Spectroscopic and computational characterization of the HCO···H2O complex.

The complexes of HCO with water are prepared in a Kr matrix and characterized by IR spectroscopy with the aid of ab initio calculations. The calculations at the UCCSD(T)/aug-cc-pVTZ level of theory predict three structures of the HCO···H2O complex. In the "linear" structure I, a hydrogen atom of water interacts with the oxygen atom of HCO. In structure II, the hydrogen atom of HCO interacts with the oxygen atom of water. The "cyclic" structure III has the C-H···O and O-H···O hydrogen bonds simultaneously. In the experiment, the HCO···H2O complex is produced by photolysis of HCOOH/HY/Kr (Y = Br and Cl) matrices followed by thermal annealing at about 30 K, which promotes the H + CO···H2O → HCO···H2O reaction. The analysis of the spectroscopic data shows that the main product has structure III whereas the formation of structure II is less efficient. The experiments show no evidence of the weakest structure I. The experiments with deuterated formic acid (DCOOH) provide additional support of the proposed assignment.