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Abstract:
Since the first instrument design was published over a decade ago[1], chirped-pulse Fourier transform microwave (CP-FTMW) spectroscopy has ushered in significant advances in molecular spectroscopy, improving its capabilities in studying intermolecular interactions and gas-phase molecular clusters, astrochemistry, chirality, and chemical dynamics. CP-FTMW spectroscopy probes the rotational degrees of freedom in molecules, so at zeroth order the observed spectra depend only on the size and the shape of the molecular system. Rotational selection rules depend only on the presence of a permanent electric dipole moment, so most molecular systems are observable. These inherent features make CP-FTMW spectroscopy an extremely useful tool for studying molecular structure. Canonically, microwave spectroscopy has found great success in studying non-covalent molecular complexes since the 1970s, thanks to the cold, collision-free environment of molecular beam expansions, but the order-of-magnitude improvements in sensitivity afforded by CP-FTMW instrumentation over older methods have led to a bounty of successful structurally-resolved spectroscopy studies that would have been intractable merely a decade ago.[2]
In this talk, I use the context of my graduate and postdoctoral research to illustrate these significant advances in microwave spectroscopy, focusing on two nearly diametrically opposed applications. The first are molecular studies enabled by low-frequency (2-18 GHz) microwave instrumentation, probing large molecular clusters that show structural features that bridge the gap between gas-phase and bulk behavior.[3] The second application will be the use of millimeter wave (60-300 GHz) instrumentation to study molecular reaction dynamics of small molecules and fuels within pyrolysis and photolysis reactors.[4]
Underlying these differing studies is a common challenge, in that broadband spectra acquired using CP-FTMW instrumentation can be incredibly complicated and feature resolvable spectra from a seemingly intractable number of molecular emitters due to the naturally high dynamic range. Since nearly all forms of isotopic substitution and stereoisomerism lead to resolvable spectra, the computational effort to fully interpret CP-FTMW spectra can be extraordinarily difficult. Thankfully, there has been an effort from a small community of spectroscopists, including myself, to develop software and ideas built to both ease and automate assignment of these dense broadband spectra.[5] In this light, I will briefly summarize our effort to build modern algorithms for spectroscopic identification, as well as the critical challenges and bottlenecks that the community currently faces.
Thanks to the huge advances provided by CP-FTMW methods, the future of microwave spectroscopy now shares common ground with all scientific fields in the digital era: thanks to rapid technological innovation, the important rhetorical question is no longer, “What problems can we solve?” Rather, the critical question is, “What can’t we solve?”
[1] Brown, G. G.; Dian, B. C.; Douglass, K. O.; Geyer, S. M.; Shipman. S. T; Pate, B. H., Rev. Sci. Instrum. 2008, 79, 053103.
[2] Park, G. Barratt; Field, R. W., J. Chem. Phys. 2016, 144, 200901.
[3] Pérez, C.; Lobsiger, S.; Seifert, N. A.; et al., Chem. Phys. Lett. 2013, 571, 1-15.
[4] Zaleski, D. P.; Sivaramakrishnan, R.; Weiler, H. R.; Seifert, N. A., et al., J. Am. Chem. Soc. 2021, 143, 3124.
[5] Seifert, N.A.; Finneran, I. A.; Pérez, C., et al., J. Mol. Spectrosc. 2015, 312, 13.