Silvia P. Centeno

How a resonant charge transfer mechanism determines the relative intensities in the SERS spectra of 4-methylpyridine

Abstract The surface-enhanced Raman scattering (SERS) spectra of 4-methylpyridine (4-MP) have been recorded at different electrode potential versus saturated Ag/AgCl/KCl reference electrode. By comparing the relative intensity of the SERS with the Raman spectrum of the aqueous solution it is possible to determine that 12, 6a, δ(CH) and especially 8a modes are strongly enhanced. In this work, it is demonstrated that these vibrations are closely related to Franck–Condon factors of a resonant photoinduced charge transfer (CT) mechanism similar to a resonance Raman (RR) process. The theoretically calculated ΔQ displacements represent the differences between geometries of the potential energy minima of the states involved in the resonant process, allowing to calculate the relative intensities in RR from the Peticolas’ equation. These calculated intensities are in a perfect agreement with the experimental behavior.

Selection rules for the charge transfer enhancement mechanism in SERS: dependence of the intensities on the L-matrix

Given the similarity between the charge transfer mechanism in the SERS effect and the resonance Raman (RR) process, the strongest enhanced bands in the SERS spectra can be explained through the Franck‐Condon factors (A-term in RR). These factors are numerically related to the DQ vector, which represents the displacement of the potential energy minimums of the two states involved in the resonant process along each normal coordinate. DQ is calculated through the transformation DQa L 21 DR and related to the intensity of the band by Peticolas’ equation. The dependence of the DQ displacements and therefore of the calculated intensities on the L-matrix has been studied for the SERS spectra of pyridine. q 2001 Elsevier Science B.V. All rights reserved.

Photoinduced charge transfer processes in the surface-enhanced Raman scattering of 2,4,6-trimethylpyridine recorded on silver electrode

Abstract Surface-enhanced Raman scattering spectra of 2,4,6-trimethylpyridine recorded on silver at different electrode potentials have been analyzed on the basis of a charge transfer enhancement mechanism. The main feature of surface-enhanced Raman scattering is the enhancement of the band corresponding to the 8a mode at negative electrode potentials. This behavior can be explained by Franck–Condon factors related to a photoinduced electron transfer process between the metal and the adsorbate. The selective surface-enhanced Raman scattering enhancement is related to the differences between the ab initio equilibrium geometries of the electronic levels involved in the resonant mechanism, i.e., the ground states of the neutral adsorbate and the respective radical anion.

A method to improve the agreement between calculated and observed vibrational frequencies after scaling of a quantum mechanical force field

A systematic method to fit calculated to observed vibrational frequencies has been developed and implemented in a computer program. The procedure consists of the refinement of a scaled quantum mechanical force field (SQMFF) previously obtained according to Pulay’s method. The key step in the process is the generation of an intermediate matrix, CΛCT, which is then refined. The above step produces only small corrections to the scaled force constants, yielding a considerable improvement of the fitted frequencies. This scheme of refinement can be carried out using any kind of coordinates. To show the reliability and performance of the proposed method, the force fields of two very different systems, as benzene and tetranitromethane, have been chosen as example tests.

Raman spectroscopic study of a genetically altered kidney cell

A Raman spectroscopic investigation of a genetically altered Human Embryonic Kidney Cell (HEK293) along with a pathologically normal cell has been carried out by a conventional method. The genetic alteration was carried out with a standard protocol by using a Green Fluorescence Protein (GFP). Raman spectra show that there are dramatic differences between the spectrum obtained from a genetically altered cell and that obtained from a pathologically normal cell. The former shows three broad bands; meanwhile the latter shows several sharp peaks corresponding to the ring vibrational modes of Phen, GFP and DNA. The present analysis provides an indication that the force field near Phen located at 64, 65 and 66 was altered during the genetic transformation. The Raman spectrum could be a direct experimental evidence for substantial modifications triggered due to the expression of specific genes.