Direct Calculation of Activation Energies and Entropies for Chemical Dynamics

Ward Thompson of the University of Kansas is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry to develop and apply methods for determining how the rates of dynamical processes in chemistry change with temperature. The effect of temperature gives insight into the factors that determine the rate of a chemical transformation.

The newly developed methods will provide a way to decompose these on a molecule-by-molecule basis. The influences of the chemical composition and entropy on chemical timescales will also be examined. The proposed methods will be tested in investigations of dynamics in three chemical systems relevant to materials and biology: Water confined in nanoscale amorphous silica pores, water in the hydration shell of proteins, and water molecules participating in and associating with hydrogen-bonded supramolecular assemblies.

The approaches developed in the proposed work will provide new insight into these problems by addressing important open questions about the energetic and entropic driving forces for the dynamics. The Thompson group will also create new course materials that improve student understanding of, and appreciation for, dynamics in chemistry.

The Thompson group will develop and apply theoretical methods for directly calculating activation energies and entropies for dynamical processes important in chemistry. Methods will be investigated for 1) determining the molecule-by-molecule contributions to the activation energy to yield detailed mechanistic insight into the underlying dynamics; 2) directly determining the composition dependence of dynamical timescales; 3) the rigorous, direct calculation of the activation entropy along with new molecular-level insight.

These approaches are based on evaluating derivatives with respect to temperature (and other thermodynamic variables) of the time correlation functions from which dynamical timescales, such as rate constants or diffusion coefficients, can be obtained. These derivatives are themselves time correlation functions and can be evaluated straightforwardly from the same molecular dynamics simulations.

In this way, an activation energy or entropy that is normally obtained from calculations at multiple temperatures through an Arrhenius analysis can be determined from simulations at a single temperature. A key advantage is that this approach enables a rigorous decomposition of the activation energy or entropy into contributions from the interactions and motions present in the system, providing otherwise unavailable physical insight.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.