Browse Articles

Concentrated aqueous lithium chloride solution dynamics: The role of chemical exchange on anisotropy and vibrational population relaxations

Ultrafast polarization-selective pump–probe experiments, conducted on the OD stretch of dilute HOD, are reported for LiCl/H2O solutions ranging from 1–24 to 1–128 (ion pairs–water molecules), 2.3–0.4 m. The results are compared to prior and revised experiments on 1–4 to 1–16 concentrations, 13.9–3.5 m. Vibrational population relaxation and anisotropy decays were measured for hydroxyls hydrogen-bonded to chlorides (HBCs). In contrast to higher salt concentrations, at ≤∼1–32 (1.7 m salt), the HBC population relaxation times and anisotropy decays are concentration independent. 1–32 marks a transition from high concentrations of ion pairs, clusters, and ion networks to concentrations of ion pairs low enough not to affect observable molecular level dynamics. At a concentration of approximately 1–32 and lower salt concentrations, chemical exchange is responsible for HBC anisotropy decay and plays a role in population relaxation. Wavelength-dependent population relaxation was used to obtain lifetime amplitude spectra (LAS), which show distinct species that are not observable with FT-IR. At very high salt concentrations, e.g., 1–6, there are no “pure” water regions, and the LAS has two bands: HBCs and hydroxyls of water oxygens solvating Li+. At lower salt concentrations, there is also a “pure” water band in the LAS. The HBC band shape is concentration independent from 1–4 to 1–128.

1H-detected measurement of spin relaxation constants in ultrawide solid-state NMR line shapes via progressive saturation of the proton reservoir

Nuclear magnetic resonance relaxation time constants provide valuable insights into the dynamic processes and structure of a system. However, determination of these relaxation parameters is often challenging for low-γ nuclides, which exhibit low sensitivity and are typically subject to large anisotropic interactions. Here, we introduce two new pulse sequences, PROSPR-T1 and PROSPR-T2, for measuring longitudinal and transverse relaxation time constants in solids. These sequences are modifications of the PROgressive Saturation of the Proton Reservoir (PROSPR) experiment, developed to facilitate the detection of insensitive nuclides. Following an initial analysis of the adiabatic requirements of the PROSPR experiment, the performance of PROSPR to measure T1/T2′ is thoroughly evaluated on a range of spins, including 119Sn (I = 1/2), 35Cl (I = 3/2), and 14N (I = 1). Experimental results demonstrate that optimized versions of PROSPR can be up to an order of magnitude more sensitive than their original implementation. These new methods offer accurate relaxation time constants, even for spectra spanning over 1 MHz, without demanding frequency stepping throughout the powder pattern.

Solvation governs cation transference in glyme-based lithium battery electrolytes

The efficacy of electrochemical systems is governed by the cation transference number, which represents the fraction of current carried by the working ion. Energy is wasted when field-induced motion also drives anions and solvent molecules, decreasing the transference number to near-zero. We present a systematic study of cation transference in a series of electrolytes: tetraglyme (TG), octaglyme (OG), and poly(ethylene oxide) (PEO) mixed with lithium bis(trifluoromethanesulfonyl)imide. In all three electrolytes, starting from the dilute salt concentration limit, the experimentally measured cation transference number decreases with increasing concentration, reaching a minimum between −0.1 and −0.2, before rising back to positive values. Explicit measurements of field-induced species’ velocities by electrophoretic nuclear magnetic resonance indicate that negative cation transference numbers in TG and OG electrolytes are dictated by solvation interactions with minimal contribution from anion-cation interactions. Simulation-based solvation structures indicate that OG serves as a bridge between TG and PEO. Multi-charge positive clusters, which are negligible in TG, become increasingly important at higher chain lengths (OG and PEO). As migrating cations drag their solvation shells, this solvation-induced motion is amplified in glyme electrolytes because of covalent interactions between solvating glyme molecules and free glyme molecules.

