RUI: Discerning the Balance of Intermolecular Forces and the Enantiomeric Composition of Chiral Samples from the Structures of Gas-Phase Heterodimers

在化学系化学结构、动力学和机理A （CSDM-A）项目的支持下，阿默斯特学院的Helen Leung教授和Mark Marshall教授正在使用傅立叶变换微波（FTMW）光谱和计算方法研究由proproacids和halo-烯烃形成的气相异二聚体之间的分子间作用力，并正在开发一种手性标记方法。分子间作用力对化学和物理过程有着深远的影响，因此了解不同作用力之间的微妙平衡和竞争非常重要，例如静电和空间效应。梁教授和马歇尔教授及其本科生将产生仅通过分子间相互作用结合在一起的分子物种，并使用两种互补光谱仪检查它们的旋转光谱，一种是大带宽（几千兆赫），另一种是窄带（1兆赫），也提供了更高的灵敏度。实验工作在计算方法的指导和补充下，有望揭示复合物的形状，从而揭示分子间力的本质。这项工作可以更好地理解分子间力之间微妙的相互作用，从而产生首选的结合几何形状，并可以识别有用的手性标签，从而有效地将对映体（彼此互为镜像的分子）样品转化为具有不同旋转光谱的不同分子物种。本研究为本科生提供了有意义的现代、最先进的物理化学实践经验。利用最新的、相关的、现代的仪器，这项研究将以有效的教学方式吸引下一代科学家，更好地为他们在日益科技化的世界中成为负责任的公民做好准备，并更好地为他们在STEM（科学、技术、工程和数学）领域做出贡献。建立在对卤代乙烯和质子酸分子间相互作用的基本理解的基础上，从卤代乙烯到卤代丙烯的延伸导致了更大的结构灵活性，并有可能更好地理解导致质子酸首选结合几何形状的力之间的微妙相互作用。正在研究的手性标记技术依赖于具有相同微波旋转光谱的对映体，通过非共价相互作用形成具有已知手性的标记分子的异源二聚体，将其转化为光谱上不同且易于识别的非对映体。给定标签的已知绝对立体化学，分析物的绝对构型和样品的对映体过量可以以无背景的方式确定。通过对这些配合物的旋转光谱和跃迁强度的详细研究，这项工作可以为采用手性标记作为确定手性分析样品对映体纯度的常规分析方法提供关键的基准。啁啾脉冲技术能够在一致的源条件下获得大带宽的旋转频谱，因此对这些研究特别有利。计算方法和窄带Balle-Flygare光谱仪的使用为这项工作增加了强大的互补力量。该奖项反映了美国国家科学基金会的法定使命，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, Professors Helen Leung and Mark Marshall of Amherst College are using Fourier transform microwave (FTMW) spectroscopy and computational methods to study intermolecular forces operative between gas-phase heterodimers formed by protic acids and halo-olefins and are developing a chiral tagging method. Intermolecular forces have profound influence on chemical and physical processes, making it important to understand the delicate balance and competition between different contributions, such as electrostatic and steric effects. Professors Leung and Marshall and their undergraduate students will generate molecular species held together solely by intermolecular interactions and examine their rotational spectra using two types of complementary spectrometers, one with large bandwidth (several thousand MHz) and a narrowband (1 MHz) instrument that also provides enhanced sensitivity. The experimental work, guided and supplemented by computational methods, is expected to reveal the shape of the complexes and hence the nature of the intermolecular forces. This work could lead to a better understanding of the subtle interplay among the intermolecular forces that results in a preferred binding geometry, and could identify useful chiral tags that enable effective conversion of a sample of enantiomers (molecules that are mirror images of each other) into distinct molecular species that have different rotational spectra. This research provides meaningful hands-on experience in modern, state-of-the-art physical chemistry for undergraduate students. With up-to-date, relevant, modern instrumentation, this research will engage the next generation of scientists in pedagogically effective ways, better prepare them to be responsible citizens in an increasingly technological world, and better position them to contribute in STEM (science, technology, engineering and mathematics) fields.Building on a foundational understanding of the intermolecular interactions between haloethylenes and protic acids, the extension from haloethylenes to halopropenes results in greater structural flexibility, with the potential for a finer understanding of the subtle interplay among the forces that lead to a preferred binding geometry for the protic acids. The chiral tagging technique being investigated relies on the conversion of enantiomers, which have identical microwave rotational spectra, into spectroscopically distinct and readily identifiable diastereomers upon complexation via non-covalent interactions to form a heterodimer with a tag molecule of known chirality. Given the known absolute stereochemistry of the tag, the absolute configuration of the analyte and the enantiomeric excess of a sample can be determined in a background free manner. Through a detailed study of the rotational spectrum and the intensities of the transitions of these complexes, this work could provide the benchmarking critical for adoption of chiral tagging as a routine analytical method for determining the enantiomeric purity of a chiral analyte sample. With the ability to obtain a large bandwidth rotational spectrum with consistent source conditions, the chirped pulse technique is particularly advantageous for these studies. The use of computational methods and the narrowband Balle-Flygare spectrometer adds powerful, complementary strength to the work.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.