Exploring the Connections between the Nonlinear Spectroscopy and the Potential Energy Landscapes of Liquids

布朗大学的理查德·M·斯特拉特得到了化学系化学理论、模型和计算方法计划的支持，他研究了液体混合物、液晶和过冷液体中异常缓慢的分子动力学与这些系统势能景观的全球特征之间的关系。虽然由于需要重新排列个别化学键或克服势能面上的其他局部瓶颈，缓慢的动力学通常是可以理解的，但在许多最有趣的情况下，问题更多的是在景观中定位和穿越最有效的全球路径的困难。这项工作研究了如何使用计算机模拟来表征这些最有效的(测地线)路径，并试图在从这些路径获得的信息和从新的非线性光谱获得的信息之间建立定量的联系。光谱研究的一半集中在溶质泵/溶剂探测光谱上，特别是因为这些光谱有可能揭示在新的电子激发溶质存在的情况下，溶剂结构的宽幅重排。通过计算优先溶解混合物、染料掺杂液晶和易碎玻璃形成液体的预期光谱，并将结果与测地线景观分析结果进行比较，研究人员希望了解如何使用光谱来发现这些特别慢的系统的势能景观的独特之处。斯特拉特教授和他的研究小组正在开发计算方法，以探索液体混合物、液晶和过冷液体中的慢分子动力学与这些系统势能景观的某些特征之间的关系。势能图景是系统中原子的相互作用能，它是原子间距离的函数。斯特拉特的研究重点是如何在这些景观中找到最有效的路径；这些路径决定了原子的实际运动。斯特拉特还开发了预测和分析光谱的理论方法，帮助我们了解溶剂如何与处于激发电子态的分子反应。这项研究的最终成果可能是对涉及缓慢分子重排的两个极其重要的实际问题的有用洞察：确定玻璃和无定形材料的性质，以及了解蛋白质如何以及何时错误折叠。无定形物质的韧性和易加工性通常使它们成为理想的商业材料，但令人惊讶的是，对于玻璃性的许多科学基础，人们几乎没有达成共识。同样，抵抗折叠成有序结构的蛋白质对生命有机体是必不可少的，但其中一些以无定形方式折叠的蛋白质是关键人类健康问题的标志，包括阿尔茨海默病。这项研究让本科生作为研究合作者，从而丰富了他们的科学教育。

Richard M. Stratt of Brown University is supported by the Chemical Theory, Models, and Computational Methods program in the Chemistry division for his research on the relationships between the unusually slow molecular dynamicsseen in liquid mixtures, liquid crystals, and supercooled liquids, and the global features of the potential-energy landscapes of these systems. While slow dynamics is often understandable as a consequence of the need to rearrange individual chemical bonds or surmount other local bottlenecks on the potential energy surface, in many of the most interesting cases, the issue is more the difficulty in locating and traversing even the most efficient global pathways through the landscape. This work investigates how computer simulations can be used to characterize these most efficient ("geodesic") pathways and attempts to forge quantitative connections between the information garnered from these pathways and that available from novel nonlinear spectroscopies. The spectroscopic half of the research is focused on solute pump/solvent-probe spectra, in particular, because those spectra have the potential to reveal how broad swaths of solvent structure can rearrange in the presence of a newly electronically excited solute. By computing the spectra expected from preferentially solvating mixtures, dye-doped liquid crystals, and fragile-glass-forming liquids, and comparing the outcomes with the results of a geodesic landscape analysis, the researchers hope to learn how spectroscopy can be used to discover what is so unique about the potential energy landscapes of these especially slow systems. Professor Stratt and his research group are developing computational methods to explore the relationship between the slow molecular dynamics seen in liquids mixtures, liquid crystals and supercooled liquids and certain features of the potential energy landscapes of these systems. A potential energy landscape is the interaction energy of the atoms in the system as a function of the distances between the atoms. Stratt's research focuses on how to find the most efficient pathways through these landscapes; such pathways determine the actual motions of the atoms. Stratt also develops theoretical methods to predict and analyze spectra that help us to understand how solvents react to molecules that are in an excited electronic state. The eventual payoff for this research may be useful insight into two enormously important practical problems involving slow molecular rearrangements: determining the properties of glassy and amorphous materials, and understanding how and when proteins fold incorrectly. The toughness and ease of processing of amorphous substances often makes them ideal commercial materials, but there is surprisingly little consensus about many of the scientific fundamentals of glassiness. Similarly, proteins that resist folding into well-ordered structures are essential to living organisms, but some of those that fold in an amorphous fashion are markers of critical human health problems, including Alzheimer's disease. The research involves undergraduates as research coworkers, thereby enriching their scientific education.