C-H···O Hydrogen Bond Anchored Water Bridge in 1,2,4,5-Tetracyanobenzene-Water Clusters.

The hydrogen-bonded chain of water molecules is known to play a very important role in proton/H transfer in chemistry and biology. This kind of water chain mainly starts from a conventional hydrogen bond (HB) donor/acceptor site. Here, we report the experimental evidence of water chain formation on an unconventional C-H HB donor site in 1,2,4,5-tetracyanobenzene (TCNB). Laser-induced fluorescence (LIF) spectra and fluorescence dip IR (FDIR) spectra of 1: n ( n = 1-3) clusters of TCNB with water prepared in a supersonic molecular jet are presented. Quantum chemical calculations of several intuitive conformers of 1: n ( n = 1-3) clusters were performed, and the computed IR spectra were compared with the experimental FDIR spectra in order to get the structural information on the experimentally observed clusters. We find that the first water molecule binds to the C-H moiety of TCNB (1:1 cluster) and the subsequent water molecules form a water chain (1:2 and 1:3 clusters) that constitutes a bridge between the C-H and the proximal -CN moiety of TCNB, forming a cyclic ring structure. NBO calculations show that upon addition of water molecules significant change in the charge distribution takes place, mainly on the atoms which are involved in cyclic ring structure. This redistribution of charges causes a cooperativity effect in the higher water clusters of TCNB.