Topological insights into dense frictional suspension rheology: Third-order loops drive discontinuous shear thickening

Dense suspensions exhibit a significant change in viscosity under external deformation, a phenomenon known as shear thickening. Recent studies have identified a stress-induced transition from lubricated, unconstrained interactions to frictional contacts, which play a crucial role in shear thickening. This work investigates the rheological behavior and frictional contact network evolution during continuous and discontinuous shear thickening (DST) in two-dimensional simulations. We find that at low stress, during weak thickening, the frictional contact network is composed of quasilinear chains along the compression axis. With increasing stress, the unweighted contact network becomes more isotropic and forms loop-like structures. We show that third-order loops within the frictional contact network are key to the DST. Our findings revealed a strong correlation between the number of third-order loops and the viscosity of the suspension. Notably, this relationship remains independent of the packing fraction, applied stress, and interparticle friction, highlighting the fundamental role of the mesoscale network topology in governing macroscopic rheology and connecting to the microscopic physics.

Erratum: “Quantum corrections to the kinetic energy and the <i>ab initio</i>-based prediction of the thermodynamic properties and vapor–liquid equilibria of hydrogen” [J. Chem. Phys. 162, 124502 (2025)]

Ultrafast dynamics of transient exciton state in methylammonium lead iodide perovskite at room temperature

Metal-halide perovskites are promising materials for photovoltaics and other optoelectronic applications. Understanding their photophysical properties, especially the energy landscape and dynamics of photoexcited carriers, is essential. This work focuses on studying ultrafast dynamics of photoexcited carriers in a methylammonium lead iodide (MAPI) perovskite thin film at room temperature. We first performed steady-state spectroscopic measurements to characterize the sample and then implemented optical 2D coherent spectroscopy in the non-collinear geometry to probe the ultrafast dynamics of photoexcited carriers. A series of rephasing one-quantum 2D spectra at different waiting times revealed that a transient exciton resonance coexists with the free-carrier resonance for hundreds of femtoseconds before the excitons dissociate into free carriers. The free-carrier resonance persists for a significantly longer time, up to at least 1 ns, and exhibits the signs of spectral diffusion. We then characterized the spectral diffusion by calculating the frequency–frequency correlation function. The acquired 2D spectra revealed unique transient dynamics in MAPI perovskites that might be difficult to probe with one-dimensional techniques.

Top–down optimization of aqueous electrolyte force fields to model chemical potentials and solubilities

Electrolyte solutions are vital to the development of technologies such as batteries and carbon sequestration methods. Accurate but efficient simulation models are crucial in guiding the development of such technologies. In this work, we investigate how the inclusion of atomic polarizability, by means of Drude oscillators, affects the ability of efficient, classical force fields to model the temperature dependence of the aqueous solubility and activity coefficients of alkali-halide salts. To achieve this, we propose a new method to efficiently and accurately compute derivatives of the salt chemical potential with respect to force field parameters, enabling gradient-based fitting directly to chemical potential data. Using this method, we attempt to refine polarizable models to better predict solid–solution solubility limits as functions of temperature. We find that while solubility predictions can be improved, polarizable models are incapable of reproducing the slope of the solid–solution coexistence line for NaCl. This implies that classical polarizability (with constant atomic charges) alone is an insufficient description of system many-body interactions, and “first-principles” descriptions of the energy landscape are necessary to achieve true predictive power in electrolyte modeling.

Low-frequency Raman spectra of amyloid fibrils

We report on how low-frequency Raman measurements can be used as a facile tool to investigate the supramolecular structure of amyloid fibrils. We investigate the low-frequency Raman spectra (&amp;lt;500 cm−1) of six different amyloid fibrils exhibiting parallel β-sheet structures prepared from amyloid-β1–40, amylin, amyloid-β25–35, and amylin20–29 peptides. We propose band assignments using a combination of semi-empirical tight-binding calculations and insights gleaned from previously published studies on model polypeptides in β-sheet conformations. We discuss how low-frequency Raman modes can be used to probe the interactions, packing, and ordering of strands and side chains within fibril β-sheets to gain insights into their supramolecular structures.

Cluster perturbation theory. XII. Parallel implementation of variational excitation energy series for the coupled cluster singles and doubles model

An efficient implementation of the variational cluster perturbation excitation energy series through fifth order is described. The series has the coupled cluster singles excitation energies as zeroth order and targets the coupled cluster singles and doubles (CCSD) excitation energies. The implementation utilizes the resolution of the identity approximation for the two-electron integrals. The perturbation series uses the 2n + 1/2n + 2 rules to eliminate all parameters higher than second order. The computational efficiency is illustrated by performing excitation energy calculations on a single node for systems with up to 1000 basis functions and comparing with CCSD calculations. Using the hybrid parallel open multiprocessing/message-passing interface implementation with graphics processing unit offloading, fifth-order cluster perturbation excitation energies for systems with 1750 basis functions can be calculated in 8 h using 20 nodes on the Frontier supercomputer at the Oak Ridge National Laboratory. A benchmark of 131 excitation energies for a diverse set of molecules is presented. For the fourth and fifth order models, a few prominent outliers are present. We demonstrate how these cases may be identified using a simple diagnostic, with the remaining values being indistinguishable from CCSD excitation energies in practice.

