Conformational Space of 3-Chloropropionic Acid in Gas Phase Explored by Rotational Spectroscopy

The conformational space of 3-chloropropionic acid has been studied under the isolated conditions of a supersonic expansion using Stark-modulated free-jet absorption millimeter-wave and centimeter-wave chirped-pulse Fourier transform microwave spectroscopy techniques. The rotational spectra originating from the three most stable conformers including ³⁵Cl and ³⁷Cl isotopologues were observed in both experiments using helium expansion while a partial conformational relaxation involving skeletal rearrangements takes place in an argon expansion. The rotational parameters, geometries, and energy order were determined from the experiment, allowing a comparison with quantum chemical predictions. B3LYP-D3(BJ)/def2-TZVP performs slightly better than MP2/aug-cc-pVTZ in reproducing the molecular geometries. The deviations are largerfor conformers showing intramolecular interactions. It is also shown that the Douglas-Kroll-Hess second-order scalar relativistic core Hamiltonian approach in the point nuclear approximation is needed to accurately reproduce the electronic properties encoded in the fully determined nuclear quadrupole coupling constant tensors.