High resolution IR spectroscopy of the carbonyl stretch of (DCOOD)2

We report the measurement of the IR high resolution spectrum of the CO stretch of deuterated formic acid dimer (DCOOD)2. The dimers were aggregated in a supersonic jet expansion. We were able to assign 93 rovibrational transitions which were fit within experimental uncertainty (10−3 cm−1) to an asymmetric top Watson S-reduced Hamiltonian. The vibrational frequency is determined to be 1717.5177(4) cm−1. Within our experimental frequency resolution we were not able to resolve the proton tunneling transfer splitting, which yields an upper limit of 0.002 cm−1 for the tunneling splitting of (DCOOD)2. We have systematically compared the rotational constants of the various isotopomers including: (DCOOD)2, (DCOOH)2, and (HCOOH)2. This allowed us to deduce information on the structure of formic acid dimer, which is directly compared with the predictions from previous ab initio studies. Assuming that the geometry of the complex remains unchanged upon deuteration an intermolecular distance of 2.680(2) Å and an angle O-H⋯O of 175.8(5) Å could be deduced from the experimental data.