# Remembering the Old Propensity Rules of the Electromagnetic Enhancement Mechanism of SERS: Reorientation of Pyridine on a Silver Electrode Induced by the Applied Potential

## TL;DR

The paper explores how pyridine molecules reorient on a silver electrode under different potentials, affecting SERS spectra and linking it to old electromagnetic enhancement rules.

## Contribution

The study provides new insights into molecular reorientation of pyridine on silver electrodes and its impact on SERS under varying potentials.

## Key findings

- Molecular reorientation of pyridine on silver electrodes affects ν(CH) bands in SERS spectra.
- At negative potentials, both Ag-NPy and Ag-HPy species coexist in SERS.
- Pyrazine's SERS does not show this behavior, supporting old electromagnetic enhancement rules.

## Abstract

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.