Communication: The highest frequency hydrogen bond vibration and an experimental value for the dissociation energy of formic acid dimer

The highest frequency hydrogen bond fundamental of formic acid dimer, ν 24 (B u ), is experimentally located at 264 cm −1 . FTIR spectra of this in-plane bending mode of (HCOOH) 2 and band centers of its symmetric D isotopologues (isotopomers) recorded in a supersonic slit jet expansion are presented. Comparison to earlier studies at room temperature reveals the large influence of thermal excitation on the band maximum. Together with three B u combination states involving hydrogen bond fundamentals and with recent progress for the Raman-active modes, this brings into reach an accurate statistical thermodynamics treatment of the dimerization process up to room temperature. We obtain D 0 = 59.5(5) kJ/mol as the best experimental estimate for the dimer dissociation energy at 0 K. Further improvements have to wait for a more consistent determination of the room temperature equilibrium constant.