Knowledge Bank

In the gas phase, $HF2-$ is a hydrogen bonded species with a linear FHF geometry. The F-H distance is short and the hydrogen bond is strong. In the aqueous phase, it is known that the hydrogen bond in $HF2-$ is weakened due to hydrogen bonding between $HF2-$ and water molecules. We have examined the effect on the hydrogen bond in $HF2-$ due to hydrogen bonding between $HF2-$ and a single water molecule. For the $HF2--H2O$ complex we have calculated geometries and vibrational frequencies for the equilibrium structure and several transition state structures. We find that the equilibrium structure is a mere 0.2 kcal/mole below the transition state structures indicating that the complex is highly fluctional. We further find that whereas the F-H distance increase for the F atom hydrogen bonded to $H2O$, the other F-H distance decreases. The F-F distance is essentially unchanged and the linearity of the FHF geometry is strictly maintained.