Hydrogen bonding to multifunctional molecules: spectroscopic and ab initio investigation of water complexes of fluorophenylacetylenes.

The water complexes of 4-fluorophenylacetylene and 2-fluorophenylacetylene were investigated using IR-UV double resonance spectroscopy. Both 4-fluoro- and 2-fluorophenylacetylenes form a cyclic complex with water incorporating C-H...O and O-H...pi hydrogen bonds. These structures are similar to the phenylacetylene-water complex, implying that the fluorine substitution on phenylacetylene does not alter the intermolecular structure. Further, the presence of fluorine enhances the interaction of water with the acetylenic pi electron density. This behavior of fluorophenylacetylenes is dramatically different from that of fluorobenzene and fluorostyrene. A second water complex was also observed in the case of 2-fluorophenylacetylene in which water interacts with fluorine atom and acetylenic C-C triple bond in a double-donor fashion. Additionally, two distinct 2-fluorophenylacetylene-(water)(2) complexes were also observed. The first is a cyclic complex in which two water molecules bridge the hydrogen bond donor and acceptor sites present in 2-fluorophenylacetylene. The second is a kinetically trapped higher energy structure in which one water molecule acts as a double-acceptor.