Ultrafast 2DIR Studies of Dynamics in Dense Gas and Supercritical Fluid Solutions

在化学系化学结构、动力学和机理(CSDM-A)项目的支持下，波士顿大学的Lawrence Ziegler教授和他的团队正在研究分子在超临界流体(SCF)中的行为。超临界流体是在高压和高温下发现的一种特殊的物质状态，在这种状态下，液体和气体之间的差异消失。其他分子在超临界流体中的溶解程度很容易调节，因为流体的性质敏感地依赖于温度和压力。超临界流体的可调特性已经在能源、食品和制药行业中得到了广泛的应用。然而，超临界流体作为一种廉价、高效和环境清洁的替代品，有可能产生更大的影响，取代对环境和人类健康造成负面影响的有机溶剂。齐格勒团队正在用非常短的红外激光脉冲进行实验，以确定分子如何在超临界流体中溶解和失去能量。这些测量揭示了在更像液体或气体的空间区域中是否发现了溶解的分子，它们在这些区域停留了多长时间，以及什么分子特性决定了这些行为。该研究小组还使用计算方法，通过与实验相比较，对SCF中分子的行为有了更多的了解。这项研究挑战了在普通化学课堂上教授学生关于超临界流体的基本概念。除了研究活动，研究团队还分别指导波士顿大学正在进行的NSF本科生研究体验(REU)和科学与工程研究(RISE)项目的本科生和高中参与者，以扩大这些研究的影响，并为下一代分子科学家的发展做出贡献。这个项目的目标是利用超快二维红外光谱(2DIR)来更详细地了解稠密气体和超临界流体中的局部溶剂化环境。虽然以前已经对凝聚相溶液中的振动分子进行了超快2DIR研究，但本项目利用稠密气体和超临界流体的2DIR光谱的特殊特性来了解稠密流体中的旋转和振动弛豫，超临界流体相中的特殊溶剂化效应，以及液体相性质随流体密度的变化。二维红外光谱具有类似回声的性质，可以在高密度和超临界流体溶液中精确测定溶质的转动能弛豫速率，这些溶液中的旋转结构在线性红外光谱中是完全无法分辨的。与泵浦-探测光谱不同，2DIR直接报告了溶剂的波动，从而为临界点区域分子的局部溶剂化动力学中涨落的作用提供了一个窗口。此外，2DIR光谱区分了给定状态点的自由转子和类液体特征，从而提供了对这些相空间的高压和高温区的新的分子水平描述，可以与先前建立的超临界区域的不均匀描述相比较。通过这些研究对比了不同的振动和转动能弛豫速率，突出了在稠密、高激发的分子系统中恢复平衡的量子瓶颈。最后，研究人员正在与波士顿大学教学制作服务团队合作，开发一系列视频讲座，伴随着一本关于线性和非线性光谱学原理的书的每一章，这本书是为研究生和高级本科生以及更广泛的科学、技术、工程和数学(STEM)社区而设计的。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

With support from the Chemical Structure, Dynamics, and Mechanisms-A (CSDM-A) Program in the Division of Chemistry, Professor Lawrence Ziegler and his group at Boston University are investigating how molecules behave in supercritical fluids (SCFs). SCFs are a special state of matter found at high pressure and temperature, where the difference between liquid and gas vanishes. How well other molecules dissolve in a SCF is readily tunable, because the fluid properties are sensitively dependent on temperature and pressure. The tunable properties of SCFs are already being used for a wide range of applications in the energy, food, and pharmaceutical industries. However, SCFs have the potential for even greater impact as an inexpensive, efficient, and environmentally clean alternative to organic solvents that have negative environmental and human health consequences. The Ziegler group is conducting experiments with very short-duration pulses of infrared laser light to determine how molecules dissolve and lose their energy in SCFs. These measurements reveal whether dissolved molecules are found in spatial regions that are more liquid- or gas-like, how long they stay in such regions, and what molecular properties determine these behaviors. The research group also uses computational methods to gain additional insight into the behavior of molecules in a SCF by comparison with the experiments. This research challenges fundamental concepts that students in beginning general chemistry classes are taught about SCFs. Aside from the research activities, the research team mentors undergraduate and high school participants in Boston University’s ongoing NSF Research Experience for Undergraduates (REU) and Research in Science and Engineering (RISE) programs, respectively, in order to expand the impact of these studies and contribute to the development of the next generation of molecular scientists. The goal of this project is to exploit ultrafast two-dimensional infrared spectroscopy (2DIR) to gain a more detailed understanding of the local solvation environments in dense gases and supercritical fluids. While previous ultrafast 2DIR studies have been carried out for vibrating molecules in condensed-phase solutions, this project exploits the special characteristics of 2DIR spectra of dense gases and supercritical fluids to learn about rotational and vibrational relaxation in dense fluids, special solvation effects in the supercritical fluid phase, and the evolution of liquid phase character as a function of fluid density. The echo-like properties of 2DIR allow rotational energy relaxation rates of solutes to be precisely determined in high density and supercritical fluid solutions, where the rotational structure is completely unresolvable in the linear IR spectra. Unlike pump-probe spectroscopy, 2DIR directly reports on solvent fluctuations, and thus offers a window on the role of fluctuations in the local solvation dynamics of molecules in the region of the critical point. Additionally, 2DIR spectra distinguish free rotor and liquid-like character at a given state point, thus providing new molecular-level descriptions of these high-pressure and high-temperature regions of phase space that can be compared with previously established descriptions of inhomogeneities in the supercritical region. The different rates of vibrational and rotational energy relaxation are contrasted by these studies, which highlights the quantum bottlenecks for restoring equilibrium in dense, highly excited molecular systems. Finally, the researchers are working with the Boston University Instructional Production Services Team to develop a series of video lectures to accompany each chapter of a book on the principles of linear and nonlinear spectroscopy that is intended for beginning graduate and upper-division undergraduate students, as well as the greater science, technology, engineering, and mathematics (STEM) community.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.