Collaborative: Terahertz Spectroscopy of Clathrates

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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.