Ground-State Long-Range Proton Transfer Controlled by Proton-Accepting Ability of Hydrogen-Bonded Chains: A Theoretical Study

The ground-state triple proton transfer (GSTPT) reactions in HCOOH complexing with H₂O, CH₃OH, C₂H₅OH and mixed water–alcohol molecules were studied by quantum mechanical methods in the gas phase and in heptane. The triple proton transfer in HCOOH–S₁–S₂ (S₁, S₂ = H₂O, CH₃OH, C₂H₅OH) systems all occurred in an asynchronous but concerted protolysis mechanism. The formation pattern of the hydrogen-bonded chain was important to reduce the barrier height of the proton transfer process. When the hydrogen-bonded chain consisted of two identical CH₃OH or C₂H₅OH molecules in the HCOOH–S₁–S₂ complexes, the GSTPT barrier height of HCOOH–S₁–S₂ decreased by more than 2 kcal mol⁻¹ compared to that of HCOOH–H₂O–H₂O both in the gas phase and in heptane, because CH₃OH and C₂H₅OH had larger proton-accepting abilities than had H₂O. When the two solvent molecules in the hydrogen-bonded chain in the HCOOH–S₁–S₂ complexes were different, the barrier height of the proton transfer process varied depending on the proton-accepting ability (basicity) of the hydrogen-bonded chain. The bigger the proton-accepting ability (basicity) of the hydrogen-bonded chain, the lower the barrier height of the proton transfer process. Mixed bridging solvent molecules could accumulate their proton-accepting abilities and thus speeded up proton transfer. The solvent effect evidently decreased the zero point energy-corrected barrier heights of HCOOH clusters and increased the asynchronicity of the proton transfer, while the proton transfer mechanisms did not change in heptane.