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

Amherst College, Amherst MA

Investigators

Abstract

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.