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-A（CSDM-A）项目的支持下，波士顿大学的劳伦斯齐格勒教授和他的小组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了。其他分子在SCF中的溶解程度很容易调节，因为流体性质敏感地依赖于温度和压力。SCF的可调特性已经被广泛应用于能源、食品和制药行业。然而，SCF作为具有负面环境和人类健康后果的有机溶剂的廉价、高效和环境清洁的替代品具有甚至更大的影响的潜力。齐格勒小组正在进行实验，用非常短的红外激光脉冲来确定分子如何溶解和失去SCF中的能量。这些测量揭示了溶解的分子是否存在于更像液体或气体的空间区域中，它们在这些区域中停留多久，以及什么样的分子特性决定了这些行为。该研究小组还使用计算方法，通过与实验进行比较，获得对SCF中分子行为的额外见解。这项研究挑战的基本概念，学生在开始普通化学课教授有关SCF。除了研究活动外，研究团队还分别指导波士顿大学正在进行的NSF本科生研究经验（REU）和科学与工程研究（RISE）计划的本科生和高中生参与者，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。该项目的目标是利用超快二维红外光谱（2D）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。虽然以前的超快2S研究已经进行了凝聚相溶液中的振动分子，该项目利用致密气体和超临界流体的2S光谱的特殊特性，以了解致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，以及作为流体密度函数的液相特征的演变。2 H2O的回波性质允许在高密度和超临界流体溶液中精确确定溶质的旋转能量弛豫速率，其中旋转结构在线性IR光谱中完全不可分辨。与泵浦-探测光谱不同，2S直接报告溶剂波动，从而提供了一个窗口，在临界点区域的分子的局部溶剂化动力学的波动的作用。此外，2 π谱在给定的状态点区分自由转子和类液体特征，从而提供了这些相空间的高压和高温区域的新的分子水平的描述，可以与先前建立的描述超临界区域中的不均匀性进行比较。这些研究对比了振动和旋转能量弛豫的不同速率，突出了在密集的、高度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，以伴随一本关于线性和非线性光谱学原理的书的每一章，该书适用于初级研究生和高年级本科生，以及更大的科学，技术，工程，该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.