The authors of this paper describe making vibrational frequency calculations for an Ag10-pyridine vertex complex with density functional theory (DFT) for static simulated spectra and with time-dependent DFT (TD-DFT) for pre-resonance and resonance simulated spectra using both B3LYP/LANL2DZ and BP86/TZP methodologies as well as their calculations of 40 excited states of the complex and assigned symmetry based on a C2v symmetry of the optimized complex found with B3LYP/LANL2DZ.
This article presents the authors’ calculations, methodology, and outcomes of a study on the charge-transfer surface enhanced Raman scattering (SERS) model from time-dependent density functional theory calculations for an Ag10-pyridine vertex complex. They report that molecular orbital isosurfaces showed the excited states involved both Ag10 intercluster excitations and charge-transfer (CT) excitations between the Ag nanocluster and the pyridine molecule. For the complex, they calculated simulated spectra at a variety of excitation wavelengths. In the case of 514 nm excitation, the simulated Raman cross section from the TD-DFT calculations 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 showed many contributions from non-totally 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 Ag10-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 a2 mode at 388 cm-1 and the moderately strong b2 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. Publisher Abstract Provided