Relationship between the Angular Characteristics of a H-bond and the Energetics of Proton Transfer Occurring Within

For a number of different H-bonded complexes, the protontransfer potential is computed by ab initio methods for the optimal geometry as well as for various angular distortions. In all cases, it is found that a rocking of one subunit A so as to turn its dipole moment away from the other subunit B causes the equilibrium position of the proton to shift toward A, even if B has a higher proton affinity than A. This shift may be explained on the basis of a better charge-dipole attraction in the preferred AH··B configuration as compared to A··HB. This principle is straightforward for the N bases where the dipole moment is approximately coincident with the N lone pair. The presence of two O lone pairs, neither of which points along the direction of the molecular dipole moment, adds a second factor which results in a net magnification of the above rule