Rotational Detection of the Missing Conformers of 2-Chloropropionic Acid

The conformational space of 2-chloropropionic acid was reinvestigated using millimeter-wave and chirp-polarization Fourier transform microwave spectroscopies under isolated conditions in supersonic expansions. Besides the global minimum (carbonyl oxygen eclipsed with the methyl group), two additional conformers were identified: one with the carbonyl eclipsed relative to the α-C-H bond, and another stabilized by an intramolecular OH···Cl hydrogen bond. The latter interaction is evidenced by a decrease in |χzz|: the chlorine nuclear quadrupole coupling constant associated with the principal axis lying along the C-Cl bond and an increase in the asymmetry parameter η = (χxx - χyy)/χzz of the chlorine quadrupole coupling tensor. Rotational constants, geometries, and relative energies were determined and compared with quantum-chemical calculations, revealing larger deviations for conformers with intramolecular interactions. Calculations also show that chlorination markedly modifies the conformational landscape relative to propionic acid, affecting both minima and interconversion barriers. The accurate reproduction of the electronic properties encoded in the determined complete nuclear quadrupole coupling tensors requires the inclusion of relativistic approaches.