Cation Dynamics Research Articles

Cation Dynamics in Supercooled and Solid Alkyl Methylimidazolium Bromide Ionic Liquids

The molecular dynamics of alkyl methylimidazolium bromide ionic liquids with different side groups of the cation are studied over a wide range of temperatures, covering the supercooled and crystalline states. Nuclear magnetic resonance relaxation dispersion (NMRD) at different magnetic field strengths was combined with NMR pulsed field gradient (PFG) diffusion measurements in order to obtain a description of the temperature dependence of the cationic mobility. While an Arrhenius dependence of the correlation times was found at high temperatures, a deviation is observed below a critical temperature of Tdyn ∼ 275 K which corresponds to about 1.25 Tg for two of the substances. The macroscopic diffusion coefficient, on the other hand, is best described by a VFT dependence down to a similar temperature, and a much weaker temperature dependence below. Measurements carried out in the crystalline state of 1-butyl-3-methylimidazolium bromide (Bmim Br) exhibit a dramatically increased self-diffusion coefficient in agreement with earlier reports of strong dynamic heterogeneity in the presence of minute amounts of water.

Effects of zero point vibration on the reaction dynamics of water dimer cations following ionization.

Reactions of water dimer cation (H2O)2+ following ionization have been investigated by means of a direct ab initio molecular dynamics method. In particular, the effects of zero point vibration and zero point energy (ZPE) on the reaction mechanism were considered in this work. Trajectories were run on two electronic potential energy surfaces (PESs) of (H2O)2+: ground state (2 A″-like state) and the first excited state (2 A'-like state). All trajectories on the ground-state PES lead to the proton-transferred product: H2 O+ (Wd)-H2 O(Wa) → OH(Wd)-H3 O+ (Wa), where Wd and Wa refer to the proton donor and acceptor water molecules, respectively. Time of proton transfer (PT) varied widely from 15 to 40 fs (average time of PT = 30.9 fs). The trajectories on the excited-state PES gave two products: an intermediate complex with a face-to-face structure (H2 O-OH2 )+ and a PT product. However, the proton was transferred to the opposite direction, and the reverse PT was found on the excited-state PES: H2 O(Wd)-H2 O+ (Wa) → H3 O+ (Wd)-OH(Wa). This difference occurred because the ionizing water molecule in the dimer switched between the ground and excited states. The reaction mechanism of (H2O)2+ and the effects of ZPE are discussed on the basis of the results. © 2017 Wiley Periodicals, Inc.

Dynamics of water-soluble base cations in a forest soil measured by an in situ combined IER method

Dynamics of water-soluble base cations in a forest soil measured by an in situ combined IER method

Effects of micro-solvation on the reaction dynamics of biphenyl cations following hole capture

Biphenyl (Bp) and its related compounds are widely applied in single-molecule electronic devices. In this study, the effects of micro-solvation on the hole capture (ionization) dynamics of Bp were investigated by means of direct ab initio molecular dynamics (AIMD) simulations. The micro-solvation of Bp was simulated using one and two water molecules (i.e., Bp(H2O)n (n=0–2)). The reaction dynamics of Bp+(H2O)n following hole capture were investigated via direct AIMD simulation. In the case without H2O (n=0), the twist angle of Bp+ periodically vibrated without decay. In contrast, when water molecules were near Bp+, the amplitude of the twist angle vibration decayed periodically. The formation of hydrogen bonds between Bp+ and water molecules prevents periodic vibration by generating friction. The electronic states and reaction mechanism were investigated based on the theoretical results.

Solid-State NMR Spectroscopy Study of Cation Dynamics in Layered Na2Ti3O7 and Li2Ti3O7

Abstract Ti-based materials exhibit suitable properties for usage in secondary Li- and Na-ion batteries and were in the focus of several electrochemical and ion conductivity studies. A material of such interest is layer-structured, monoclinic Na2Ti3O7. Additionally, the sodium in Na2Ti3O7 can be replaced completely with lithium to achieve monoclinic Li2Ti3O7, whose electrochemical properties were already investigated as well. Both materials exhibit interesting properties such as zero-strain behavior upon intercalation and high cycling stability. However, there is still a lack of fundamental understanding of the ion diffusivity of both Na and Li in the corresponding host structure. Solid-state nuclear magnetic resonance (NMR) spectroscopy is used here for the first time to reveal the cation dynamics in layered Na2Ti3O7 and Li2Ti3O7. This includes activation energies for the ionic motion and jump rates on the microscopic scale from NMR spin-lattice relaxation (SLR), spin-alignment echo (SAE), and 2D NMR exchange techniques. Moreover, the dimensionality of the ionic motion is investigated by frequency-dependent NMR SLR. Structural details are studied using magic-angle spinning (MAS) NMR spectroscopy. Results for the electric field gradient at the Na and Li site, respectively, are compared with those from theoretical calculations performed within this study. The dynamics are similar for both cations, and the frequency-dependence of the 7Li NMR SLR rate indicates Li motion confined to two dimensions. Thus, these two materials may be regarded a model system for low-dimensional diffusion of two different cations.

