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 XH⋅⋅⋅Y hydrogen bond (HB) is stronger in the triads than in the corresponding dyads, while the XH 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 ClH 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.