A charge-transfer surface enhanced Raman scattering model from time-dependent density functional theory calculations on a Ag10-pyridine complex

Vibrational frequency calculations were made for a Ag-10-pyridine vertex complex with density functional theory (DFT) for static simulated spectra and with time-dependent DFT (TD-DFT) for preresonance and resonance simulated spectra using both B3LYP/LANL2DZ and BP86/TZP methodologies. In addition, 40 excited states of the complex were calculated and assigned symmetry based on a C-2v symmetry of the optimized complex found with B3LYP/LANL2DZ. Molecular orbital isosurfaces show that the excited states involve both Ag-10 intercluster excitations and charge-transfer (CT) excitations between the Ag nanocluster and the pyridine molecule. An excitation around 500 nm involving CT from the Ag cluster to pyridine was found in both calculations. For free pyridine, the relative average deviations between unscaled calculated and experimental results were 1.5 cm(-1) for BP86 and 3.1 cm(-1) for the B3LYP calculations. For the complex, simulated spectra at a variety of excitation wavelengths were calculated. In the case of 514 nm excitation, the simulated Raman cross section from the TD-DFT calculations (near the CT resonance) was plotted versus Raman shift frequency and compared with an experimental surface enhanced Raman scattering (SERS) spectrum obtained on an oxidation-reduction cycle, ORC roughened Ag electrode. The BP86 TD-DFT calculation with finite damping term showed a better fit to experimental spectrum with respect to both relative intensities and frequencies. The average deviation of the unscaled BP86 calculations for 16 bands in the experimental spectrum was 13.0 cm(-1). The calculated spectrum in both cases shows many contributions from nontotally symmetric as well as totally symmetric modes, indicating the contribution of Herzberg-Teller (HT) scattering. The simulated intensities of the Raman modes of different symmetry from Ag-10-pyridine can be correlated with HT intensity borrowing from excited states of given symmetry and decent oscillator strength. These results explain the appearance of the a(2) mode at 388 cm(-1) and the moderately strong b(2) mode at 1573 cm(-1) found in the SERS experimental spectrum. The good agreement between experimental and simulated results indicates that a small Ag nanocluster-pyridine vertex complex can be used to simulate spectra with good relative intensities, if not absolute intensities, and accurate frequencies for most of the SERS bands for adsorbed pyridine on roughened bulk Ag.