Organic Cation Rotation and Immobilization in Pure and Mixed Methylammonium Lead-Halide Perovskites.

Three-dimensional lead-halide perovskites have attracted a lot of attention due to their ability to combine solution processing with outstanding optoelectronic properties. Despite their soft ionic nature these materials demonstrate a surprisingly low level of electronic disorder resulting in sharp band edges and narrow distributions of the electronic energies. Understanding how structural and dynamic disorder impacts the optoelectronic properties of these perovskites is important for many applications. Here we combine ultrafast two-dimensional vibrational spectroscopy and molecular dynamics simulations to study the dynamics of the organic methylammonium (MA) cation orientation in a range of pure and mixed trihalide perovskite materials. For pure MAPbX3 (X = I, Br, Cl) perovskite films, we observe that the cation dynamics accelerate with decreasing size of the halide atom. This acceleration is surprising given the expected strengthening of the hydrogen bonds between the MA and the smaller halide anions, but can be explained by the increase in the polarizability with the size of halide. Much slower dynamics, up to partial immobilization of the organic cation, are observed in the mixed MAPb(ClxBr1-x)3 and MAPb(BrxI1-x)3 alloys, which we associate with symmetry breaking within the perovskite unit cell. The observed dynamics are essential for understanding the effects of structural and dynamical disorder in perovskite-based optoelectronic systems.

Influence of cellulose gel matrix on BMIMCl ionic liquid dynamics and conductivity

Fast field-recycling magnetic resonance relaxometry (FFC NMR) was applied to measure the spin-lattice relaxation time, T 1, of protons in pure ionic liquid (IL) 1-butyl-3-methylimidazolium chloride (BMIMCl) and when confined in cellulose (Cell) ion gel (Cell/BMIMCl) at different temperatures and different Larmor frequencies. The rotational and translational contributions were taken to interpret the relaxation data of neat BMIMCl and were described by Woessner’s and Torrey’s theoretical models, respectively. The ionic liquid–cellulose matrix interaction detected in 10 wt% polymer-ion gel was interpreted on the basis of a dynamical process called reorientation mediated by translational displacements (RMTD), which allow explanation of the significant slowing of the dynamics of IL cations at cellulose surfaces. Two types of cation diffusion were identified in this gel: a long-range translational diffusion within large cavities of the Cell matrix and diffusion occurring at the polymer surface. The correlational time constants and self-diffusion coefficients of the BMIMCl ionic liquid in bulk-like state and interaction with the cellulose matrix surface were determined. The conductivity measurements performed for pure IL and that confined in the Cell/BMIMCl ion gel show that the gelation only results in a small decrease of the ionic conductivity.

Direct grid-based quantum dynamics on propagated diabatic potential energy surfaces

We present a method for performing non-adiabatic, grid-based nuclear quantum dynamics calculations using diabatic potential energy surfaces (PESs) generated “on-the-fly”. Gaussian process regression is used to interpolate PESs by using electronic structure energies, calculated at points in configuration space determined by the nuclear dynamics, and diabatising the results using the propagation diabatisation method reported recently (Richings and Worth, 2015). Our new method is successfully demonstrated using a grid-based approach to model the non-adiabatic dynamics of the butatriene cation. Overall, our scheme offers a route towards accurate quantum dynamics on diabatic PESs learnt on-the-fly.

Structural implications on the excitation dynamics of fluorescent 3H-indolium cations.

