RUI: Shape Matters: Using the Preferred Structures of Gas-Phase Heterodimers to Investigate Intermolecular Forces and for Chiral Analysis

在这项由化学系化学结构动力学与机制(CSDM-A)计划资助的项目中，阿默斯特学院的梁海伦教授和马歇尔教授使用最先进的高速方法测量了两个单独的分子在太空中旋转时形成的络合物所发出的微波辐射。分析发射的特定频率的模式以确定复合体的详细形状。这项研究提供了关于这两个分子如何相互排列以及精确的距离和角度的基本信息。这些信息反过来又表明了分子相互作用的力的性质。这种作用力对许多化学和生物系统有重要的影响，但很难与更强的化学键的更大影响分开。这个项目试图提供对分子间相互作用的更好的理解。这项研究还利用这些相互作用来区分具有相同物理特性但不同生物效应的镜像分子。从事这项研究的本科生将获得最先进的微波光谱仪、复杂的数据分析和现代量子化学程序包的宝贵实践经验。此外，他们还学习如何制定问题和设计研究方法。本项目同时使用腔增强和啁啾脉冲傅里叶变换微波光谱来获得气相双分子异二聚体的转动光谱。通过微波辐射脉冲，旋转的二聚体以自由感应衰变(FID)的特征频率重新发射，在自由喷射的气体样品中诱导出宏观的相干偏振。FID被数字化和傅立叶变换以揭示频谱，该频谱被分析以提供分子结构。对质酸-卤代乙烯络合物的研究测试了关于静电和分散相互作用与空间要求的相互作用的假设，这些相互作用导致了新颖和意想不到的结构。通过表征手性分析物和已知手性的小标记分子之间形成的非共价键合异二聚体，将分析物的对映体转化为光谱可分辨的非对映异构体，该项目有助于开发一种新的、改进的手性分析方法，其应用遍及化学，特别是制药行业。这项技术被称为手性标记，通过提供分析物的绝对立体化学以及以无背景的方式提供样品的过量对映体，有望产生广泛的影响。这项工作的其他更广泛的影响包括鼓励在STEM领域追求职业生涯的本科生的教育。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

In this project funded by the Chemical Structure Dynamics and Mechanisms (CSDM-A) program of the Chemistry Division, Professors Helen O. Leung and Mark D. Marshall of Amherst College use high speed, state-of-the-art methods to measure the microwave radiation given off by complexes formed between two individual molecules rotating together in space. The patterns of the specific frequencies emitted is analyzed to determine the detailed shape of the complex. The research gives fundamental information into how the two molecules are arranged with respect to each other along with precise distances and angles. This information, in turn, indicates the nature of the forces that molecules exert upon each other. Such forces have important consequences for many chemical and biological systems, but are difficult to separate from the larger effects of the much stronger chemical bonds. This project seeks to provide a greater understanding of intermolecular interactions. The research also use these interactions to differentiate between mirror image molecules that have identical physical properties but different biological effects. The undergraduate students engaged in this research receive valuable hands-on experience with state-of-the art microwave spectrometers, sophisticated data analysis, and modern quantum chemistry program packages. Additionally, they learn how to formulate questions and design the research methods. This project uses both cavity-enhanced and chirped-pulse Fourier transform microwave spectroscopy to obtain the rotational spectrum of gas-phase bimolecular heterodimers. A macroscopic, coherent polarization is induced in the free-jet, gaseous sample via a pulse of microwave radiation, which the rotating dimers re-emit at characteristic frequencies in a free-induction decay (FID). The FID is digitized and Fourier transformed to reveal the frequency spectrum that is analyzed to provide molecular structures. The investigation of protic acid-haloethylene complexes tests hypotheses regarding the interplay of electrostatic and dispersion interactions with steric requirements that result in novel and unexpected structures. Through the characterization of non-covalently bonded heterodimers formed between a chiral analyte and a small tag molecule of known chirality, which converts enantiomers of the analyte into spectroscopically-distinguishable diastereomers, the project contributes to the development of a new, improved method of chiral analysis with applications across chemistry and particularly to the pharmaceutical industry. Called chiral tagging, the technique promises to have broad impact by providing the absolute stereochemistry of the analyte and, in a background-free manner, the enantiomeric excess of a sample. Additional broader impacts of this work include the education of undergraduates who are encouraged to pursue careers in STEM fields.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.