ChemPhysChem, 6, 1411-1418 (2005)

DOI: 10.1002/cphc.200500089

Cooperativity and Proton Transfer in Hydrogen-Bonded Triads

Ab initio MP2/6-311+G(3df,2pd) and MP2/aug-cc-pVTZ calculations have been carried out to investigate the structures and properties of AH⋅XH⋅YH3 (A=F, Cl; X=F, Cl; Y=N, P) hydrogen-bonded complexes. Significant cooperative effects are observed in the XH⋅YH3 dyads in the triads due to the presence of the polar near-neighbor AH. These effects are greater when the polar partner is HF, which is a better proton donor than HCl. Structural changes, red shifts of proton-donor stretching frequencies, nonadditive interaction energies, and electron density redistributions unambiguously demonstrate that the X[BOND]H⋅⋅⋅Y hydrogen bond (HB) is stronger in the triads than in the corresponding dyads, while the X[BOND]H bond of the proton donor becomes weaker. Even more pronounced cooperative effects are observed in the AH⋅XH dyads due to the presence of the YH3 partner. These effects are weaker in complexes having PH3 rather than NH3as the proton acceptor, since NH3 is a stronger base. Cooperativity also enhances the proton-donating ability of the YH3 moiety, with the result that all complexes except FH⋅FH⋅PH3 are cyclic. Cooperativity, together with the ease of breaking the Cl[BOND]H bond in ClH⋅ClH⋅NH3and FH⋅ClH⋅NH3, leads to proton transfer (PT), so that these two complexes are better described as approaching hydrogen-bonded ClHCl+HNH3 and FHCl+HNH3 ion pairs.