arXiv subjects

Ethan Abraham

Publications and source records attributed to Ethan Abraham.

10 recordsLinked to original sources

arXiv2026

Chiral vibrational modes and vibrational circular dichroism

The recent interest in chiral phonons in a variety of physical phenomena and their hypothesized role in the chiral-induced spin selectivity effect [Phys. Rev. Research, 5, L022039 (2023)] call for further investigation into the chirality of molecular vibrations. Although molecular chirality has conventionally been quantified using structural properties, recent work has highlighted the role of dynamical response properties as chirality metrics. In this work, we examine an inter-atom helicity pseudoscalar as a complementary measure of vibrational chirality, associated with the vibrational circular dichroism (VCD) intensity in the fixed partial charge (FPC) approximation. This pseudoscalar is translationally and rotationally invariant, can distinguish between opposite enantiomers, and unlike an atomic pseudoscalar measure considered in our earlier work [Phys. Rev. Lett., 133, 268001 (2024)] does not rely on a predefined symmetry axis. For a twisted ethane model as well as several small molecules, this pseudoscalar correlates well with structural descriptors based on the continuous chirality measure. Overall, our results support response-based metrics as a physically meaningful and practically useful characterization of vibrational chirality. Importantly, while the FPC-based VCD estimate provides a useful quantifier of vibrational chirality, we show that it is a rather poor predictor of the actual molecular VCD response because the latter is strongly influenced by the (vibrational configuration-dependent) molecular electronic response.

physics.chem-ph↗

arXiv2025

A Resolution to the Marcus-Rehm-Weller Paradox in Electron Transfer

Marcus theory famously predicts that electron-transfer rates decrease once the thermodynamic driving force exceeds the reorganization energy. Yet many systems instead exhibit Rehm-Weller kinetics characterized by rate saturation rather than decrease. Here we show that these apparently contradictory phenomenologies emerge as opposite physical limits of the same two-state quantum Hamiltonian. In the normal region, the model recovers both Marcus and Rehm-Weller behavior. In the inverted region, however, it predicts Marcus's decreasing rate in the nonadiabatic limit but Rehm-Weller saturation in the adiabatic limit. Using physically realistic reorganization energies and electronic coupling values, we show that Rehm-Weller's data can be quantitatively reproduced within a microscopic quantum model without invoking phenomenological corrections.

physics.chem-ph↗

arXiv2025

Ab Initio Free Energy Surfaces for Coupled Ion-Electron Transfer

Although coupled ion-electron transfer (CIET) has emerged as a powerful framework to rationalize the kinetics of Faradaic reactions, its mechanism has lacked a fully microscopic, first-principles description. Here we show that the coupling of electron-transfer to ion-transfer can be understood as an extension of Marcus theory in which the ensemble of diabatic nuclear configurations is conditioned on a classical collective variable describing the interfacial anisotropy. This formalism enables direct construction of the CIET free-energy surface from constrained ab initio trajectories, providing a first-principles route to electrochemical current-overpotential relations. We demonstrate this method for CO2 redox on a gold electrode and find that the resulting two-dimensional saddle-point barriers differ substantially from one-dimensional treatments that consider only electron- or ion-transfer coordinates individually.

physics.chem-ph↗

arXiv2025

Effective reorganization energy for electron transfer

The Marcus theory expression for the rate of non-adiabatic electron transfer is widely used across a range of physical conditions. Although Marcus theory defines the reorganization energy classically, here we show that the reorganization parameter appearing in the activation barrier for normal-region electron transfer is most generally a quantum mechanical object that depends on the electronic coupling, coinciding with the Marcus picture only in the limit of vanishing electronic coupling. This result unifies the physical description of electron-transfer activation barriers across the adiabatic and non-adiabatic regimes and formally predicts that Marcus-like rate expressions remain accurate beyond their traditional non-adiabatic domain of validity. These insights allow us to derive a closed-form expression for the curvature of the current-overpotential relation for electron-transfer-limited reactions at the electrochemical interface, now formally applicable to both inner-sphere and outer-sphere processes.

quant-ph↗

arXiv2025

Rectification of Vibrational Energy Transfer in Driven Chiral Molecules

We show that the combination of molecular chirality and phase-controlled driving can lead to rectification of vibrational energy transfer. We demonstrate this effect using classical models of (1) a single helical chain and (2) a more realistic model of polyethylene double helix. We examine the effect of the driving frequency, polarization, and temperature on this phenomenon. Notably, we find that the direction and magnitude of the observed directionality preference depend on the driving frequency and phase, and that the effect persists at room temperature.

