The observed shifts of the vibrational SERS frequencies with respect to those of the Raman spectrum of pyrazine have allowed us to conclude that the molecule adsorbs to the metal through the unshared electron pair of one of the nitrogen atoms. The same conclusion is reached by observing the behavior of the molecular vibrations when the electrode potential is changed. Only one type of molecules has been found with orientation perpendicular to the metal surface in the whole range of potentials studied. These conclusions are supported by the comparison of the experimental data with the ab initio results obtained for the vibrational frequencies of pyrazine isolated and coordinated to several conveniently selected silver clusters.