Hydrogen bonding to multifunctional molecules: spectroscopic and ab initio investigation of 4-ethynylbenzonitrile-(water)(1-3) complexes.

The water complexes of 4-ethynylbenzonitrile (4EBzN) were investigated with IR-UV double resonance spectroscopy. Water interacts with the pi electron density of the C[triple bond]N group in 4EBzN, leading to the formation of a quasiplanar cyclic complex incorporating C-H...O and O-H...pi hydrogen bonds. The (H(2)O)(2) and (H(2)O)(3) complexes of 4EBzN are characterized by the presence of hydrogen bonding bridges between the hydrogen bond donor (aromatic C-H group) and acceptor (C[triple bond]N group) sites present in 4EBzN. The present structures of the water complexes are similar to the corresponding complexes of benzonitrile but are drastically different from water complexes of phenylacetylene and fluorophenylacetylenes. The infrared spectra in the O-H stretching region indicate that the stability of all the water complexes of 4EBzN is lower than that of the corresponding complexes of benzonitrile, which can be attributed to the mild electron withdrawing ability of the ethynyl group present in the para position relative to the nitrile group. Hierarchally, the C[triple bond]N group has the higher propensity to form hydrogen bonding relative to the acetylenic group and the benzene ring in 4EBzN.