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1.
Phys Chem Chem Phys ; 20(46): 29430-29439, 2018 Nov 28.
Artigo em Inglês | MEDLINE | ID: mdl-30452048

RESUMO

Surface-enhanced Raman scattering (SERS) spectroscopy of pyridazine shows the selective enhancement of the bands recorded at about 1570, 1450 and 380 cm-1, which are assigned to two different types of vibrations. The first two correspond to in-plane 8a;νring and 19b;δ(CH) totally symmetric A1 modes, respectively, while the last band is assigned to the out-of-plane 16b;τring,B1 vibration. The selective enhancement has been analyzed on the basis of a resonant Raman process involving photoexcited metal (M)-to-molecule (A) charge transfer (CT: M-A + hν → M+-A-) states of the metal-adsorbate surface complex, which have also been related to the doublet electronic states of the corresponding radical anion of the adsorbate (A-). According to the selection rules of the electromagnetic/plasmonic SERS enhancement mechanism, the simultaneous enhancement of in-plane and out-of-plane modes could be attributed to different orientations of the adsorbate (perpendicular and parallel, respectively, or tilted) with respect to the metallic surface. The calculated resonance Raman-CT spectra (SERS-CT) and the vibrational wavenumbers of isolated pyridazine and of the pyridazine-Ag2 complex obtained from electronic structure calculations suggest a single type of molecule adsorbed with perpendicular orientation. The relative SERS enhancements of both in-plane and out-of-plane modes are due to Franck-Condon factors related to differences between the equilibrium geometries (A1 vibrations, ΔQ ≠ 0) and gradients (B1 vibrations, Δν ≠ 0 and ΔQ ≠ 0), respectively, of the potential energy surfaces of the involved ground and photoinduced CT electronic states. Therefore, the selective enhancement of the SERS bands of pyridazine is controlled by a general metal-to-molecule resonant CT mechanism. This conclusion supports the usefulness of SERS in studying the subtle electronic structure of charged interfaces as well as key processes such as electron transfer at the nanoscale.

2.
J Phys Chem C Nanomater Interfaces ; 128(30): 12566-12574, 2024 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-39109355

RESUMO

Electrochemical SERS of pyridine adsorbed on a silver electrode has been analyzed by comparing the spectra to the calculated normal Raman and resonance Raman intensities of model systems of pyridine bonded to linear silver clusters with different densities of charge through the nitrogen (Ag-NPy) or flipped through the hydrogen in the para-position (Ag-HPy). The changes observed in the ν(CH) region of the SERS have been investigated for the first time and related to a molecular reorientation at negative surface excess of charge of the metal in such a way that the ν(CH) bands with the highest (mode 2) and lowest (mode 13) wavenumber dominate this spectral region at positive or negative electrode potentials, respectively. The calculations support that the ν(CH) region is dominated by a specific vibration depending on pyridine orientation and suggest that both species coexist in the SERS recorded at negative potentials. This conclusion is supported by the SERS of centrosymmetric pyrazine which do not show this behavior and remembers the predictions from the old propensity rules of the so-called electromagnetic mechanism of SERS.

3.
Front Chem ; 7: 423, 2019.
Artigo em Inglês | MEDLINE | ID: mdl-31231641

RESUMO

Vibrational wavenumbers of pyridine adsorbed on a silver electrode have been correlated to the calculated ones from different theoretical approaches based on DFT methods. The vibrational tuning caused by the electrode potential has been simulated by means of pyridine-silver clusters with different densities of charge or, alternatively, under applied external electric fields. Both methodologies predict correctly a qualitative red-shift of the vibrational wavenumbers at negative potentials. As a result, harmonic frequency calculations performed at the B3LYP/LanL2DZ level of theory by using a linear [Ag n Py] q complex model with different densities of charge (q eff = q/n) have exhibited the best agreement with the experimental observations although the tuning amplitudes are overestimated. Electric fields calculations are unable to account for subtle details observed in the spectra related to the differentiated chemical nature of the metal-molecule bond at positive or negative potentials with respect to the potential of zero charge of the electrode.

4.
Nanomaterials (Basel) ; 9(9)2019 Aug 28.
Artigo em Inglês | MEDLINE | ID: mdl-31466241

RESUMO

A Surface-Enhanced Raman Scattering (SERS) spectrum of 4-cyanopyridine (4CNPy) was recorded on silver plasmonic nanoparticles and analyzed by using Density Functional Theory (DFT) calculations. Two simple molecular models of the metal-4CNPy surface complex with a single silver cation or with a neutral dimer (Ag+-4CNPy, Ag2-4CNPy), linked through the two possible interacting sites of 4CNPy (aromatic nitrogen, N, and nitrile group, CN), were considered. The calculated vibrational wavenumbers and intensities of the adsorbate and the isolated species are compared with the experimental Raman and SERS results. The analysis of the DFT predictions and the experimental data indicates that 4CNPy adsorbs preferentially on neutral/charged active sites of the silver nanoparticles through the nitrogen atom of the aromatic ring with a perpendicular orientation.

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