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，阿默斯特学院的梁海伦教授和马克马歇尔教授正在利用傅里叶变换微波（FTMW）光谱和计算方法研究由质子酸和卤代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。分子间力对化学和物理过程有着深远的影响，因此理解不同贡献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、马歇尔教授和他们的本科生将利用两种互补的光谱仪，一种具有大带宽（几千兆赫），另一种具有更高灵敏度的窄带（1兆赫）仪器，生成仅通过分子间相互作用结合在一起的分子种类，并检测它们的旋转光谱。在计算方法的指导和补充下，实验工作有望揭示复合物的形状，从而揭示分子间力的性质。这项工作可以导致更好地理解分子间力之间的微妙相互作用，从而产生优选的结合几何形状，并可以识别有用的手性标签，使对映体样品（彼此镜像的分子）有效转化为具有不同旋转光谱的不同分子种类。这项研究为本科生提供了现代，最先进的物理化学有意义的实践经验。 随着最新的，相关的，现代化的仪器，这项研究将从事下一代科学家在教学上有效的方式，更好地准备他们是一个越来越技术化的世界负责任的公民，并更好地定位他们在干贡献（科学、技术、工程和数学）领域。基于对卤代乙烯和质子酸之间分子间相互作用的基本理解，从卤代乙烯到卤代丙烯的延伸导致更大的结构灵活性，具有更好地理解导致质子酸的优选结合几何形状的力之间的微妙相互作用的潜力。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异构体在通过非共价相互作用络合后转化为光谱上不同且易于识别的非对映异构体，以与已知手性的标记分子形成异二聚体。给定标签的已知绝对立体化学，可以以无背景的方式确定分析物的绝对构型和样品的对映体过量。通过对这些配合物的转动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测定手性分析物样品的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谱，因此对这些研究特别有利。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, 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.