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Simulation of SERS by a DFT study: a comparison of static and near-resonance Raman for 4-mercaptopyridine on small Ag clusters

We have used density functional theory at the B3LYP/LANL2DZ level to calculate the static (DFT) and frequency-dependent (TDDFT) Raman spectra of complexes of 4-mercaptopyridine (4-Mpy) binding with Ag10 and Ag13 clusters via the thiolate Ag-S bond. Geometric optimization of the Ag10-4-Mpy complexes with plus one or neutral charge had a bridging surface structure with the bond between two Ag atoms of the cluster; whereas, the minus charged Ag10-4-Mpy complex had an on-top structure directly bound to one Ag atom. A neutral Ag13-4-Mpy complex showed the on-top structure also. The static (DFT) Raman tensor for these complexes showed the spectra of the structures with bridging geometry were very similar and the static spectra of the on-top structures were also very similar. New assignments for 4-Mpy anion and 4-Mpy bound to the Ag13 cluster were made using the assignment method of Gardner and Wright (2011 J. Chem. Phys. 135 114305) which proves to be more accurate than Wilson numbers. The calculated frequencies of the normal vibrational modes were found to be sufficiently close to several experimental surface enhanced Raman scattering spectra on a variety of different Ag substrates to clarify the previous assignments. The frequency-dependent (TDDFT) Raman calculations were carried out in the infinite life-time approximation. Calculation of the UV–VIS spectra of the isolated 4-Mpy and of the Ag13-4-Mpy complex showed that binding shifts the spectra from deep in the UV to the near-IR and visible region. Calculation of the frequency-dependent (TDDFT) near-resonance Raman spectrum of this complex at 637 nm (resonance at 632 nm) showed a strong enhancement. However, within the infinite life-time approximation, the actual enhancement cannot be estimated. The transition involves a charge transfer from the 4-Mpy molecule to the Ag cluster with a weak oscillator strength, f = 0.0162. The relative intensities for the near-resonance spectrum of Ag13-4-Mpy complex are found to be very different from the static spectrum of the complex. Examination of the integrated enhancement factor, EF, as a function of the vibrational mode symmetry for near-resonance shows the order of relative enhancements to be b2 > b1 > a2 > a1. This order indicates that a charge-transfer resonance mechanism is operating with Herzberg–Teller scattering dominating Franck–Condon scattering for this simulation. Conditions prevailing in such a case are also discussed.