Effects of External Ions on the Energetics of Proton Transfers across Hydrogen Bonds

Ions of both positive and negative charge are placed in various locations around H bonds and energetics of proton transfer within the bonds calculated by ab initio molecular orbital methods. Ions examined include Li+, Na+, C1-, (NH4)+, HCOO-, and point charges of both signs. These ions are found capable of producing profound effects upon the transfers, with the most marked changes OcCwTing when the ions are positioned along the H-bond axis. Approach of each ion toward the symmetric (H20-H-OH2)+ system introduces progressively greater degrees of asymmetry into the transfer potential. As a cation approaches the nitrogen atom of (H3N-H-OH2)+, a second well appears in this potential which, in the absence of external charges, contains only one minimum. The new well becomes progressively stabilized under closer approach of the ion until the (H3N- -HOH2)+ configuration is equal in energy to (H3NH- -OH2)+ at a distance of 3 A. The effects of all ions, including the point charges, are quite similar to one another, particularly for distances greater than 3 A. The perturbations in the transfer potentials may be modeled quite closely by including the Coulombic interaction between a point charge and the proton undergoing the motion; however, the remaining atoms of the H-bonded system act to magnify to nearly double this effective field