Using an ultrahigh vacuum scanning tunneling microscope (STM), we have explored the interactions of isolated five-membered heterocycles, pyrrole, thiophene, pyrrolidine, and tetrahydrothiophene, with the Cu(001) surface at 9 K. Pyrrolidine was also studied on the Ag(001) surface. Important distinctions in bonding, vibrational spectra, and vibrationally mediated negative differential resistance were observed with the aid of single-molecule inelastic electron tunneling spectroscopy (STM-IETS).