Characterization of Acetonitrile Isotopologues as Vibrational Probes of Electrolytes

Nanoclustering in non-ideal ethanol/heptane solutions alters solvation dynamics

Alcohol/alkane solutions widely used in chemical synthesis and as transportation fuels are highly non-ideal due to the nanoscale clustering of the amphiphilic alcohol molecules within the nonpolar alkanes. Besides impacting reactivity, such as combustion, non-ideal solutions are likely to exhibit unusual solvation dynamics on ultrafast time scales arising from the structurally heterogeneous nature of molecular-scale association. Using a convenient transition metal carbonyl vibrational probe [(C5H5)Mn(CO)3, CMT], linear absorption and nonlinear two-dimensional infrared (2D-IR) spectroscopy reveal composition-dependent solvation dynamics as reported by the frequency fluctuation correlation function in a series of ethanol/heptane solutions. Slow spectral diffusion with dilute ethanol indicates preferential solvation of the polar solute by the alcohol with a mechanism largely dominated by solvent exchange. Comparison with an ethanol/acetonitrile solution series yields no substantial preferential solvation or solvent exchange signatures in the linear or 2D-IR spectra. In ethanol/heptane solutions, increasing the ethanol concentration speeds up the solvation dynamics, which is largely consistent with a model that includes solvent exchange and single-solvent spectral diffusion. Detailed analysis of the deviation from the experimental time constants from the model's optimal parameters yields a remarkable resemblance of the concentration-weighted Kirkwood-Buff integrals for ethanol/heptane solutions. This trend indicates that solution non-ideality alters the spectral diffusion dynamics of the probe solute. Given that nanoscale clustering drives the non-ideality, these experiments reveal a dynamical consequence of nanoscale heterogeneity on the ultrafast dynamics of the solution. Refined understanding of the structural and dynamical aspects of mixed solvents will be necessary for predictive solution strategies in chemistry.

Measuring the Electric Fields of Ions Captured in Crown Ethers

Controlling reactivity with electric fields is a persistent challenge in chemistry. One approach is to tether ions at well-defined locations near a reactive center. To quantify fields arising from ions, we report crown ethers that capture metal cations as field sources and a covalently bound vibrational Stark shift probe as a field sensor. We use experiments and computations in both the gas and liquid phases to quantify the vibrational frequencies of the probe and estimate the electric fields from the captured ions. Cations, in general, blue shift the probe frequency, with effective fields estimated to vary in the range of ∼0.2-3 V/nm in the liquid phase. Comparison of the gas and liquid phase data provides insight into the effects of mutual polarization of the molecule and solvent and screening of the ion's field. These findings reveal the roles of charge, local screening, and geometry in the design of tailored electric fields.

Molecular Photothermal Effect on the 2D-IR Spectroscopy of Acetonitrile-Based Li-Ion Battery Electrolytes

Advancements in Li-ion battery (LIB) technology hinge on an understanding of Li-ion solvation and charge transport dynamics. Ultrafast two-dimensional infrared (2D-IR) spectroscopy has been used to investigate these dynamics in electrolytes by probing chemical exchange processes through time-dependent cross-peak analysis. However, accurate interpretation is complicated by factors such as vibrational energy transfer and molecular photothermal effect (MPTE), affecting cross-peak evolution. Pinpointing the precise origin of these cross-peaks has posed a significant challenge in time-resolved IR spectroscopic studies of LIB electrolytes. Here, we trace the origin of 2D-IR cross-peaks of LIB electrolytes utilizing acetonitrile as a solvent. Time-dependent analysis of LiSCN and CH3SCN mixtures in CD3CN revealed distinctive MPTE features. Furthermore, direct observation of intermolecular MPTE through two-color IR pump-probe spectroscopy lends support to the findings. Our results emphasize the non-negligible artifacts induced by MPTE and the necessity of considering these effects to accurately observe the ultrafast dynamics within LIB electrolytes.

