SURFACE-ENHANCED RAMAN-SPECTROSCOPIC STUDY OF ISOMERIC FORMYL PYRIDINES IN SILVER COLLOID

Surface enhanced Raman (SER) spectra of ortho, meta, and para formyl p yridines in silver sol are compared with their normal Raman spectra in bulk and aqueous solution. Significant enhancement of Raman bands has been observed. The SER spectra suggest an almost normal orientation o f the adsorbate molecular planes to the metal surface and also reveal that ortho and para isomers exist in a hydrate form in the surface ads orbed state. The meta isomer, however, retains its original nicotinald ehyde form. On addition of the formyl pyridines (FP), the nu(max) of t he bands that appear on the longer wavelength side of the electronic a bsorption band of stable, neat silver sol show a parallel relationship with the Hammett sigma (sigma(H)) values of the isomers. Raman excita tion wavelength dependence suggests that resonance to the aggregation band contributes significantly to SER spectra. The concentration depen dence of the enhancement ratio shows a maximum, suggesting monolayer f ormation at a specific concentration for each isomer. This ''first-lay er effect'' and also the excitation wavelength dependence of SER spect ra imply a charge transfer contribution to enhancement. (C) 1996 Acade mic Press, Inc.