A quantum chemical study of bonding interaction, vibrational frequencies, force constants, and vibrational coupling of pyridine-M-n (M = Cu, Ag, Au; n=2-4)
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The binding interactions between the pyridine and small noble metal clusters in different sizes (n = 2-4) have been investigated by using quantum chemical methods. The binding energies of Py-M-2 complexes are obtained at the levels of the Hartree-Fock method (HF), the second-order Moller-Plesset perturbation theory (MP2), the local density functional method (SVWN), the nonlocal density functional method (BLYP, BPW91, G96LYP, G96PW91), and the hybrid density functional method (B3LYP and B3PW91). All calculated results Shaw that the bonding is stronger in pyridine/copper and pyridine/gold than that in pyridine/silver. The bonding mechanism is explored in terms of the bonding molecular orbital properties. The donation interaction of the lore-pair electrons on nitrogen of the pyridine molecule to the unoccupied orbital of each metal cluster plays an important role. The force constants of the internal coordinates of interests are presented. The vibrational frequency shift has been analyzed on the basis of the coupling between the internal vibrational modes of pyridine and the nitrogen-metal stretching modes as well as the metal-metal stretching modes. For low-frequency Raman spectra of pyridine-small silver cluster complexes, we propose a new assignment to the N-Ag and Ag-Ag stretching vibrations. The calculated infrared intensities of vibrational modes are compared with the experimental spectra.