Rotational spectroscopy and modelling of halogenated carboxylic acids and their hydrated complexes

The rotational spectra of several halogenated carboxyl acids and their 1:1 and 1:2 water complexes, namely 3-chloropropionic acid, 2-chloropropionic acid, 3-bromopropionic acid, 3-chloropropionic acid monohydrate and dihydrate and 2-chloropropionic acid monohydrate, were studied in the microwave and millimeter wave ranges using Chirp Pulse-Fourier Transform Microwave and Stark-modulated Free-Jet Absorption Millimeter-Wave spectroscopy respectively. For 3-chloropropionic acid, three conformations with a Z-COOH arrangement were identified in the gas expansion, while the spectra belonging to the 37Cl isotopologue of all three conformers were observed in nature abundance. For 2-chloropropionic acid, in total three conformations were observed using a helium expansion, two of which were not observed in a previous reported study. For 3-bromopropionic acid, transitions from one Z-COOH conformation were assigned, and 81Br isotopologue was identified in nature abundance. Two conformations of 3-chloropropionic acid monohydrate were observed, in which water acts as both a proton donor and a proton acceptor, forming hydrogen bonded cyclic structures with the carboxyl groups. For the 3-chloropropionic acid dihydrate complexes, only one conformation was revealed, in which the global minimum conformation of 3ClPA forms a ring structure with three HBs with a slightly distorted water dimer. For 2-chloropropionic acid monohydrate, only one conformation was observed, in which water forms HBs with the carboxyl groups of the global minimum conformations of the monomer. The last chapter employs implicit and explicit models to describe the structural and energetic changes of monochloropropionic acids in aqueous environments compared to those in the gas phase. This study provides a theoretical basis for understanding the behavior of such water pollutants and holds significant implications for their degradation.