Oxidatively-induced C(sp3)-C(sp3) bond formation at a tucked-in iron(iii) complex

Carbon-carbon (C-C) bond formation is a cornerstone of synthetic chemistry, relying on routes such as transition-metal mediated cross-coupling for the introduction of new carbon-based functionality. For {[M] n+-C} (M = metal) structural units, studies that offer well-defined relationships between metal oxidation state, hydrocarbon strain, and {[M] n+-C} bond thermochemistry are thus informative, providing a means to reliably access new product classes. Here, we show that one-electron oxidation of the iron tucked-in complex [(η6-C5Me4[double bond, length as m-dash]CH2)Fe(dnppe)] (dnppe = 1,2-bis(di-n-propylphosphino)ethane) results in C(sp3)-C(sp3) bond formation giving unique {Fe2} dimers. Freeze-quenched CW X-band EPR spectroscopy allowed for spectroscopic identification of the reactive [(η6-C5Me4[double bond, length as m-dash]CH2)Fe(dnppe)]+ intermediate. Density functional theory (DFT) calculations reveal a primarily Fe-centered radical and a weak {[Fe]-C} bond (BDE[Fe]-C = 24.5 kcal mol-1, c.f. BDEC-C(ethane) = 90 kcal mol-1). For comparison, a structurally analogous Fe(iii) methyl complex was prepared, [Cp*Fe(dnppe)(CH3)]+ (Cp* = C5Me5 -), where C(sp3)-C(sp3) coupling was not observed, consistent with a larger calculated BDE[Fe]-C value of 47.8 kcal mol-1. These data are analogized to the simple hydrocarbons ethane and cyclopropane, where a strain-induced BDEC-C decrease of 33 kcal mol-1 is witnessed on cyclization.

Direct Probe of Vibrational Fingerprint and Combination Band Coupling

Vibrational fingerprints and combination bands are a direct measure of couplings that control molecular properties. However, most combination bands possess small transition dipoles. Here we use multiple, ultrafast coherent infrared pulses to resolve vibrational coupling between CH3CN fingerprint modes at 918 and 1039 cm-1 and combination bands in the 2750-6100 cm-1 region via doubly vibrationally enhanced (DOVE) coherent multidimensional spectroscopy (CMDS). This approach provides a direct probe of vibrational coupling between fingerprint modes and near-infrared combination bands of large and small transition dipoles in a molecular system over a large frequency range.

Dynamics of Formamide-Water Mixtures Investigated by Linear and Nonlinear Infrared Spectroscopy

Formamide (FA) exhibits complete miscibility with water, offering a simplified model for exploring the solvation dynamics of peptide linkages in biophysical processes. Its liquid state demonstrates a three-dimensional hydrogen bonding network akin to water, reflecting solvent-like behavior. Analyzing the microscopic structure and dynamics of FA-water mixtures is expected to provide crucial insights into hydrogen bonding dynamics─a key aspect of various biophysical phenomena. This study is focused on the dynamics of FA-water mixtures using linear and femtosecond infrared spectroscopies. By using the intrinsic OD stretch and extrinsic probe SCN-, the local vibrational behaviors across various FA-water compositions were systematically investigated. The vibrational relaxation of OD stretch revealed a negligible impact of FA addition on the vibrational lifetime of water molecules, underscoring the mixture's water-like behavior. However, the reorientational dynamics of OD stretch slowed with increasing FA mole fraction (XFA), plateauing beyond XFA > 0.5. This suggests a correlation between OD's reorientational time and the strength of the hydrogen bond network, likely tied to the solution's changing dielectric constant. Conversely, the vibrational relaxation dynamics of SCN- was strongly correlated with XFA, highlighting a competition between water and FA molecules in solvating SCN-. Moreover, a linear relationship between rising viscosity and the prolonged correlation time of SCN-'s slow dynamics indicates that the solution's macroscopic viscosity is dictated by the extended structures formed between FA and water molecules. The relation between the reorientation dynamics of the SCN- and the macroscopic viscosity in aqueous FA-water mixture solutions was analyzed by using the Stokes-Einstein-Debye equations. The direct viscosity-diffusion coupling is observed, which can be attributed to the homogeneous dynamics feature in FA-water mixture solutions. The inclusion of these intrinsic and extrinsic probes not only enhances the comprehensiveness of our analysis but also provides valuable insights into various aspects of the dynamics within the FA-water system. This investigation sheds light on the fundamental dynamics of FA-water mixtures, emphasizing their molecular-level homogeneity in this binary mixture solution.

