Pure rotational spectroscopy of the H2O-trans-HOCO complex

Pure rotational spectra of the H2O-trans-HOCO complex have been observed by Fourier transform microwave (FTMW) spectroscopy and millimeter-wave FTMW double resonance spectroscopy. The complex was produced in a supersonic jet by discharging a mixture gas of CO and H2O diluted in Ar. The observed rotational lines consist of two groups of transitions with different hyperfine patterns. This is explained by considering the internal rotation of the H2O monomer in the complex. The molecular constants including the fine and hyperfine coupling constants have been determined for the two groups of lines. The hydrogen bond distance between H2O and the trans-HOCO monomer has also been determined with other structural parameters fixed to ab initio values. The hydrogen bond distance, 1.794 angstrom, is much shorter than that of the water dimer, and similar to those of water-acid complexes. The Fermi coupling constant of the proton of HOCO is compared with that of the trans-HOCO monomer, leading to the conclusion that there is an induced effect on the spin density on the proton of HOCO by the complex formation. (C) 2013 AIP Publishing LLC.