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 cm−1 have been recorded for (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 (DCOOH)2 is determined to be 1244.8461 (2) cm−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 (DCOOH)2 established a proton transfer time of 5.8 ns [tunneling splitting of 0.00286(25) cm−1]. 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) cm−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 C2h structure which indicates a high tunneling barrier. The observed spin statistical weights follow a description of formic acid dimer in the molecular symmetry group G8, which is isomorphic to the point group D2h. Our results suggest an out-of-plane contribution to the proton transfer mechanism for the otherwise planar dimer.

The infrared spectrum of formic acid dimers in helium nanodroplets has been observed corresponding to excitation of the "free" OH and CH stretches. The experimental results are consistent with a polar acyclic structure for the dimer. The formation of this structure in helium, as opposed to the much more stable cyclic isomer with two O-H...O hydrogen bonds, is attributed to the unique growth conditions that exist in helium droplets, at a temperature of 0.37 K. Theoretical calculations are also reported to aid in the interpretation of the experimental results. At long range the intermolecular interaction between the two monomers is dominated by the dipole-dipole interaction, which favors the formation of a polar dimer. By following the minimum-energy path, the calculations predict the formation of an acyclic dimer having one O-H...O and one C-H...O contact. This structure corresponds to a local minimum on the potential energy surface and differs significantly from the structure observed in the gas phase.

Rotationally resolved spectra of h2- and d1-formic acid embedded in liquid helium droplets have been recorded in the spectral region of the O–H (ν1) and C–H (ν2) stretch vibrations. Accidental resonant mixing between the ν1-band of h2-formic acid and the combination bands (ν2+ν7) and (ν2+ν9) has been observed. The fundamental ν1 band can interact via Fermi resonance with the (ν2+ν7), and the (ν2+ν7) via Coriolis coupling with the (ν2+ν9) band. Examination of the resonance induced line-broadening effects suggests that the helium environment modifies vibrational relaxation dynamics. The 2ν3 C=O stretch overtone is also observed and was assigned by a density functional theory (DFT) ab initio calculation. The spectroscopic constants are determined by fitting all spectra with a standard gas phase Hamiltonian.

The dynamical behavior of single OCS molecules embedded in large (N4=6000) superfluid 4He droplets has been investigated with high resolution double resonance infrared-microwave spectroscopy. The individual lines for the rotational transitions 2↔1, 3↔2, 4↔3, and 5↔4 are clearly resolved in the difference double resonance spectra and reveal a fine structure. This is especially well resolved for the 3↔2 transition which exhibits five separate lines having widths of about 50 MHz and separated by 200 MHz. The possible origin of the splitting is discussed. © 2000 American Institute of Physics.

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