Proton Transfer in (HCOOH)₂:  An IR High-Resolution Spectroscopic Study of the Antisymmetric C−O Stretch^†

We report the fully analyzed high-resolution spectrum of the carboxylic acid dimer (HCOOH)₂ in the gas phase. High-resolution IR spectra in the region of the antisymmetric C−O stretch vibration have been recorded at 1221.0−1226.7 cm^-1. The data could be fit within experimental uncertainty to a rigid rotor Watson S-reduced Hamiltonian. The vibrational frequency of the C−O vibration is 1225.3430(3) cm^-1. On the basis of the measurement of a tunneling splitting of 0.0158(4)cm^-1 for the lower state and 0.0100(3) cm^-1 for the upper state, we determine a proton-transfer time of 1.0 and 1.7 ns for the ground and the vibrationally excited state, respectively.