CAREER: Tiny Drops of Acid: Microwave Spectroscopy and Isomer-resolved IR Spectroscopy of Hydrohalic Acid-Water Clusters

With support from the Chemical Structure, Dynamics, and Mechanisms A (CSDM-A) program in the Division of Chemistry, G. Barratt Park of Texas Tech University is developing broadband microwave spectroscopy-based tools for the automated assignment and structure determination of clusters in complex mixtures. The microwave structures will be combined with isomer-resolved IR measurements to obtain detailed information about small hydrohalic acid-water clusters. Microwave spectroscopy is a powerful method for determining the precise three-dimensional structure of a molecule or cluster of molecules, but spectra obtained from mixtures of unknown molecular species are extremely challenging to interpret because of their sheer complexity. Dr. Park and his students will use automated microwave and infrared spectroscopy techniques to unravel the complicated patterns and learn about the structure and vibrational dynamics of the world’s tiniest drops of acid, consisting of a single acid molecule dissolved by a handful of water molecules. Their studies could provide crucial benchmarks for the theoretical description of aqueous clusters and could lead to significant advances in chemical structure determination from microwave spectra of mixtures. The Park group will develop a microwave spectrometer to provide hands-on instruction for undergraduate students and will perform community outreach with future-scientist middle school girls. Technological advances in frequency-flexible broadband microwave spectroscopy enable entirely new approaches to microwave structure determination in complex mixtures. The current capabilities of automated assignment algorithms will be extended to more complex, non-rigid Hamiltonians. The primary target of this investigation will be hydrohalic acid-water clusters with a single HX molecule and 3-10 water molecules. To date, existing spectroscopic approaches have only enabled microwave structures and unambiguous, undisputed IR assignment of very small clusters with fewer than three water molecules. A key goal of this investigation will be to obtain high-resolution measurements of the structure and vibrational dynamics of the smallest clusters that lead to dissociation. The isotopic substitution of the clusters will be varied in a controlled way, facilitating the assignment of a complete set of substitution structures, which will allow the determination of the microwave structure. Isomer-resolved IR spectra of neutral clusters will be obtained by IR-microwave double resonance. Advances in methodology that will be enabled by the proposed work could make progress towards the elusive goal of allowing chemical structures to be determined from microwave spectra of mixtures in an efficient way by a non-expert. 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.