Triple-Bond Vibrations: Emerging Applications in Energy and Biological Sciences

Triple bonds, such as that formed between two carbon atoms (i.e., C≡C) or that formed between one carbon atom and one nitrogen atom (i.e., C≡N), afford unique chemical bonding and hence vibrational characteristics. As such, they are not only frequently used to construct molecules with tailored chemical and/or physical properties but also employed as vibrational probes to provide site-specific chemical and/or physical information at the molecular level. Herein, we offer our perspective on the emerging applications of various triple-bond vibrations in energy and biological sciences with a focus on C≡C and C≡N triple bonds.

The hydrogen-bonding dynamics of water to a nitrile-functionalized electrode is modulated by voltage according to ultrafast 2D IR spectroscopy

We report the hydrogen-bonding dynamics of water to a nitrile-functionalized and plasmonic electrode surface as a function of applied voltage. The surface-enhanced two-dimensional infrared spectra exhibit hydrogen-bonded and non-hydrogen-bonded nitrile features in similar proportions, plus cross peaks between the two. Isotopic dilution experiments show that the cross peaks arise predominantly from chemical exchange between hydrogen-bonded and non-hydrogen-bonded nitriles. The chemical exchange rate depends upon voltage, with the hydrogen bond of the water to the nitriles breaking 2 to 3 times slower (>63 vs. 25 ps) under a positive as compared to a negative potential. Spectral diffusion created by hydrogen-bond fluctuations occurs on a ~1 ps timescale and is moderately potential-dependent. Timescales from molecular dynamics simulations agree qualitatively with the experiment and show that a negative voltage causes a small net displacement of water away from the surface. These results show that the voltage applied to an electrode can alter the timescales of solvent motion at its interface, which has implications for electrochemically driven reactions.

Life Aging Effect as a Conditioning Process that Regulates the Performance of the Halogen-Free Mg Electrolyte

Studying the interplay between the electrochemical performance and the electrolyte conditioning process is crucial for building an efficient magnesium battery. In this work, we use halogen-free electrolyte (HFE) based on Mg(NO3)2 in acetonitrile (ACN) and tetraethylene glycol dimethyl ether (G4) to study the effect of the aging time calendar on its electrochemical properties. The characterization techniques confirm apparent changes occurring in the bulk speciation and the Mg2+ solvation barrier of the aging HFE relative to the as-prepared fresh HFE. The overpotential of Mg plating/stripping and bulk resistance of the aging HFE is reduced relative to the as-prepared fresh HFE. Mg-S cells using aged HFE deliver high specific capacities (586 mA h/g), higher Coulombic efficiencies, and higher cycle life (up to 30 cycles at 25 °C) relative to Mg-S cells with fresh HFE that deliver a specific capacity of ∼535 mA h g-1, low Coulombic efficiency, and short cycle life at a current density of 0.02 mA cm-2. The present findings provide a new concept describing how the aging process regulates the electrochemical performance of the HFE and enhances the cycle life of Mg-S batteries.

Chain-like Structures Facilitate Li+ Transport in Concentrated Aqueous Electrolytes: Insights from Ultrafast Infrared Spectroscopy and Molecular Dynamics Simulations

Highly concentrated aqueous electrolytes have attracted attention due to their unique applications in lithium ion batteries (LIBs). However, the solvation structure and transport mechanism of Li+ cations at concentrated concentrations remain largely unexplored. To address this gap in knowledge, we employ ultrafast infrared spectroscopy and molecular dynamics (MD) simulations to reveal the dynamic and spatial structural heterogeneity in aqueous lithium chloride (LiCl) solutions. The coupling between the reorientation dynamics of the extrinsic probe and the macroscopic viscosity in aqueous LiCl solutions was analyzed using the Stokes-Einstein-Debye (SED) equations. MD simulations reveal that the Cl- and Li+ form chain-like structures through electrostatic interactions, supporting the vehicular migration of Li+ through the chain-like structure. The concentration dependent conductivity of the LiCl solution is well reproduced, where Li(H2O)2+ and Li(H2O)3+ are the dominant species that contribute to the conduction of Li+. This study is expected to establish correlations between ion pair structures and macroscopic properties.

