  We propose a model for a molecular junction with internal anharmonic
torsional vibrations interacting with an electric current. The
Wangsness-Bloch-Redfield master equation approach is used to determine the
stationary reduced density matrix of the molecule. The dependence of the
current, excitation energy and angular momentum of the junction on the applied
voltage is studied. Negative differential conductance is observed in the
current-voltage characteristics. It is shown that a model with vibrationally
dependent coupling to the electrodes, asymmetric with respect to the
interchanging of electrodes, leads to a strong correlation between the applied
voltage and the angular momentum of the junction. The model thus works as a
molecular motor, with the angular momentum controlled by the size and sign of
the voltage.
