Intermolecular structure on binary complexes of water with phenylacetylene and its substituted analogs: a combined spectroscopic and ab initio investigation

The hydrogen-bonded complexes of phenylacetylene, 4-fluorophenylacetylene, 2-fluorophenylacetylene and 4-cyanophenylacetylene with water were investigated using IR-UV double resonance spectroscopy. Phenylacetylene, 4-fluorophenylacetylene, 2-fluorophenylacetylene and 4-cyanophenylacetylene form quasi-planar cyclic complexes with water incorporating C-H center dot center dot center dot O and O-H center dot center dot center dot pi hydrogen bonds. In the case of phenylacetylene, 4-fluorophenylacetylene and 2-fluorophenylacetylene complexes the pi electron density of the acetylenic Ca parts per thousand C bond acts as an acceptor to the O-H center dot center dot center dot pi hydrogen bond, while in the case of 4-cyanophenylacetylene complex it's the pi electron density of the Ca parts per thousand N bond is the acceptor. A second water complex was also observed for 2-fluorophenylacetylene in which water interacts with fluorine atom and acetylenic Ca parts per thousand C bond rendering a double donor motif for the water molecule. One of the most interesting features of water complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene is the preference of pi hydrogen bond over sigma hydrogen bond