Two-D imensional Model of a Double-Well Potential: Proton Transfer when a Hydrogen Bond is Deformed
The potential energy cross-section profile along a hydrogen bond may contain two minima in certain conditions; it is so-called a double well potential. The H -bond double well potential is essential for proton transfer along this hydrogen bond. We have considered the two-dimensional model of such double well potential in harmonic approximation, and we have also investigated the proton tunneling in it. In real environments thermal motion of atoms or conformational changes may cause reorientation and relative shift of molecule fragment forming the hydrogen bond and, as a result, the hydrogen bond is deformed. This deformation is liable to change the double well potential form and, hence, the probability of the proton tunneling is changed too. As it is shown the characteristic time of proton tunneling is essentially increased by even small relative shift of heavy atoms forming the H -bond and also rotational displacement of covalent bond generated by one of heavy atoms and the proton (hydrogen atom). H owever, it is also shown, at the certain geometry of the H -bond deformation the opposite effect occurred, i.e. the characteristic time is not increased and even decreased. N otice that such its behavior arises from two-dimensionality of potential wells; this and other properties of our model are discussed in detail.