Direct Calculation of Activation Energies and Entropies for Chemical Dynamics

堪萨斯大学的沃德·汤普森得到了化学系化学理论、模型和计算方法计划的支持，该计划开发和应用方法来确定化学中动态过程的速率如何随温度变化。温度的影响使我们能够洞察决定化学转变速度的因素。新开发的方法将提供一种在分子基础上分解这些物质的方法。还将考察化学成分和熵对化学时间尺度的影响。所提出的方法将在与材料和生物相关的三个化学体系的动力学研究中进行测试：限制在纳米级无定形二氧化硅孔中的水，蛋白质水化壳层中的水，以及参与和结合氢键超分子组装的水分子。拟议工作中开发的方法将通过解决有关动力学的能量和熵驱动力的重要开放问题，为这些问题提供新的见解。汤普森小组还将创建新的课程材料，以提高学生对化学动力学的理解和欣赏。汤普森小组将开发和应用理论方法，直接计算化学中重要的动力学过程的活化能和熵。将研究以下方法：1)确定每个分子对活化能的贡献，以获得对潜在动力学的详细机制洞察；2)直接确定动态时间尺度的组成相关性；3)严格、直接地计算活化熵以及新的分子水平洞察力。这些方法的基础是评估时间关联函数关于温度(和其他热力学变量)的导数，从时间关联函数可以获得动态时间尺度，如速率常数或扩散系数。这些导数本身就是时间关联函数，可以从相同的分子动力学模拟中直接计算出来。这样，通常通过Arrhenius分析从多个温度下的计算中获得的活化能或熵可以从单一温度下的模拟中确定。一个关键的优势是，这种方法能够将激活能或熵严格分解为来自系统中存在的相互作用和运动的贡献，提供其他方面无法获得的物理见解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.