Density functional study and normal-mode analysis of the bindings and vibrational frequency shifts of the pyridine-M (M = Cu, Ag, Au, Cu+, Ag+, Au+, and Pt) complexes
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Raman and infrared (IR) spectroscopies have been used extensively to study the adsorption of pyridine on various surfaces for more than 40 years. However, no satisfactory assignment on all the vibrational modes has been achieved. In the present study, density functional theory at the level of B3LYP/6-311+G** (for C, N, H)/LANL2DZ (for metals) has been used for the normal coordinate calculations of the neutral and cationic species of the pyridine-metal atom (ion) complexes. Based on the present calculations, new assignments of the fundamental frequencies for the bands attributed to nu(17a), nu(3), nu(5), and nu(18b) modes of the free pyridine in the reported IR and Raman spectra have been suggested. The calculated frequency shifts indicate that the coupling among nu(1), nu(12), and nu(18a) modes and the coupling between these mode,, and the N-metal (Cu, Ag, Au, or Pt) bond depend on the strength of the N-metal bond and its bonding properties. The fundamental frequency of the nu6(a) mode has a nearly linear relationship with the change of the force constant of the N-M bond, Therefore, its frequency shift can be used to directly correlate to the strength of the interaction of pyridine with the metal atom (surface).