  A novel mechanism for switching a molecular junction based on a proton
transfer reaction triggered by an external electrostatic field is proposed. As
a specific example to demonstrate the feasibility of the mechanism, the
tautomers [2,5-(4-hydroxypyridine)] and {2,5-[4(1H)-pyridone]} are considered.
Employing a combination of first-principles electronic structure calculations
and Landauer transport theory, we show that both tautomers exhibit very
different conductance properties and realize the "on" and "off" states of a
molecular switch. Moreover, we provide a proof of principle that both forms can
be reversibly converted into each other using an external electrostatic field.
