Investigation of the Failure of Marcus Theory for Hydrated Electron Reactions.

Reactions of the hydrated electron with a wide variety of substrates have been found to exhibit unusually similar activation energies in a manner incompatible with Marcus electron transfer theory. Given the fundamental linear response assumption of Marcus theory, one possible explanation for this apparent failure is that the underlying free energy surfaces governing the reactions are not harmonic; i.e., hydrated electron structural fluctuations exhibit non-Gaussian behavior. In this work, we test this hypothesis by using simulations to calculate the hydrated electron vertical detachment energy distribution. We consider both cavity and noncavity models for the hydrated electron, between which the actual hydrated electron behavior is expected to lie. Our results identify a possible origin for non-Gaussian behavior of the hydrated electron but show that it is not of sufficient magnitude to explain the failure of Marcus theory to describe its reactions. Thus, other explanations must be sought.

Role of magnetization on catalytic pathways of non-oxidative methane activation on neutral iron carbide clusters.

Methane has emerged as a promising fuel due to its abundance and clean combustion properties. It is also a raw material for various value-added chemicals. However, the conversion of methane to other chemicals such as olefins, aromatics, and hydrocarbons is a difficult task. In recent years, ionic iron carbide clusters have been explored as potential catalysts for efficient direct methane conversion. Herein, we have investigated the gas-phase methane conversion process on various neutral iron carbide clusters with different Fe:C ratios using density functional theory. Reaction pathways were studied on mononuclear and trinuclear iron carbide clusters in the three lowest energy spin multiplicity channels. Three descriptors - methane binding energy, the effective energy barrier for C-H bond activation, and the effective energy required for methyl radical evolution - were chosen to identify the best catalyst among the clusters considered. Isomers of Fe3C6 (Fe3C6-iso) and Fe3C9 (Fe3C9-iso) are recognized as being the most promising catalysts among all the clusters considered here because they require the least methyl radical evolution energy, a step that is crucial in methane conversion to higher hydrocarbon but also requires the most energy.

Temperature Dependence of Peptide Conformational Equilibria from Simulations at a Single Temperature.

Understanding the structure of proteins is key to unraveling their function in biological processes. Thus, significant attention has been paid to the calculation of conformational free energies. In this paper, we demonstrate a simple extension of fluctuation theory that permits the calculation of the temperature derivative of the conformational free energy, and hence the internal energy and entropy, from single-temperature simulations. The method further enables the decomposition into the contribution of different interactions present in the system to the internal energy surface. We illustrate the method for the canonical test system of alanine dipeptide in aqueous solution, for which we examine the free energy as a function of two dihedral angles. This system, like many, is most effectively treated using accelerated sampling methods and we show how the present approach is compatible with an important class of these, those that introduce a bias potential, by implementing it within metadynamics.

Water plays a dynamical role in a hydrogen-bonded, hexameric supramolecular assembly.

The hexameric resorcin[4]arene supramolecular assembly has attracted significant interest as a self-assembled capsule that exhibits dynamic host-guest chemistry. Many studies have been carried out to investigate the structure and thermodynamics of the assembly, but considerably less is known about its dynamical properties. Here, molecular dynamics simulations are used to investigate the timescales of water encapsulation in this assembly in wet chloroform. We have previously shown [A. Katiyar et al., Chem. Commun. 2019, 55, 6591-6594] that at low water content there are three distinct populations of water molecules present, while at higher water content an additional population, long water chains interacting with the assembly, appears. The relative free energies of these different water positions are calculated and time correlation functions are used to determine the timescales for interconversion between the populations. This analysis demonstrates that the water molecules are in rapid exchange with each other on timescales of tens of ps to a few ns, and suggests that water molecules might be acting as a critical component in the guest exchange mechanism.

Water plays a diverse role in a hydrogen-bonded, hexameric supramolecular assembly.

The interactions between water and a hexameric resorcin[4]arene assembly formed in wet chloroform are examined by molecular dynamics simulations of the diffusion coefficients. It is found that the water diffusion coefficients provide a route to understanding the degree of water association with the assembly. The simulated diffusion coefficients are in excellent agreement with prior measurements and the diffusion data are well described by a simple adsorption model. This analysis demonstrates that a significant number of waters are encapsulated within the assembly or hydrogen-bonded to its exterior, consistent with and elucidated by a direct examination of the water molecules in the simulations.