Collaborative: Terahertz Spectroscopy of Clathrates

该项目由布朗大学的丹尼尔·米特曼教授和佛蒙特大学的迈克尔·鲁杰罗教授合作的化学部化学结构动力学和机理（CSDM-A）计划资助，重点研究笼形材料的振动运动。笼形物是由小分子组成的多孔材料，通常称为“客人”，保持在由“宿主”分子网络形成的笼中。宿主分子可以是许多不同的类型，包括由水分子形成的宿主笼结构的关键例子。这些水包合物可以包裹各种客体分子，如甲烷和其他温室气体。这种类型的甲烷笼形物可以在环境中自然形成，并且在永久冻土层和海洋底部丰富。它们也是天然气管道堵塞的主要来源之一。现在可以理解，这些大分子结构的振动模式，特别是那些以相对低的频率振荡的振动，与包合物的许多性质密切相关，包括它们的形成和解离动力学以及它们的化学反应性。该合作研究小组正在使用太赫兹范围内的辐射（频率高于微波，但低于大多数红外测量）来研究这些振动运动如何受到温度，压力和当地化学环境的影响。他们正在理解涉及包合物的关键化学反应，例如现有客体分子（例如，甲烷）与新的甲烷（例如，二氧化碳），从而在提取甲烷的同时储存二氧化碳。在这些研究工作的同时，项目人员正在协调布朗大学和佛蒙特大学为高中生提供的暑期讲习班，将这项研究的范围扩大到下一代早期职业科学家。 该研究计划的目标是研究有关动力学和动力学的基本问题，这些动力学和动力学使用太赫兹（THz）光谱（0.3 - 4 THz）驱动包合物的形成和性质，并开发一个新的理论框架来解释这些测量，准确地解释相关模式的非谐性。 笼形化合物的热力学和结构性质一般都很好地表征，但这些宏观现象的微观起源仍然未知。因此，有许多关于它们的形成和解离动力学，振动动力学，结构相变和稳定性的公开问题。这项研究采用了最近开发的压力和温度相关的太赫兹光谱技术，以表征各种笼形化合物的低频模式，并观察振动景观的演变，如客体交换反应展开。研究人员正在开发新的理论工具来从头预测这些光谱。这种方法将非谐性的严格描述纳入振动分析中，这对于准确描述相关的低频模式以及许多热力学量至关重要。最后，项目人员还为本科生和研究生开发了一门新的跨学科课程，涵盖化学科学中的超快光谱学，为两个机构以及更大的STEM社区提供服务，该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的评估来支持。影响审查标准。

This project funded by the Chemical Structure Dynamics and Mechanism (CSDM-A) program of the Chemistry Division, a collaboration between Professor Daniel Mittleman of Brown University and Professor Michael Ruggiero of the University of Vermont, focuses on the study of the vibrational motions of clathrate materials. Clathrates are porous materials consisting of small molecules, often called ‘guests’, held within a cage formed by a network of ‘host’ molecules. The host molecules can be of many different types, including the crucial example in which the host cage structure is formed by water molecules. These water clathrates can enclose various guest molecules such as methane and other greenhouse gases. Methane clathrates of this type can form naturally in the environment, and are plentiful in the permafrost and at the bottom of the ocean. They are also one of the primary sources of clogs in natural gas pipelines. It is now understood that the vibrational modes of these macromolecular structures, especially those vibrations which oscillate at relatively low frequency, are intimately related to many properties of clathrates including their formation and dissociation dynamics and their chemical reactivity. The collaborative research team is using radiation in the terahertz range of the spectrum (higher frequency than microwaves, but lower than most infrared measurements) to study how these vibrational motions are influenced by temperature, pressure, and the local chemical environment. They are understanding critical chemical reactions involving clathrates, such as the reaction in which an existing guest molecule (e.g., methane) is exchanged with a new one (e.g., carbon dioxide), thus storing the carbon dioxide while extracting the methane. In parallel to these research efforts, the project personnel are coordinating summer workshops for high school students that are offered at both Brown and the University of Vermont, expanding the reach of this research to the next generation of early career scientists. The goal of this research program is to investigate fundamental questions about the kinetics and dynamics that drive the formation and properties of clathrates using terahertz (THz) spectroscopy (0.3 – 4 THz), and to develop a new theoretical framework for interpreting these measurements which accurately accounts for the anharmonicity of the relevant modes. The thermodynamic and structural properties of clathrates are generally well characterized, but the microscopic origins of these macroscopic phenomena remain unknown. As a result, there are many open questions concerning their kinetics of formation and dissociation, vibrational dynamics, structural phase transitions, and stability. This research is employing recently developed techniques for pressure- and temperature-dependent terahertz spectroscopy to characterize the low-frequency modes of various clathrate compounds, and to observe the evolution of the vibrational landscape as reactions such as the guest exchange reaction unfold. The researchers are developing new theoretical tools for the ab initio prediction of these spectra. This approach incorporates a rigorous description of anharmonicity into the vibrational analysis, which is critical for accurately describing the relevant low-frequency modes, as well as many thermodynamic quantities. Finally, the project personnel are also developing a new interdisciplinary course for undergraduate and graduate students covering ultrafast spectroscopy in the chemical sciences, which serves both institutions, as well as the greater STEM community, by filling in this important area of modern chemical research.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.