Conformational\nSpace of 3‑Chloropropionic\nAcid in Gas Phase Explored by\nRotational Spectroscopy

The conformational space of 3-chloropropionic acid has\nbeen studied\nunder the isolated conditions of a supersonic expansion using Stark-modulated\nfree-jet absorption millimeter-wave and centimeter-wave chirped-pulse\nFourier transform microwave spectroscopy techniques. The rotational\nspectra originating from the three most stable conformers including <sup>35</sup>Cl and <sup>37</sup>Cl isotopologues were observed in both\nexperiments using helium expansion while a partial conformational\nrelaxation involving skeletal rearrangements takes place in an argon\nexpansion. The rotational parameters, geometries, and energy order\nwere determined from the experiment, allowing a comparison with quantum\nchemical predictions. B3LYP-D3(BJ)/def2-TZVP performs slightly better\nthan MP2/aug-cc-pVTZ in reproducing the molecular geometries. The\ndeviations are largerfor conformers showing intramolecular interactions.\nIt is also shown that the Douglas–Kroll–Hess second-order\nscalar relativistic core Hamiltonian approach in the point nuclear\napproximation is needed to accurately reproduce the electronic properties\nencoded in the fully determined nuclear quadrupole coupling constant\ntensors.