Prediction of increased tunneling current by bond length stretch in molecular break junctions

We use ab initio calculations of the tunneling current through a 1,4-phenylenedimethanethiol (XYL) molecule adsorbed between Au(1 1 1) electrodes to show that there are circumstances under which tunneling currents can be increased by bond stretching. The effect is dependent upon the geometry of the gold–sulfur bond and does not occur for the physisorbed thiol, that is, a thiol with the hydrogen atom still in place. However, we predict that when the hydrogen atom is removed, causing the sulfur atom to be strongly bound to the gold surface, stretching this bond away from equilibrium will actually increase the tunneling current.