Surface enhanced Raman spectroscopic study of isomeric methylthiophenes insilver colloid

Surface Enhanced Raman Spectra (SERS) of 2-methylthiophene (2-MT) and 3-met hylthiophene (3-MT) in saver colloid are compared with their normal Raman s pectra in the bulk and in acetonitrile solution. Experimental results indic ate that both the methylthiophene molecules are adsorbed on a metal surface through the sulfur atom. Assuming the image dipole theory to be valid, the observed surface enhancement factor of the C-N stretching mode in 2-MT ind icates that the C-H bond of this isomer makes an angle of 51 degrees with t he surface normal. The experimental results further suggest that the 3-MT m olecule is adsorbed with its ring plane almost vertical to the metal surfac e; the 2-MT molecule, on the other hand, is tilted more than 45 degrees to the surface normal. The concentration dependence of the relative enhancemen t factor indicates that a monomolecular layer is formed on the metal surfac e at a specific concentration of the methylthiophenes in silver sol. No oli gomerization of the two methylthiophenes on the silver surface is observed. (C) 1999 Academic Press.