Rotational Spectroscopy of 2-Furoic Acid and Its Dimer: Conformational Distribution and Double Proton Tunneling.

Structural and tunneling properties of the 2-furoic acid (FA) monomer and dimer were investigated using rotational spectroscopy and DFT calculations. CREST, a conformational ensemble space exploration tool, was used to identify all possible low-energy conformations of the FA monomer and dimer, followed by the DFT geometry optimization and harmonic frequency calculations. Broadband rotational spectra in the 2-6 and 8-12 GHz regions were recorded in a supersonic jet expansion. The monomeric FA was found to exist dominantly as three different conformers: I, II, and III in a jet, with I and II taking on the cis-COOH configuration while III having the trans-COOH configuration. For the FA dimer, only the I-II conformer was observed experimentally, whereas the symmetric I-I and II-II conformers were not observed because of their zero dipole moments. The analysis of the splittings in the rotational transitions of I-II allowed one to extract the tunneling splitting to be 1056.0(12) MHz. The barrier height was determined to be ∼442 cm -1 using the scaled potential energy scans at several different levels of theory.