Cascade at local yield strain for silica and metallic glass

We report observations of unusual first plastic events in silica and metallic glasses in the shear startup regime at applied strain two orders of magnitude smaller than yield strain. The (non-affine) particle displacement fields during these events have complex real space structure with multiple disconnected cores of high displacement appearing at the same applied strain under athermal quasistatic simple shear deformation and identified by using a “cell based cluster analysis” method. By monitoring the stress relaxation during the first plastic event by Langevin dynamics simulation, we directly show the cascade nature of these events. Thus, these first plastic events are reminiscent of avalanches in the post-yielding steady state, but unlike the steady state avalanches, we show that these events are not system spanning. To understand the nature of these events, we tune three factors that are known to affect brittleness of a glass. These are (i) sample preparation history, (ii) inter-particle interactions, and (iii) rigidity of the background matrix applying a “soft matrix” probe recently developed by some of us. In each case, we show that such first plastic events are more probable in more ductile glasses. Our observations are consistent with the picture that more ductile materials are softer, implying that understanding the role of softness may be a promising route to develop microscopic quantifiers of brittleness and thus clarifying the physical origin of brittle-to-ductile transition.

Microwave-free nuclear spin hyperpolarization through photo-CIDNP in static and rotating solids

This study advances the theoretical foundation of photo-chemically induced dynamic nuclear polarization (photo-CIDNP)—a powerful mechanism for enhancing nuclear spin sensitivity without microwave irradiation. Using an operator-based effective Hamiltonian approach, we derive precise resonance matching conditions and identify key dipolar scaling factors governing the photo-CIDNP Hamiltonian under both static and magic-angle spinning conditions. Our analytical formulation of coherent evolution of photoexcited singlet state exhibits strong agreement with numerical simulations, reinforcing the validity of our theoretical framework. By unraveling the intricate interplay of spin parameters in the radical-pair mechanism, our findings provide critical insights into optimizing photo-CIDNP efficiency and guiding the rational design of tailored molecular systems. The ability to develop highly efficient photo-CIDNP sensitizers marks a crucial step toward harnessing hyperpolarized nuclear magnetic resonance and magnetic resonance imaging, paving the way for next-generation advancements in biomedical imaging and materials science.

Energy dissipation in ensembles of catalytic Janus particles

The conversion of chemical energy into mechanical energy, which drives the motion of active particles, inherently involves energy dissipation. Dissipation plays a crucial role in transport efficiency, structure formation in self-organizing systems, and the thermodynamic properties of active particle suspensions. In this work, we present a thermodynamic analysis that derives the energy dissipation of coupled irreversible processes occurring in particles, substrate, and solvent. Dissipation in chemical reactions is examined under conditions where the reaction flux follows a nonlinear dependence on affinity, as described by the law of mass action. Our approach considers concentration-dependent reaction rates, in contrast to some previous descriptions of active particles, which assume a constant reaction rate and, consequently, a constant active velocity of the particles. We analyze entropy production for both cases, highlighting significant discrepancies and demonstrating that assuming a constant active velocity overlooks key thermodynamic contributions. Our framework provides a more accurate and self-consistent characterization of entropy production, capturing the inherent nonlinearities of active particle dynamics.

Fundamental bounds on many-body spin cluster intensities

Multiple-quantum coherence (MQC) spectroscopy is a powerful technique for probing spin clusters, offering insights into diverse materials and quantum many-body systems. However, prior experiments have revealed a rapid decay in MQC intensities as the coherence order increases, restricting observable cluster sizes to the square root of the total system size. In this work, we establish fundamental bounds on observable MQC intensities in the thermodynamic limit (N ≫ 1) outside the weak polarization region. We identify a sharp decay in the observable MQC intensities as the coherence order grows. This transition regime fragments the state space into two components consisting of observable and unobservable multiple-quantum coherences. Notably, we find that the center of the transition region is directly proportional to size N and polarization p of the system, suggesting that the aforementioned square root limitation can be overcome through hyperpolarization techniques. Our results provide important experimental guidelines for the selective observation of large spin cluster phenomena.

