  The influence of vibrational motion on electron conduction through single
molecules bound to metal electrodes is investigated employing first-principles
electronic-structure calculations and projection-operator Green's function
methods. Considering molecular junctions where a central phenyl ring is coupled
via (alkane)thiol-bridges to gold electrodes, it is shown that -- depending on
the distance between the electronic $\pi$-system and the metal --
electronic-vibrational coupling may result in pronounced vibrational
substructures in the transmittance, a significantly reduced current as well as
a quenching of negative differential resistance effects.
