Spectroscopic investigations of hydrogen bonding interactions in the gas phase. IV. The heterodimer H 2 0 • • • HF: the observation and analysis of its microwave rotational spectrum and the determination of its molecular geometry and electric dipole moment

The microwave rotational spectra of the isotopic species H 2 18 0 • • • H 19 F, H 2180 • • • H 19 F, HD 16 0 • •. HF, D 2 16 0 • • • DF and D 2 18 0 • • • DF of the hydrogen bonded heterodimer formed between water and hydrogen fluoride have been observed and analysed to give rotational constants. Evidence from the effects of nuclear spin statistical weights in the spectra leads to the conclusion that the dimer H 20 • • • HF is either planar with C2V symmetry or pyramidal with Cg symmetry but with a low barrier to inversion of the configuration at oxygen. r0-values of the heavy atom distance 0 * • • F have been derived from the observed rotational constants, the value for H 2160 • • • H 19 F being 2.662 Aj. The measured values of electric dipole moment in the vibrational ground state and in the state having v^{0) = 1 are 4.07 and 3.80 D respectively. These values show that excitation of the out-of-plane hydrogen bonding bending mode vP(0) leads to a large decrease in the dipole moment.