Raman optical activity (ROA) and surface-enhanced ROA (SE-ROA) of (+)-(R)-methyloxirane adsorbed on a Ag20 cluster

Abstract

Chirality is ubiquitous in nature and plays an important role in biochemistry because biological function is largely dependent on the handedness of chiral molecules. Methods for determining chirality essentially measure optical activity. Surface plasmons have been demonstrated to hugely enhance the optical activity of chiral molecules. As with surface-enhanced Raman spectroscopy, when chiral molecules are adsorbed on the surface of plasmonic particles, chemical enhancement of the Raman optical activity (ROA) also occurs. Herein, we studied the theoretical Raman optical activity and surface-enhanced Raman optical activity of the chiral molecule methyloxirane, compared its vibrational modes and elucidated the new vibrational modes resulting from chemical enhancement. We found that upon adsorption on the Ag₂₀ cluster, three modes exhibited a weaker ROA (even none at all), whereas two modes exhibited a stronger ROA (which was zero without adsorption on Ag₂₀). This was attributed to changes in the symmetry of methyloxirane upon adsorption on the Ag₂₀ cluster. A chiral molecule adsorbed on metal particles leads to changes in the dipole moment such that the intensity of some Raman vibrational modes is enhanced, and the symmetry broken responses for the changing ROA.