Surface-enhanced Raman spectroscopy of pyridine derivatives: Effects of adsorption on electronic structure

Abstract

The surface-enhanced Raman (SER) spectra of 2-, 3-, and 4-pyridinecarboxaldehydes, methylketones and 3- and 4-carbinols are compared with the spectra of the neat compounds and aqueous solutions, and effects of the adsorbate-substrate interactions which occur when these molecules are adsorbed on silver electrodes are identified. The 4-substituted carboxaldehyde and methylketone exhibit weaker carbonyl stretching bands when adsorbed on silver than in solution, while no such effect is found for the corresponding 3-substituted compounds. This is consistent with an interaction of the nitrogen lone-pair electrons with the silver, leading to electron withdrawal from 4-substituents and increased hydration of carbonyl groups at that position. Examination of solution spectra of the N-methylpyridinium salts shows a similar trend, with N-methylation increasing hydration and reducing the carbonyl band intensity in N-methyl-4-pyridiniumcarboxaldehyde but not in N-methyl-3-pyridiniumcarboxaldehyde. The surface-enhanced spectrum of 4-pyridinecarboxaldehyde is identical to that of 4-pyridylcarbinol, indicating that the aldehyde is completely hydrated on the electrode. Thus SER spectroscopy allows identification of a surface species which is quite different from that found in bulk solution. The reduction potentials of the pyridinecarboxaldehydes and methylketones are determined to show that no electrochemical reductions take place at the potentials used.