Surface Enhanced Raman Spectroscopic Study of Isomeric Methylthiophenes in Silver Colloid

Surface Enhanced Raman Spectra (SERS) of 2-methylthiophene (2-MT) and 3-methylthiophene (3-MT) in silver colloid are compared with their normal Raman spectra in the bulk and in acetonitrile solution. Experimental results indicate 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-H stretching mode in 2-MT indicates that the C-H bond of this isomer makes an angle of 51 degrees with the surface normal. The experimental results further suggest that the 3-MT molecule is adsorbed with its ring plane almost vertical to the metal surface; the 2-MT molecule, on the other hand, is tilted more than 45 degrees to the surface normal. The concentration dependence of the relative enhancement factor indicates that a monomolecular layer is formed on the metal surface at a specific concentration of the methylthiophenes in silver sol. No oligomerization of the two methylthiophenes on the silver surface is observed. Copyright 1999 Academic Press.