Contact mechanics for layered materials: Rubber film on hard substrate

I consider the contact mechanics for a layered material, consisting of an elastically soft film glued to a hard substrate. I calculate the area of real contact for surfaces with fractal-like roughness and for surfaces with roughness in narrow length scale regions. For the fractal-like surfaces, when the product q0d of the film thickness d and the low cut-off wavenumber q0 of the surface roughness power spectrum satisfy q0d &amp;lt; 0.1, the effective modulus becomes very large. This results in large contact stresses, which can induce plastic deformation or wear, in particular during sliding contact. I also calculate the probability distributions of the normal and tangential stresses at the film–substrate interface. If the tangential (shear) stress is too high, the adhesive bond between the film and the substrate will break. I compare the thin-film contact mechanics problem with the Gent solution for a thin elastic sheet confined between two flat solid surfaces, and discuss the origin of the difference in effective elastic modulus.

The bond capacity electronegativity equilibration charge model (EEQBC) for the elements <i>Z</i> = 1–103

The accurate and efficient assignment of atomic partial charges is crucial for many applications in theoretical and computational chemistry, including polarizable force fields, dispersion corrections, and charge-dependent basis sets. Classical charge models struggle to distinguish between neutral and zwitterionic fragments because, unlike quantum mechanical methods, there are no discrete electronic states. This limitation can lead to either reduced or additional artificial charge transfer (CT) at different interfragment distances. To address this issue, we propose a new version of a bond capacity electronegativity equilibration (EEQBC) model, which limits artificial CT between distant fragments in the simple EEQ framework. EEQBC offers excellent agreement with DFT-based reference charges for elements up to lawrencium (Z = 103) with mean absolute errors as low as 0.02 and 0.07 e− for random PubChem molecules and “mindless” molecules (MLMs), respectively. Thanks to its computational efficiency for both atomic charges and their analytical nuclear gradients, EEQBC is highly suitable as an initial charge guess for next-generation tight-binding methods. For seamless accessibility, EEQBC is implemented in the upcoming 0.5.0 release of the freely available multicharge program at github.com/grimme-lab/multicharge.

Solvent-induced transformations of assemblies of structured ionizable block co-polymers

Micelles formed by ionizable co-polymers are governed by both van der Waals and electrostatic forces. Slight tweaking of the solvent characteristics often drives large structural transformations. Here, the effects of modulating the electrostatic characteristics of solvents on polymeric assemblies formed by a co-polymer with ABCBA topology and an ionizable polystyrene sulfonate center (C) tethered to polyethylene propylene (B) end-capped by t-butyl styrene (A) are probed by small angle neutron scattering in cyclohexane/propanol solutions. With increasing propanol fraction, the spherical core–shell micelles, with the ionic block in their core, elongate and transition into large “swarms.” Surprisingly, as propanol becomes the major component of the solvent, reentrant spherical assemblies with a smaller polystyrene sulfonate core are formed. The propanol partitions across the interface between the core and the corona, affecting the polymer distribution in the corona and the core-corona boundary’s curvature, resulting in morphing of the shape of assemblies.

Characteristics of photoemission from radiative and sub-radiative localized surface plasmons in metal nanostructures

Revealing the mechanism of photoemission from plasmonic nanostructures that supports different localized surface plasmon modes is crucial for designing new ultrafast photoelectric cathodes, as well as for enhancing localized surface plasmon (LSP)-based photocatalysis, energy harvest, and photoluminescence, but the investigation on this topic is still lacking. In this paper, we directly investigated the photoemission yield and photoemission mechanism from a sub-radiative Fano mode in an asymmetric nanorod dimer, a radiative dipole mode in an isolated nanorod, and a radiative coupled dipole mode in a symmetric nanorod dimer, respectively, using time-of-flight photoemission electron microscopy. We found that the photoemission yield from the sub-radiative Fano mode is almost equal to that of the radiative dipole mode but more than four times higher than that of the radiative coupled dipole mode case. We reproduced the physical process using a two temperature model and Fowler–Dubridge theory and demonstrated that the thermal effects of the electron gas play an influential role in photoemission from LSP. Interestingly, it is found that the sub-radiative nanorod dimer, although exhibiting a much lower electron temperature, maintains a similar photoemission yield to that of the radiative dipole mode, and this feature of the sub-radiative nanorod dimer makes it potentially a high-brightness electron source with strong robustness. The demonstrated results in this work help to understand the LSP-assisted photoemission process in plasmonic nanostructures, which lays the foundation for designing new ultrafast photoelectric cathodes and many other applications.

