Relative intensities in surface‐enhanced Raman spectra of 9‐methoxy‐2‐N‐methylellipticinium chloride studied by potential energy distribution of the vibrational modes

Near-infrared Fourier transform (NIR-FT) Raman and surface-enhanced Raman spectroscopic (SERS) techniques were used in studies of the vibrational spectrum of 9-methoxy-2-N-methylellipticinium chloride. NIR-FT-Raman spectra were obtained on the powder and SER spectra were measured in silver colloids using dilute aqueous solutions of the drug (2 × 10−6 M, pH 7.5). Some Raman bands in the SER spectrum show increased relative intensities compared with the corresponding bands in the NIR-FT-Raman spectrum, whereas other bands show decreased relative intensities. A general valence force field was used in the calculation of the potential energy distribution of the vibrational modes. This calculation reveals that bands from the indole moiety of the molecule show intensity enhancements, whereas bands arising from modes localized in the isoquinolinium part of the molecule show decreasing intensities in the SER spectra. These results strongly indicate that the indole part of the molecule is close to or attached to the silver surface.