Theoretical Studies of Chemical Systems and Reactions at High Pressures

压力是决定化学反应选择性、方向和速率的重要物理参数。压力对化学动力学的影响得到了很好的研究，并在过去的六十年里收集了大量的实验数据。通常根据反应物转化为产物的过渡态的性质来讨论反应机理。传统上，实验测量的压力对反应速率的影响是用所谓的活化体积来表示的，活化体积代表过渡态和反应物的体积之差。由于反应物的体积很容易通过实验获得，因此活化体积提供了过渡态体积的直接测量，从而提供了对这些瞬时物种的独特一瞥。这需要一种可靠的技术，将微观系统的体积与其几何参数联系起来。直到最近，这样的技术还不存在。在过去的几年里，我们开发了一种基于计算机模拟的精确方法，称为分子动力学。在我们的方法中，微观反应体系的体积被确定为体系在溶剂中的阿基米德置换体积，并且被发现是纯溶剂(N个粒子)和含有该体系的溶剂(N+1个粒子)的体积之差。计算的体积对分子几何形状、溶质-溶剂相互作用的强度和类型以及温度和压力等物理参数很敏感，并与在给定条件下获得的实验体积非常接近。我们现在正在开发这种方法在高压化学、生物化学和光谱学问题上的各种应用。我们准确计算分子体积的能力是这些应用的核心。

Pressure is an important physical parameter that determines selectivity, direction, and rates of chemical reactions. The effects of pressure on chemical kinetics are well studied and a vast amount of experimental data has been collected over the last sixty years. Reaction mechanisms are usually discussed in terms of properties of transient states (transition states), through which reactants transform into products. Traditionally, the experimentally measured effects of pressure on reaction rates are expressed in terms of so-called volumes of activation, which represent the difference in volumes of transition states and reactants. Since the volumes of reactants are readily available experimentally, the volumes of activation provide a direct measure of the transition state volumes and thus a unique glimpse at these transient species. This requires a reliable technique relating the volume of a microscopic system to its geometrical parameters. Until recently, such a technique did not exist. Over the past few years we developed a precise method based on computer simulations known as molecular dynamics. Within our approach, the volume of a microscopic reaction system is determined as an Archimedean displacement volume of the system in a solvent and is found as a difference between the volumes of the pure solvent (N particles) and the solvent containing the system (N+1 particles). The calculated volumes are sensitive to molecular geometry, strength and type of solute-solvent interactions, and physical parameters such as temperature and pressure, and closely reproduce the experimental volumes obtained at given conditions. We are now developing various applications of this method to problems of high pressure chemistry, biochemistry, and spectroscopy. Our ability to accurately calculate molecular volumes is at the heart of each of these applications.