rocks

Water of hydration plays an important role in minerals, determining their crystal structures and physical properties. Here ultrafast nonlinear infrared (IR) techniques, two-dimensional infrared (2D IR) and polarization selective pump–probe (PSPP) spectroscopies, were used to measure the dynamics and disorder of water of hydration in two minerals, gypsum (CaSO₄·2H₂O) and bassanite (CaSO₄·0.5H₂O). 2D IR spectra revealed that water arrangement in freshly precipitated gypsum contained a small amount of inhomogeneity. Following annealing at 348 K, water molecules became highly ordered; the 2D IR spectrum became homogeneously broadened (motional narrowed). PSPP measurements observed only inertial orientational relaxation. In contrast, water in bassanite’s tubular channels is dynamically disordered. 2D IR spectra showed a significant amount of inhomogeneous broadening caused by a range of water configurations. At 298 K, water dynamics cause spectral diffusion that sampled a portion of the inhomogeneous line width on the time scale of ∼30 ps, while the rest of inhomogeneity is static on the time scale of the measurements. At higher temperature, the dynamics become faster. Spectral diffusion accelerates, and a portion of the lower temperature spectral diffusion became motionally narrowed. At sufficiently high temperature, all of the dynamics that produced spectral diffusion at lower temperatures became motionally narrowed, and only homogeneous broadening and static inhomogeneity were observed. Water angular motions in bassanite exhibit temperature-dependent diffusive orientational relaxation in a restricted cone of angles. The experiments were made possible by eliminating the vast amount of scattered light produced by the granulated powder samples using phase cycling methods

Extraordinary Slowing of Structural Dynamics in Thin Films of a Room Temperature Ionic Liquid

The role that interfaces play in the dynamics of liquids is a fundamental scientific problem with vast importance in technological applications. From material science to biology, e.g., batteries to cell membranes, liquid properties at interfaces are frequently determinant in the nature of chemical processes. For most liquids, like water, the influence of an interface falls off on a ∼1 nm distance scale. Room temperature ionic liquids (RTILs) are a vast class of unusual liquids composed of complex cations and anions that are liquid salts at room temperature. They are unusual liquids with properties that can be finely tuned by selecting the structure of the cation and anion. RTILs are being used or developed in applications such as batteries, CO₂ capture, and liquids for biological processes. Here, it is demonstrated quantitatively that the influence of an interface on RTIL properties is profoundly different from that observed in other classes of liquids. The dynamics of planar thin films of the room temperature ionic liquid, 1-butyl-3-methylimidazolium bis­(trifluoromethylsulfonyl)­imide (BmimNTf₂), were investigated using two-dimensional infrared spectroscopy (2D IR) with the CN stretch of SeCN^– as the vibrational probe. The structural dynamics (spectral diffusion) of the thin films with controlled nanometer thicknesses were measured and compared to the dynamics of the bulk liquid. The samples were prepared by spin coating the RTIL, together with the vibrational probe, onto a surface functionalized with an ionic monolayer that mimics the structure of the BmimNTf₂. Near-Brewster’s angle reflection pump–probe geometry 2D IR facilitated the detection of the exceedingly small signals from the films, some of which were only 14 nm thick. Even in quarter micron (250 nm) thick films, the observed dynamics were much slower than those of the bulk liquid. Using a new theoretical description, the correlation length (exponential falloff of the influence of the interfaces) was found to be 28 ± 5 nm. This very long correlation length, ∼30 times greater than that of water, has major implications for the use of RTILs in devices and other applications

Water Dynamics in Polyacrylamide Hydrogels

Polymeric hydrogels have wide applications including electrophoresis, biocompatible materials, water superadsorbents, and contact lenses. The properties of hydrogels involve the poorly characterized molecular dynamics of water and solutes trapped within the three-dimensional cross-linked polymer networks. Here we apply ultrafast two-dimensional infrared (2D IR) vibrational echo and polarization-selective pump–probe (PSPP) spectroscopies to investigate the ultrafast molecular dynamics of water and a small molecular anion solute, selenocyanate (SeCN^–), in polyacrylamide hydrogels. For all mass concentrations of polymer studied (5% and above), the hydrogen-bonding network reorganization (spectral diffusion) dynamics and reorientation dynamics reported by both water and SeCN^– solvated by water are significantly slower than in bulk water. As the polymer mass concentration increases, molecular dynamics in the hydrogels slow further. The magnitudes of the slowing, measured with both water and SeCN^–, are similar. However, the entire hydrogen-bonding network of water molecules appears to slow down as a single ensemble, without a difference between the core water population and the interface water population at the polymer–water surface. In contrast, the dissolved SeCN^– do exhibit two-component dynamics, where the major component is assigned to the anions fully solvated in the confined water nanopools. The slower component has a small amplitude which is correlated with the polymer mass concentration and is assigned to adsorbed anions strongly interacting with the polymer fiber networks