Periodic trends in the bonding and vibrational coupling: Pyridine interacting with transition metals and noble metals studied by surface-enhanced Raman spectroscopy and density-functional theory

Pyridine adsorbed on the group-VIIIA transition metal and the coinage metal (IB) electrode surfaces is employed as a model system to explore the metal-dependent nature of surface bonding. The periodic trends of the surface-enhanced Raman spectra are interpreted with the density-functional theory calculation results of bond geometries, bond energies, vibrational frequencies, and the force constants. On the basis of the normal-mode analysis the vibrational frequency shifts of the pyridine internal modes with the total symmetry are discussed in detail from the coupling between these internal modes and the N–M bonds. We show that the spectral properties of the ring breath mode $(ν1)$ and the ring asymmetric deformation mode $(ν6a)$ are sensitive to the interaction between metal and pyridine molecule. Although the frequency shift of the $ν1$ mode is generally regarded as an indicator of the bonding strength, we find that the frequency shift of the $ν6a$ mode has a nearly linear dependence on the force constant of the N–M bond; and thus conclude that $ν6a$ is a better indicator than $ν1$ for the strength of pyridine binding with the metals.