Fluorescent 3H-indolium cations are valuable components for the realization of activatable fluorophores for bioimaging applications. Their relatively poor fluorescent quantum yields in organic solvents, however, appear to be in contradiction to their good performance in analytical methods based on single-molecule detection. The elucidation of the structural factors governing the excitation dynamics of these compounds is, therefore, essential to rationalize these effects and possibly guide the future design of activatable probes with improved performance. In this context, the structural, photochemical and photophysical properties of a model compound, consisting of coumarin and 3H-indolium heterocycles separated by a [C-C[double bond, length as m-dash]C-C] bridge, were characterized with a combination of experimental and theoretical analyses. These studies demonstrate that the fast rotation about the [C-C] bond adjacent to the coumarin component competes with the radiative deactivation of the excited state in nonviscous environments. This geometrical change dislodges the coumarin and 3H-indolium cations out of planarity to allow the population of a weakly-emissive twisted intramolecular charge-transfer (TICT) state and produce fluorescence with low quantum yield. In viscous environments, the conformational change is slow and cannot compete effectively with the radiative deactivation of the excited state, which instead produces fluorescence with high quantum yield. These results indicate that structural modifications aimed at the restriction of the rotation of this [C-C] bond are essential to improve considerably the fluorescence quantum yield of this chromophoric platform. Should a synthetic strategy for the implementation of these design guidelines be identified, activatable fluorophores, based on the 3H-indolium platform, with improved brightness will ultimately emerge.

A velocity map imaging study of the photodissociation of the methyl iodide cation.

The photodissociation dynamics of the methyl iodide cation has been studied using the velocity map imaging technique. A first laser pulse is used to ionize methyl iodide via a (2 + 1) REMPI scheme through the 5pπ → 6p Rydberg state two-photon transition. The produced CH3I+(X[combining tilde]2E3/2) ions are subsequently excited at several wavelengths between 242 and 260 nm. The reported translational energy distributions for the methyl and iodine ions present a Boltzmann-type unstructured distribution at low excitation energies as well as a recoiled narrow structure at higher excitation energies highlighting two different dissociation processes. High level ab initio calculations have been performed in order to obtain a deeper understanding of the photodissociation dynamics of the CH3I+ ion. Direct dissociation on a repulsive state from the manifold of states representing the B[combining tilde] excited state leads to CH3+(X[combining tilde]1A1') + I*(2P1/2), while the CH3 + I+(3P2) channel is populated through an avoided crossing outside the Franck-Condon region. In contrast, an indirect process involving the transfer of energy from highly excited electronic states to the ground state of the ion is responsible for the observed Boltzmann-type distributions.

Molecular dynamics of 1-ethyl-3-methylimidazolium triflate ionic liquid studied by 1H and 19F nuclear magnetic resonances.

The molecular dynamics of an ionic liquid (IL) composed of a 1-ethyl-3-methylimidazolium cation and a triflate (trifluoromethanesulfonate) anion, abbreviated as [Emim][TfO], were studied by NMR spectroscopy. By measuring the temperature-dependent high-field 1H and 19F spin-lattice relaxation (SLR) rates, the frequency-dependent 1H and 19F SLR dispersion curves using fast-field-cycling relaxometry, and the temperature-dependent 1H and 19F diffusion constants, and by utilizing the fact that the primary NMR-active nucleus on the Emim cation is 1H, whereas on the TfO anion it is 19F, the cationic and anionic dynamics were studied separately. A single theoretical relaxation model successfully reproduced all the experimental data of both types of resonant nuclei by fitting all the data simultaneously with the same set of fit parameters. Upon cooling, [Emim][TfO] exhibited a supercooled liquid phase between TSL = 256 K and the crystallization temperature TCr ≈ 227-222 K, as confirmed by differential scanning calorimetry (DSC) experiments. Theoretical analysis revealed that within the liquid and the supercooled liquid states of [Emim][TfO], the 1H and 19F relaxation rates are affected by both the rotational and translational diffusional processes with no discontinuous change at TSL. While the rotational diffusion is well described as an Arrhenius thermally activated process, the translational diffusion undergoes strong freezing dynamics that are well described by the Vogel-Fulcher model assuming a freezing temperature of T0 = 157 K. The existence of the supercooled liquid region in the [Emim][TfO] IL should be taken into account when using this IL for a specific application.

Nanostructuration of ionic liquids: impact on the cation mobility. A multi-scale study.

When probed at the macroscopic scale, Ionic Liquids (ILs) behave as highly dissociated (i.e. strong) electrolytes while, at the molecular scale, they show clear characteristics of weak ionic solutions. The multi-scale analysis we report in this paper reconciles these apparently at odds behaviors. We investigate by quasi-elastic neutron scattering (QENS) and neutron spin-echo (NSE), the nanometer/nanosecond dynamics of OMIM-BF4, an imidazolium-based IL showing strong nanostructuration. We also probe the same IL on the microscopic (μm and ms) scale by pulsed field gradient NMR. To interpret the neutron data, we introduce a new physical model to account for the dynamics of the side-chains and for the diffusion of the whole molecule. This model describes the observables over the whole and unprecedented investigated spatial ([0.15-1.65] Å-1) and time ([0.5-2000] ps) ranges. We arrive at a coherent and unified structural/dynamical description of the local cation dynamics: a localized motion within the IL nanometric domains is combined with a genuine long-range translational motion. The QENS, NSE and NMR experiments describe the same long-range translational process, but probed at different scales. The associated diffusion coefficients are more than one order of magnitude different. We show how this apparent discrepancy is a manifestation of the IL nanostructuration.

