Knowledge Bank

The rotational spectra of ethylene-water and acetylene-water complexes were observed using the molecular beam electric resonance technique. The following spectroscopic constants were obtained. [FIGURE] For ethylene-water, the rotational constants and low dipole moment are consistent with a structure in which the water is hydrogen bonded to the double bond of the ethylene. The center of mass separation of the two molecules is calculated to be 3.41 {\AA} for $C2H4-H2O$. The acetylene-water complex is effectively planar with the acetylenic hydrogen pointed toward the water oxygen. The molecular center of mass separation in $C2H2-H2O$ is 3.96 {\AA} which gives a hydrogen bond length of 2.23 {\AA}. The possibility of the presence of a low barrier at the planar configuration will be discussed.