Extra hydrogen-bonding and water chain altering water-mediated ground-state multiple proton transfer in 2-aminopyridine: a theoretical study

A systematic study on the effect of extra hydrogen bonding (H-bonding) and water chain on ground-state multiple proton transfer (GSMPT) in 2-aminopyridine (2AP) complexes bonded with 1–3 water molecules was explored at the M06-2X/6-311+G(d, p) level. In 2AP-(H2O)n+m (n = 1–3, m = 1) complexes, n and m represented the number of water molecules which involved in PT process and formed an extra H-bonding with 2AP-(H2O)1–3, respectively. The analyses of structural parameter and correlation plot indicated that the H-bonded chain did not influence the GSPT mechanism, but affected the barrier height of GSPT. The H-bonded chain composed of water dimer in 2AP complex is more favorable to PT process than that of one water or water trimer. However, the extra H-bonding formed at different region had different effects on GSPT process. Evidently, when the extra H-bonding is formed in the proton donating and proton accepting regions, the concerted but asynchronous solvolysis pathway is not changed, but the asynchronicity of proton transfer is enlarged. When the extra H-bonding is formed at the bridging water region, 2AP-(H2O)3 maintains the concerted but asynchronous solvolysis pathway to occur GSPT, while 2AP-H2O undergo GSPT process in a concerted but asynchronous protolysis pathway. In addition, the extra H-bond formed in the proton accepting region is favorable to GSPT process. The GSPT mechanisms in 2AP-(H2O)1+1(B) and 2AP-(H2O)2+1(B1) in water change from protolysis pattern to solvolysis pattern. The solvation effect augments the asynchronicity of the concerted pathway and decreases the barrier height of GSPT.