Research of CO2 and N2 Adsorption Behavior in K-Illite Slit Pores by GCMC Method.

Understanding the adsorption mechanisms of CO2 and N2 in illite, one of the main components of clay in shale, is important to improve the precision of the shale gas exploration and development. We investigated the adsorption mechanisms of CO2 and N2 in K-illite with varying pore sizes at the temperature of 333, 363 and 393 K over a broad range of pressures up to 30 MPa using the grand canonical Monte Carlo (GCMC) simulation method. The simulation system is proved to be reasonable and suitable through the discussion of the impact of cation dynamics and pore wall thickness. The simulation results of the excess adsorption amount, expressed per unit surface area of illite, is in general consistency with published experimental results. It is found that the sorption potential overlaps in micropores, leading to a decreasing excess adsorption amount with the increase of pore size at low pressure, and a reverse trend at high pressure. The excess adsorption amount increases with increasing pressure to a maximum and then decreases with further increase in the pressure, and the decreasing amount is found to increase with the increasing pore size. For pores with size greater larger than 2 nm, the overlap effect disappears.

Tracking ultrafast dynamics of n-propylbenzene cations by delayed photofragmentation and photoelectron spectroscopy

Ultrafast relaxation dynamics of n-propylbenzene (n-PB) cation were studied by delayed photofragmentation and photoelectron spectroscopy. Multiphoton ionization (MPI) by the fundamental pulse at ∼800nm was used to prepare the n-PB cations. The photoelectron image for MPI was captured and characterized by the photoelectron imaging technique, and the observed photoelectron bands were assigned to the corresponding cationic states. The ultrafast relaxation dynamics of these cation states were tracked by monitoring the time-dependent parent-ion (C6H5C3H7+) depletion and the fragment-ion (C6H5CH2+) appearance. An ultrafast relaxation time of 1.1 (±0.1) ps for the cations was obtained, which was ascribed to a dissipative intramolecular vibrational redistribution process within the ground D0 state.

Unexpected Cation Dynamics in the Low-Temperature Phase of Methylammonium Lead Iodide: The Need for Improved Models.

High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the Pnma structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy.

Dynamic Phenomena at Perovskite/Electron-Selective Contact Interface as Interpreted from Photovoltage Decays

Summary Drastic changes in open-circuit voltage decay (OCVD) response time in CH 3 NH 3 PbX 3 perovskites have been systematically investigated in order to elucidate the dynamic properties of the interface. Under pre-illumination treatment, the decay times are reduced by orders of magnitude, but if left to rest for sufficient time, the solar cell evolves to its original decay kinetics. In order to explain these observations, we developed advanced modeling of the perovskite solar cell to obtain a realistic description of the immediate vicinity of the interface, including ionic variable concentration and accumulation of holes via degenerate statistics in the space charge region. The results reveal a large amount of majority carriers at the minority carrier extraction contact, assisted by additional ionic charge. The surface band bending related to accumulation gives an electrostatic contribution to the photovoltage in a manner governed by slow dynamics of cations at the electron-selective contact. The modeling of the interface allows us to describe the dynamics of the contact region dominated by surface charging and recombination. These phenomena also play an important role in operation conditions and current-voltage scans of the solar cell.

Hillslope soil water flowpaths and the dynamics of roadside soil cation pools influenced by road deicers

AbstractOver the past 60 years, road deicers (i.e. road salt) have been applied to roadways in high latitudes to improve road conditions in winter weather. However, the dissolution of road deicers in highway runoff creates waters with high concentrations of sodium, which can mobilize soil metals via soil cation‐exchange reactions. While several studies have detailed the interactions of road salt‐rich solutions and surface and ground waters, less attention has been given to how local hydrologic flowpaths can impact the delivery of these solutions to near‐road soils. Between 2013 and 2014, soil water samples were collected from a roadside transect of lysimeter nests in Pittsburgh, Pennsylvania (USA). Soil water samples were analysed for metal concentrations and resulting data used to examine cation dynamics. While patterns in soil water calcium and magnesium concentrations follow patterns in soil water sodium concentrations, additional processes influence patterns in soil water potassium concentrations. Specifically, we observe the highest calcium and magnesium concentrations in the deepest lysimeters, suggesting divalent cations are mobilized to, and potentially accumulate in, deeper soil horizons. In contrast, soil water potassium concentrations do not follow this pattern. Additionally, in all examined elements (Ca, Mg, K, Na, and Cl), the timing of concentration peaks appears be influenced by a combination of both distance from the roadside and sampling depth. These relationships not only suggest that multiple soil water flowpaths interact with our study transect but also confirm that road salt plumes persist and migrate following the road salting season. Characterizing the interactions of sodium‐rich solutions and roadside soil cation pools clarifies our understanding of metal dynamics in the roadside environment. A deeper understanding of these processes is necessary to effectively restore and manage watersheds as high total dissolved solid solutions (e.g. road deicing melt, unconventional natural gas brines, and marginal irrigation water) continue to influence hydrological systems. Copyright © 2016 John Wiley & Sons, Ltd.