physics.chem-ph↗

arXiv2025

Nuclear Angular Momentum Generation in Thermally Driven Chiral Systems

The appearance of angular momentum in the nuclear motion of molecular systems lacking inversion symmetry under imposed thermal gradients presents a novel mechanism with potential implications for spintronics, magnetic response, and energy transport in such systems. Here we explore this phenomenon, using theoretical analysis and numerical simulations to study angular momentum generation in several driven chiral molecular models. We demonstrate that significant vibrational angular momentum can be induced under both mechanical and thermal driving, with magnitude comparable to that induced in optically driven chiral phonons. We find that generation of angular momentum is a common and general phenomenon in driven chiral structures, highlighting the role of symmetry-breaking in the (non-equilibrium) internal atomic motion of such systems.

physics.chem-ph↗

arXiv2025

Chiral Vibrational Modes in Small Molecules

The development of quantitative methods for characterizing molecular chirality can provide an important tool for studying chirality induced phenomena in molecular systems. Significant progress has been made in recent years toward understanding the chirality of molecular normal vibrational modes, mostly focusing on vibrations of helical molecular structures. In the present study, we examine the applicability two methodologies previously used for helical structures for the quantification of the chirality of molecular normal modes across a range of small, not necessarily helical, molecules. The first approach involves the application of the Continuous Chirality Measure (CCM) to each normal mode by associating the mode with a structure formed by imposing the corresponding motion about a common origin. The second approach assigns to each normal mode a pseudoscalar defined as the product of atomic linear and angular momentum summed over all atoms. In particular, using the CCM also as a measure of the chirality of the underlying molecular structure, we establish the existence of correlation between the chirality of molecular normal modes and that of the underlying molecular structure. Furthermore, we find that normal modes associated with different frequency ranges of the molecular vibrational spectrum exhibit distinct handedness behavior.

physics.chem-ph↗

arXiv2024

Molecular chirality quantification: Tools and benchmarks

Molecular chirality has traditionally been viewed as a binary property where a molecule is classified as either chiral or achiral, yet in the recent decades mathematical methods for quantifying chirality have been explored. Here we use toy molecular systems to systematically compare the performance of two state of the art chirality measures (1) the Continuous Chirality Measure (CCM) and (2) the Chirality Characteristic ($\chi$). We find that both methods exhibit qualitatively similar behavior when applied to simple molecular systems such as a four-site molecule or the polymer double-helix, but we show that the CCM may be more suitable for evaluating the chirality of arbitrary molecules or abstract structures such as normal vibrational modes. We discuss a range of considerations for applying these methods to molecular systems in general, and we provide links to user-friendly codes for both methods. We aim this paper to serve as a concise resource for scientists attempting to familiarize themselves with these chirality measures or attempting to implement chirality measures in their own work.

physics.chem-ph↗

arXiv2023

Heat Transport with a Twist

Despite the desirability of polymers for use in many products due to their flexibility, light weight, and durability, their status as thermal insulators has precluded their use in applications where thermal conductors are required. However, recent results suggest that the thermal conductance of polymers can be enhanced and that their heat transport behaviors may be highly sensitive to nanoscale control. Here we use non-equilibrium molecular dynamics (MD) simulations to study the effect of mechanical twist on the steady-state thermal conductance across multi-stranded polyethylene wires. We find that a highly twisted double-helical polyethylene wire can display a thermal conductance up to three times that of its untwisted form, an effect which can be attributed to a structural transition in the strands of the double helix. We also find that in thicker wires composed of many parallel strands, adding just one twist can increase its thermal conductance by over 30%. However, we find that unlike stretching a polymer wire, which causes a monotonic increase in thermal conductance, the effect of twist is highly non-monotonic, and certain amounts of twist can actually decrease the thermal conductance. Finally, we apply the Continuous Chirality Measure (CCM) in an attempt to explore the correlation between heat conductance and chirality. The CCM is found to correlate with twist as expected, but we attribute the observed heat transport behaviors to structural factors other than chirality.

physics.comp-ph↗

arXiv2023

Quantifying the Chirality of Vibrational Modes in Helical Molecular Chains

Chiral phonons have been proposed to be involved in various physical phenomena, yet the chirality of molecular normal modes has not been well defined mathematically. Here we examine two approaches for assigning and quantifying the chirality of molecular normal modes in double-helical molecular wires with various levels of twist. First, associating with each normal mode a structure obtained by imposing the corresponding motion on a common origin, we apply the Continuous Chirality Measure (CCM) to quantitatively assess the relationship between the chirality-weighted normal mode spectrum and the chirality of the underlying molecular structure. We find that increasing the amount of twist in the double helix shifts the mean normal mode CCM to drastically higher values, implying that the chirality of molecular normal modes is strongly correlated with that of the underlying molecular structure. Second, we assign to each normal mode a pseudoscalar defined as the product of atomic linear and angular momentum summed over all atoms, and we analyze the handedness of the normal mode spectrum with respect to this quantity. We find that twisting the double-chain structure introduces asymmetry between right and left-handed normal modes so that in twisted structures different frequency bands are characterized by distinct handedness. This may give rise to global phenomena such as thermal chirality.

cond-mat.mes-hall↗