Rotational spectroscopy as a tool for environmental and structural chemistry: studies on PFAs, fluoroalcohols, and hydroxylamines

This thesis demonstrates chirped-pulse Fourier-transform microwave (CP-FTMW) spectroscopy as both an analytical method to detect and monitor contaminants in aqueous solutions, with emphasis on perfluorocarboxylic acids, and a structural tool to probe conformational landscapes of molecules and weakly bound complexes. In the analytical part, a commercial K-band CP-FTMW spectrometer was evaluated using headspace sampling and direct injection with complete vaporization in the measurement cell. The available headspace inlet was inefficient, so a custom injection inlet was implemented to enable direct vaporization of small injected volumes. With this configuration, the microwave signal showed good linearity with concentration and achieved a limit of detection of 0.02 g/100 mL for methanol. A systematic decrease in signal intensity with increasing molecular mass was observed, implying that detection and quantification of heavier PFAS would benefit from instrument designs implementing supersonic expansions. The spectroscopic part employs molecular-beam CP-FTMW spectrometers to investigate perfluorinated alcohols, hydroxylamine derivatives, and their water complexes. For 2,2,3,3,4,4,4-heptafluoro-1-butanol (HFB) and 3,3,4,4,5,5,6,6,6-nonafluoro-1-hexanol (NFH), conformational preferences are dominated by intramolecular OH···F interactions. Microhydration of HFB readily disrupts OH···F, leading to stronger OH···O(water) bonding and highlighting the hydrogen-bond donor character of fluoroalcohols. For NFH, two monohydrated conformers were observed, one disrupting OH···F to form OH···O(water), and one retaining OH···F with the hydroxyl group acting as a hydrogen-bond acceptor, illustrating how fluorination degree modulates hydration patterns. Two helium complexes of HFB, both based on the G+G+t monomer, were assigned and analysed using Kraitchman’s equation. For deuterated N,N-diethylhydroxylamine, combined with prior data, nuclear quadrupole coupling tensors were determined. Finally, for N,N-diethylacetyloxyamine, four low-energy conformers and 1:1, 1:2, and 1:3 water complexes were characterized, revealing site-selective hydration at the sp3 nitrogen lone pair and a sequential “water wire”.