NSF Award
2309942
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.<br/><br/>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.<br/><br/>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.