Statistical mechanics of homologous pairing of long double-stranded DNA

The ability of homologous dsDNA to recognize and attract each other is a fundamental feature in DNA recombination and repair. A major unresolved question is how homologous genes initially locate and position themselves in front of each other—whether they do so at a distance without the use of proteins or unzipping their strands. One hypothesis suggests that such recognition is an innate property of DNA’s structure. DNA is not a perfect double-helix and distortions from helical structure are correlated with the sequence of base pairs. These distortions influence the patterns of charge distribution along the molecules. Those with identical sequences exhibit matching patterns of distortion, allowing them to align in a one-to-one register, which facilitates more favorable interactions. Conversely, uncorrelated sequences are unlikely to align perfectly, resulting in weaker attraction or greater repulsion. Consequently, the pairing of homologous sequences (i.e., positioning homologous genes in front of each other at a distance corresponding to free energy minimum) is more favorable than that of heterologous sequences. But how complete and stable would the pairing be? To address this, we present a model mapped on an Ising-like framework, which provides insight into the extent of pairing and its robustness. Our findings suggest that homologous dsDNAs, spanning multiple Kuhn lengths, can pair with some “bubbles”—regions of less tightly coupled sections. We compute the fraction of these unpaired sections and analyze the stability conditions to demonstrate that under physiological salt concentrations, heterologous double-stranded DNA cannot sustain long paired segments beyond the helical coherence length, further supporting the advantage of homologous pairing.

Pure spin current modulation via Fano resonance in a copper-coordinated single-molecule junction

The generation of pure spin current is a highly sought-after objective in spintronics as it holds significant promise for enabling high-performance spintronic devices with low power consumption and high circuit integration density. However, achieving pure spin current at the single-molecule level remains a formidable challenge. In this study, we demonstrate a novel approach to generate pure spin current through spin Fano resonance in a p-Ben-CuII molecule. Our results reveal that the spin-down Fano resonance in p-Ben-CuII is associated with the β-HOMO, while the spin-up Fano resonance originates from the α-HOMO. Furthermore, by precisely tuning the chemical potential to align with the resonance peak, a well-defined pure spin current can be achieved under specific chemical potential or temperature conditions. This strategy offers a promising pathway for the precise manipulation of pure spin current in single-molecule junctions, paving the way for advanced spintronic applications.

Experimental and theoretical investigations on the vibrational and electronic spectra of 1,3-dibromobenzene

We report here a comprehensive spectroscopic study of the vibrational and electronic spectra of 1,3 dibromobenzene. The vibrational spectrum is studied using Fourier transform infrared and Raman techniques and analyzed using density functional theory (DFT) calculations incorporating anharmonicity effects via the second-order vibrational perturbation theory method, resulting in a set of consistent assignments for all the fundamentals and several overtone and combination bands. The electronic spectrum is studied using synchrotron radiation based photoabsorption spectroscopy spanning the spectral region 1150–3000 Å (86 956–33 333 cm−1), for which the spectrum in the 1150–1700 Å (86 956–58 823 cm−1) region is reported here for the first time. The electronic absorption spectrum is richly structured and comprises valence, Rydberg, and charge transfer excitations, along with distinct vibronic features. Quantum defect analysis is used to assign Rydberg series converging to the first four IPs of 1,3 dibromobenzene, while charge transfer and valence transitions are assigned using theoretical calculations at the TDDFT/CAMB3LYP/aug-cc-pVTZ level. The extensive vibrational bands accompanying the first valence transition are analyzed and assigned using Franck–Condon factor calculations incorporating the Herzberg–Teller effect. In addition, time dependent DFT studies of excited state potential energy curves yield some new insights into the role of internal conversions and intersystem crossings in the UV photodissociation dynamics of 1,3 dibromobenzene.