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Franck–Condon Dominates the Surface-Enhanced Raman Scattering of 3‑Methylpyridine: Propensity Rules of the Charge-Transfer Mechanism under Reduced Symmetry
journal contribution
posted on 2016-02-20, 06:46 authored by Silvia
P. Centeno, Isabel López-Tocón, Jessica Roman-Perez, Juan F. Arenas, Juan Soto, Juan C. OteroThe analysis of photoinduced charge transfer (CT) processes
in
a particular SERS spectrum is not a trivial task since each system
must be studied separately and no universal selection rules can be
proposed for this kind of enhancement mechanism. Despite this, we
found out in previous studies that the SERS-CT spectra of benzene-like
molecules are mainly characterized by the strong enhancement of the
totally symmetric 8a ring-stretching vibration and we have used this
as a propensity rule to recognize the presence of resonant CT processes
in the SERS of these adsorbates. However, it is necessary to cautiously
consider the relevant symmetry of the system under study before applying
this simplified rule. When the molecule shows a low symmetry (or the
relevant symmetry is reduced by the adsorption on the metal as it
was demonstrated in the case of pyrimidine), the selective intensification
of the 8a mode can be disguised by the redistribution of the CT enhancement
with another close vibration in wavenumber, namely the 8b mode, usually
a non-totally symmetric fundamental. 3-Methylpyridine is an illustrative
example of this given that similar SERS enhancement of both 8a and
8b modes can be quantitatively explained through Franck–Condon
factors without having to resort to Herzberg–Teller contributions.