High resolution spectroscopy of carboxylic acid in the gas phase: Observation of proton transfer in (DCOOH)2

We report the first fully analyzed high resolution spectrum of a carboxylic acid dimer in the gas phase. High resolution spectra in the region of the C–O stretch 1241.7–1250.7 ${\mathrm{cm}}^{\text{−1}}$ have been recorded for $(\mathrm{DCOOH}{)}_2.$ The data could be fit within experimental uncertainty to a rigid rotor Watson S reduced Hamiltonian. The vibrational frequency of the C–O stretch in $(\mathrm{DCOOH}{)}_2$ is determined to be 1244.8461 (2) ${\mathrm{cm}}^{\text{−1}}.$ Our spectra are the first direct experimental evidence for proton transfer tunneling in formic acid dimer (FAD), with FAD serving as a prototype for double hydrogen bonded organic complexes. Previous theoretical studies predicted proton transfer times covering a range of several orders of magnitude. Our measurements on $(\mathrm{DCOOH}{)}_2$ established a proton transfer time of 5.8 ns [tunneling splitting of 0.00286(25) ${\mathrm{cm}}^{\text{−1}}\mathrm{].}$ The proton transfer was found to accelerate upon vibrational excitation of the skeleton motion (the vibrational C–O mode), corresponding to a tunneling splitting of 0.00999(21) ${\mathrm{cm}}^{\text{−1}}.$ For this state the proton transfer time decreased to 1.7 ns, which corresponds to a vibrationally enhanced proton transfer mechanism. The moments of inertia are in agreement with a $C_{\text{2h}}$ structure which indicates a high tunneling barrier. The observed spin statistical weights follow a description of formic acid dimer in the molecular symmetry group $G_8,$ which is isomorphic to the point group $D_{\text{2h}}.$ Our results suggest an out-of-plane contribution to the proton transfer mechanism for the otherwise planar dimer.