Vibrational Response of Methylammonium Lead Iodide: From Cation Dynamics to Phonon-Phonon Interactions.

The dynamic evolution of the vibrational interactions in the prototypical CH3 NH3 PbI3 was studied through a comprehensive experimental and theoretical investigation with a focus on the interactions between the organic cations and the inorganic cage. To date, no clear picture has emerged on the critical and fundamental interactions between the two perovskite components, despite the relevance of phonons to the electronic properties of several classes of perovskites. For the first time, we have monitored the IR and nonresonant Raman response in the broad frequency range 30-3400 cm-1 and in the temperature interval 80-360 K. Strong changes in the energies of different vibrational modes with temperature are observed and examined in the framework of phonon-phonon interactions considering a significant anharmonic contribution to the phonon relaxation process. The vibrational relaxation of the bending modes and their reorientation activation energies identify that such mechanisms are governed by medium-to-strong hydrogen bonds in the orthorhombic phase; however, any ferroelectric ordering in the orthorhombic phase is governed mostly by dipole interactions. These changes imply that charge localization mechanisms play a primary role, and our study enriches the fundamental knowledge of phonon interactions and charge transport in CH3 NH3 PbI3 for the further development of optoelectronic applications.

Elastic Constants, Optical Phonons, and Molecular Relaxations in the High Temperature Plastic Phase of the CH3NH3PbBr3 Hybrid Perovskite.

Low frequency dynamics has been studied in a CH3NH3PbBr3 hybrid perovskite single crystal by using four different spectroscopy techniques: coherent inelastic neutron, Raman and Brillouin scatterings, and ultrasound measurements. Sound velocities were measured over five decades in energy to yield the complete set of elastic constants in a hybrid halide perovskite crystal in the pseudocubic plastic phase. The C44 shear elastic constant is very small, leading to a particularly low resistance to shear stress. Brillouin scattering has been used to study the relaxation dynamics of methylammonium cations and to evidence translation-rotation coupling associated with the cubic to tetragonal phase transition at Tc ≈ 230 K. Low frequency and highly damped optical phonons observed using both Raman and inelastic neutron below 18 meV, do not present softening close to Tc. The critical dynamics at Tc ≈ 230 K is compatible with an order-disorder character, dominated by relaxational motions of the molecules.

Understanding Magnesiation Mechanism and Cation Dynamics in Transition Metal Oxides for Magnesium-Ion Batteries

Elucidation of positive chemical and electrochemical magnesiation and favorable cation dynamics has been an ongoing challenge in rechargeable multivalent-ion battery materials. The lack of experimental methodologies to study favorable cation dynamics and positive intercalation in multivalent-ion batteries limits both theoretical approaches to confirm materials of interest and experimental materials screening processes. Despite major methodological challenges working with multivalent-ion nuclei (M= 25Mg, 67Zn, 43Ca etc.) with solid state NMR spectroscopy (one of the few methods that can directly probe cation dynamics in solids), an experimental approach to study favorable and non-favorable ion dynamics in bulk MgMn2O4 has been developed. Although the findings that will be reporting are orders of magnitude slower than Li-ion battery cathode analogs the results are in some agreement with theoretical predictions (for ex. ~200-300 meV difference predicted between Li+ and Mg+2 in Mn-spinels) and experimentally show, for the first time, favorable and controllable structure-activity relationships in high voltage multivalent-ion battery materials. Furthermore, distinct differences in magnesiation mechanisms are observed with spectroscopic methods for alpha-V2O5 and xerogel-V2O5, former involving only protonic activity and the latter via a magnesium/solvent co-intercalation1,2. This mechanism will be compared with the traditional intercalation mechanism observed for MoO2.8F0.2. 3