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Journal ArticleDOI

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

01 Feb 2017-Journal of the American Chemical Society (American Chemical Society)-Vol. 139, Iss: 6, pp 2408-2420
TL;DR: 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.
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 BmimNTf2 liquid (870 ± 20 ps) measured with optical heterodyne-detected optical Kerr effect (OHD-OKE) experiments. The orientational c...
Citations
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Journal ArticleDOI
TL;DR: This review comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.
Abstract: Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

236 citations

Journal ArticleDOI
TL;DR: The ways in which two-dimensional visible and infrared spectroscopies are being applied to elucidate fundamental details of important processes in biological and materials science are provided.
Abstract: Two-dimensional spectroscopy is a powerful tool for extracting structural and dynamic information from a wide range of chemical systems. We provide a brief overview of the ways in which two-dimensional visible and infrared spectroscopies are being applied to elucidate fundamental details of important processes in biological and materials science. The topics covered include amyloid proteins, photosynthetic complexes, ion channels, photovoltaics, batteries, as well as a variety of promising new methods in two-dimensional spectroscopy.

58 citations

Journal ArticleDOI
TL;DR: In this article, a two-step hydrothermal and in-situ generating process was used to produce 2D core-shell composites with high visible light photocatalytic performance and high stability.

44 citations

Journal ArticleDOI
TL;DR: Several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool are introduced, which will allow for groundbreaking research in many new areas of natural sciences.
Abstract: Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.

31 citations

Journal ArticleDOI
TL;DR: Small-amplitude motions within contact F-Q pairs, which gate the electronic coupling, are suggested to be the limiting dynamics, and reaction rates in ionic liquids are limited by some factor not properly accounted for in bimolecular electron transfer models based on a spherical diffusion-reaction approach.
Abstract: The present work seeks to better understand the role of solute diffusion and solvation dynamics on bimolecular electron transfer in ionic liquids (ILs). Steady-state and time-resolved measurements of the reductive fluorescence quenching of five fluorophores (“F”) by six quenchers (“Q”; electron donors) are reported in acetonitrile and two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide. Data were collected on 66 different F–Q–solvent systems, which span a 2.0 eV range in driving force and viscosities that vary 1000-fold, enabling stringent tests of bimolecular electron transfer models. A Stern–Volmer analysis yielded much larger diffusion-limited rates than simple kinetic theory predictions in the ILs and the absence of a Marcus turnover. Use of an approximate solution to the spherical diffusion-reaction equation enabled testing of several models for the reaction rate distance dependence. The Smoluchowski an...

26 citations

References
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Journal ArticleDOI
TL;DR: In this article, a new estimate minimum information theoretical criterion estimate (MAICE) is introduced for the purpose of statistical identification, which is free from the ambiguities inherent in the application of conventional hypothesis testing procedure.
Abstract: The history of the development of statistical hypothesis testing in time series analysis is reviewed briefly and it is pointed out that the hypothesis testing procedure is not adequately defined as the procedure for statistical model identification. The classical maximum likelihood estimation procedure is reviewed and a new estimate minimum information theoretical criterion (AIC) estimate (MAICE) which is designed for the purpose of statistical identification is introduced. When there are several competing models the MAICE is defined by the model and the maximum likelihood estimates of the parameters which give the minimum of AIC defined by AIC = (-2)log-(maximum likelihood) + 2(number of independently adjusted parameters within the model). MAICE provides a versatile procedure for statistical model identification which is free from the ambiguities inherent in the application of conventional hypothesis testing procedure. The practical utility of MAICE in time series analysis is demonstrated with some numerical examples.

47,133 citations

Journal ArticleDOI
TL;DR: The goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas, and to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.
Abstract: Ionic liquids are room-temperature molten salts, composed mostly of organic ions that may undergo almost unlimited structural variations. This review covers the newest aspects of ionic liquids in applications where their ion conductivity is exploited; as electrochemical solvents for metal/semiconductor electrodeposition, and as batteries and fuel cells where conventional media, organic solvents (in batteries) or water (in polymer-electrolyte-membrane fuel cells), fail. Biology and biomimetic processes in ionic liquids are also discussed. In these decidedly different materials, some enzymes show activity that is not exhibited in more traditional systems, creating huge potential for bioinspired catalysis and biofuel cells. Our goal in this review is to survey the recent key developments and issues within ionic-liquid research in these areas. As well as informing materials scientists, we hope to generate interest in the wider community and encourage others to make use of ionic liquids in tackling scientific challenges.

4,098 citations

Journal ArticleDOI
TL;DR: In this article, a model-free approach to the interpretation of nuclear magnetic resonance relaxation experiments on macromolecules in solution is presented, which uses the above spectral density to least-squares fit relaxation data by treating S/sup 2 and T/sub e/ as adjustable parameters.
Abstract: A new approach to the interpretation of nuclear magnetic resonance relaxation experiments on macromolecules in solution is presented. This paper deals with the theoretical foundations and establishes the range of validity of this approach, and the accompanying paper demonstrates how a wide variety of experimental relaxation data can be successfully analyzed by using this approach. For both isotropic and anisotropic overall motion, it is shown that the unique imformation on fast internal motions contained in relaxation experiments can be completely specified by two model-independent quantities; (1) a generalized order parameter, S, which is a measure of the spatial restriction of the motion, and (2) an effective correlation time, T/sub e/, which is a measure of the rate of motion. A simple expression for the spectral density involving these two parameters is derived and is shown to be exact when the internal (but not overall) motions are in the extreme narrowing limit. The model-free approach (so called because S/sup 2/ and T/sub e/ have model-independent significance) consists of using the above spectral density to least-squares fit relaxation data by treating S/sup 2/ and T/sub e/ as adjustable parameters. The range of validity of this approach is illustrated by analyzing error-free relaxationmore » data generated by using sophisticated dynamical models. Empirical rules are presented that allow one to estimate the of S/sup 2/ and T/sub e/ extracted by using the model-free approach by considering their numerical values, the resonance frequencies, and the parameters for the overall motion. For fast internal motions, it is unnecessary to use approaches based on complicated spectral densities derived within the framework of a model because all models that can give the correct value of S/sup 2/ work equally well.« less

3,562 citations

Journal ArticleDOI
TL;DR: In this paper, the physical and chemical properties of room temperature ionic liquids (RTILs) are reviewed from the point of view of their possible application as electrolytes in electrochemical processes and devices.

2,241 citations

Journal ArticleDOI
TL;DR: As the length of the alkyl chain increases, the nonpolar domains become larger and more connected and cause swelling of the ionic network, in a manner analogous to systems exhibiting microphase separation.
Abstract: Nanometer-scale structuring in room-temperature ionic liquids is observed using molecular simulation. The ionic liquids studied belong to the 1-alkyl-3-methylimidazolium family with hexafluorophosphate or with bis(trifluoromethanesulfonyl)amide as the anions, [Cnmim][PF6] or [Cnmim][(CF3SO2)2N], respectively. They were represented, for the first time in a simulation study focusing on long-range structures, by an all-atom force field of the AMBER/OPLS_AA family containing parameters developed specifically for these compounds. For ionic liquids with alkyl side chains longer than or equal to C4, aggregation of the alkyl chains in nonpolar domains is observed. These domains permeate a tridimensional network of ionic channels formed by anions and by the imidazolium rings of the cations. The nanostructures can be visualized in a conspicuous way simply by color coding the two types of domains (in this work, we chose red = polar and green = nonpolar). As the length of the alkyl chain increases, the nonpolar domai...

1,668 citations