Relation between microscopic structure and macroscopic properties in polyacrylonitrile-based lithium-ion polymer gel electrolytes

Polymer gel electrolytes (PGE) have seen a renewed interest in their development because they have high ionic conductivities but low electrochemical degradation and flammability. PGEs are formed by mixing a liquid lithium-ion electrolyte with a polymer at a sufficiently large concentration to form a gel. PGEs have been extensively studied, but the direct connection between their microscopic structure and macroscopic properties remains controversial. For example, it is still unknown whether the polymer in the PGE acts as an inert, stabilizing scaffold for the electrolyte or it interacts with the ionic components. Here, a PGE composed of a prototypical lithium-carbonate electrolyte and polyacrylonitrile (PAN) is pursued at both microscopic and macroscopic levels. Specifically, this study focused on describing the microscopic and macroscopic changes in the PGE at different polymer concentrations. The results indicated that the polymer-ion and polymer-polymer interactions are strongly dependent on the concentration of the polymer and the lithium salt. In particular, the polymer interacts with itself at very high PAN concentrations (10% weight) resulting in a viscous gel. However, the conductivity and dynamics of the electrolyte liquid components are significantly less affected by the addition of the polymer. The observations are explained in terms of the PGE structure, which transitions from a polymer solution to a gel, containing a polymer matrix and disperse electrolyte, at low and high PAN concentrations, respectively. The results highlight the critical role that the polymer concentration plays in determining both the macroscopic properties of the system and the molecular structure of the PGE.

Distinguishing between the Electrostatic Effects and Explicit Ion Interactions in a Stark Probe

Vibrational Stark probes are incisive tools for measuring local electric fields in a wide range of chemical environments. The interpretation of the frequency shift often gets complicated due to the specific interactions of the probe, such as hydrogen bonding and Lewis bonding. Therefore, it is important to distinguish between the pure electrostatic response and the response due to such specific interactions. Here we report a molecular system that is sensitive to both the Stark effect from a single ion and the explicit Lewis bonding of ions with the probe. The molecule consists of a crown ether with an appended benzonitrile. The crown captures cations of various charges, and the electric field from the ions is sensed by the benzonitrile probe. Additionally, the lone pair of the benzonitrile can engage in Lewis interactions with some of the ions by donating partial charge density to the ions. Our system exhibits both of these effects and therefore is a suitable test bed for distinguishing between the pure electrostatic and the Lewis interactions. Our computational results show that the electrostatic influence of the ion is operative at large distances, while the Lewis interaction becomes important only within distances that permit orbital overlap. Our results may be useful for using the nitrile probe for measuring electrostatic and coordination effects in complex ionic environments such as the electrode-electrolyte interfaces.

Ionic conduction mechanism in high concentration lithium ion electrolytes

The conduction mechanism of a family of high concentration lithium electrolytes (HCEs) is investigated. It is found in all HCEs that the molecular motions are regulated by the anion size and correlated to the HCE ionic resistivity. From the results, a mechanism involving highly correlated ionic networks is derived.

Molecular Photothermal Effects on Time-Resolved IR Spectroscopy: Solute-Solvent Intermolecular Energy Transfer

Time-resolved IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy are valuable tools for studying ultrafast chemical and biological processes in solutions. However, the corresponding signals at long times are obscured by the molecular photothermal effects resulting from the heat dissipation of vibrationally photoexcited molecules to the surroundings. Recently, a phenomenology model was used to describe molecular photothermal effects on IR-PP signals and the diagonal and cross-peaks of 2D-IR spectra at long pump-probe delay times. Here, we consider the thermal diffusion equation with a time-dependent heat source term to describe the solute-solvent energy transfer process. An approximate solution to the nonhomogeneous differential equation shows that the molecular photothermal effect is determined by the mean intermolecular distance between IR-absorbing molecules. We show that the time profile of heat dissipation from a vibrationally excited molecule to the surroundings, which provides information about the mechanisms involved in the solute-solvent intermolecular energy transfer process in solutions, can be directly measured by analyzing the molecular photothermal IR-PP and 2D-IR signals. We anticipate that the present work can be used to interpret local heating-induced time-resolved IR spectroscopic signals and understand the rate of and the mechanisms involved in the conversion from high-frequency molecular vibrational energy to solvent kinetic energy in condensed phases.

