Microwave spectra of jet-cooled bromo- and chloroacetic acids

Microwave spectra of jet-cooled bromoacetic acid (CH₂BrCOOH) and chloroacetic acid (CH₂ClCOOH) have been obtained. For bromoacetic acid, the observed spectra are definitively assigned to a conformer in which the C–Br bond is nearly perpendicular to the plane defined by the other heavy atoms. This differs from the conformer previously observed at room temperature in which the C–Br bond was found to be in the plane. MP2 and DFT calculations predict four stable conformations, and the observed form corresponds to that with the lowest calculated energy. Diagonal and off-diagonal elements of the ⁷⁹Br and ⁸¹Br nuclear quadrupole coupling tensors have been determined and are in good agreement with the calculations. Interestingly, previous room temperature work on chloroacetic acid also revealed the conformation in which the C–Cl bond is in-plane (i.e., analogous to the room temperature work on bromoacetic acid). Thus, to see if the same disparity exists between the conformers observed under room temperature and jet-cooled conditions, the spectrum of jet-cooled chloroacetic was also examined. In this case, the conformer observed in the jet was the same as that previously reported at room temperature (in-plane C–Cl bond). The spectroscopic constants obtained, however, are somewhat more accurate than those previously determined and are reported here. Due to the cooling in the supersonic expansion, the species observed in this work are likely the lowest-energy forms for both acids. Comparison with prior studies on fluoro-, chloro-, and bromoacetic acids highlights a complex conformational landscape for these systems.