Development of Methods to Simulate Resonance Raman Spectra Employing Density Functional Theory: Application to the Investigation of the Chemical Mechanism of Surface Enhanced Raman Scattering by Examination of Pyridine on Silver Clusters.

摘要

A method for the calculation of resonance Raman cross sections is presented on the basis of structural differences between optimized ground and excited state geometries using density functional theory. A vibrational frequency calculation of the molecule is employed to obtain normal coordinate displacements for the modes of vibration. The excited state displacement relative to the ground state can be calculated in the normal coordinate basis by means of a linear transformation from a Cartesian basis to the normal one. The displacements in normal coordinates are then scaled relative to the classical turning point of the molecule to calculate dimensionless displacements for use in the two -- time -- correlator formalism for the calculation of resonance Raman spectra at an arbitrary temperature. This method is valid for Franck -- Condon active modes within the harmonic approximation and is validated by calculation of resonance Raman cross sections and absorption spectra for nitrate ion, chlorine dioxide, trans -- stilbene, 1, 3, 5 -- cycloheptatriene, and the aromatic amino acids. This method permits significant gains in efficiency of calculating resonance Raman cross sections form first principles and, consequently, permits extension to large systems ( > 50 atoms).;As an application, the chemical mechanism of surface enhanced Raman scattering (SERS) was investigated by studying super -- molecules consisting of Ag clusters with a bound pyridine, Agn -- Pyridine (n = 2, 4, 8, 14, and 20). Calculation of the excited state displacements of pyridine on Ag clusters were applied to the two -- time -- correlator formalism for the calculation of resonance Raman cross sections. The goal of the study is to understand the contribution of resonance Raman scattering to the chemical mechanism enhancement in SERS. Based on three theoretical observations, it is apparent that resonance Raman enhancement is a major contributor to the chemical enhancements observed in SERS when treating the silver clusters with bound pyridine as a super -- molecule. First, structural changes in pyridine observed in the excited state displacement for all silver -- pyridine molecules were essentially equivalent, and similar to those observed for the 168 nm transition of free pyridine. Secondly, the excited state displacement leads to resonance Raman scattering with cross sections on the magnitude of ∼ 109 A2/molecule. Thirdly, enhancements of the magnitude observed for a typical resonance Raman experiment, ∼ 103 -- 106, were calculated for all silver -- pyridine clusters. Given that traditional SERS effects range from 103 -- 106, the study suggests that SERS may be dominated by the contribution from resonance Raman. This study does not rule out the role of the electromagnetic enhancement, but rather suggests that the chemical enhancement should be reconsidered as a significant contribution.