Distinct Hydrogen Bonding Dynamics Underlies the Microheterogeneity in DMF-Water Mixtures

The hydrogen bonding interaction between the amide functional group and water is fundamental to understanding the liquid-liquid heterogeneity in biological systems. Herein, the structure and dynamics of the N,N-dimethylformamide (DMF)-water mixtures have been investigated by linear and nonlinear IR spectroscopies, using the hydroxyl stretch and extrinsic probe of thiocyanate as local vibrational reporters. According to vibrational relaxation dynamics measurements, the orientational dynamics of water is not directly tied to those of DMF molecules. Wobbling-in-a-cone analysis demonstrates that the water molecules have varying degrees of angular restriction depending on their composition due to the formation of specific water-DMF networks. Because of the preferential solvation by DMF molecules, the rotational dynamics of the extrinsic probe is slowed significantly, and its rotational time constants are correlated to the change of solution viscosity. The unique structural dynamics observed in the DMF-water mixtures is expected to provide important insights into the underlying mechanism of microscopic heterogeneity in binary mixtures.

Molecular photothermal effects on time-resolved IR spectroscopy

Time-resolved IR pump-probe (IR-PP) and two-dimensional IR (2D-IR) spectroscopy are valuable techniques for studying various ultrafast chemical and biological processes in solutions. The time-dependent changes of nonlinear IR signals reflecting fast molecular processes such as vibrational energy transfer and chemical exchange provide invaluable information on the rates and mechanisms of solvation dynamics and structural transitions of multi-species vibrationally interacting molecular systems. However, due to the intrinsic difficulties in distinguishing the contributions of molecule-specific processes to the time-resolved IR signals from those resulting from local heating, it becomes challenging to interpret time-resolved IR-PP and 2D-IR spectra exhibiting transient growing-in spectral components and cross-peaks unambiguously. Here, theoretical considerations of various effects of vibrational coupling, energy transfer, chemical exchange, the generation of hot ground states, molecular photothermal process, and their combinations on the lineshapes and time-dependent intensities of IR-PP spectra and 2D-IR diagonal and cross-peaks are presented. We anticipate that the present work will help researchers using IR pump-probe and 2D-IR techniques to distinguish local heating-induced photothermal signals from genuine nonlinear IR signals.

Exchange-Mediated Transport in Battery Electrolytes: Ultrafast or Ultraslow?

Understanding the mechanisms of charge transport in batteries is important for the rational design of new electrolyte formulations. Persistent questions about ion transport mechanisms in battery electrolytes are often framed in terms of vehicular diffusion by persistent ion-solvent complexes versus structural diffusion through the breaking and reformation of ion-solvent contacts, i.e., solvent exchange events. Ultrafast two-dimensional (2D) IR spectroscopy can probe exchange processes directly via the evolution of the cross-peaks on picosecond time scales. However, vibrational energy transfer in the absence of solvent exchange gives rise to the same spectral signatures, hiding the desired processes. We employ 2D IR on solvent resonances of a mixture of acetonitrile isotopologues to differentiate chemical exchange and energy-transfer dynamics in a comprehensive series of Li⁺, Mg²⁺, Zn²⁺, Ca²⁺, and Ba²⁺ bis(trifluoromethylsulfonyl)imide electrolytes from the dilute to the superconcentrated regime. No exchange phenomena occur within at least 100 ps, regardless of the ion identity, salt concentration, and presence of water. All of the observed spectral dynamics originate from the intermolecular energy transfer. These results place the lower experimental boundary on the ion-solvent residence times to several hundred picoseconds, much slower than previously suggested. With the help of MD simulations and conductivity measurements on the Li⁺ and Zn²⁺ systems, we discuss these results as a continuum of vehicular and structural modalities that vary with concentration and emphasize the importance of collective electrolyte motions to ion transport. These results hold broadly applicable to many battery-relevant ions and solvents.