A TIME-RESOLVED SERS STUDY OF THE ADSORPTION AND ELECTROCHEMICAL REDUCTION OF 4-PYRIDINECARBOXALDEHYDE AND 4-(HYDROXYMETHYL)PYRIDINE

The electroreduction processes of both 4-pyridinecarboxaldehyde, 4-ALP Y, and 4-(hydroxymethyl)pyridine, 4-HMPY, have been studied in aqueous solution on a Ag electrode using surface-enhanced Raman spectroscopy, SERS, and cyclic voltammetry, CV. Each electrode process was investig ated at pH 3 and at pH 7 in 0.1M KCl electrolyte. SERS spectra show th at both the neutral 4-ALPY and a species which is protonated on the al dehyde group (Py-CHOH+) adsorb on the roughened Ag electrode surface w ith an end-on orientation through the pyridine nitrogen, but the speci es with a protonated ring (H+Py-CHO) prefers adsorption with a face-on orientation through its pi-system. The electrochemical reduction of A LPY produces a common product, 1,2-bis(4-pyridyl)ethane (BPE), in both acidic and basic media. The major reaction process was studied by tim e-resolved SERS, TR-SERS, in the time range of 0.1 to 30 ms. A transie nt intermediate appears at 3.0 ms and is suggested to be the pinicol P y-CH(OH)CH(OH)-Py on the basis of a Gaussian 94 ab initio frequency ca lculation. It disappears by 10 ms, at which time the BPE product spect rum is established. The reduction of HMPY also undergoes similar react ion pathways in both acidic and neutral pH conditions, and the major p roduct is the same in either condition. A striking fact is that the re duction of HMPY on Ag produces the same product as that of the ALPY, i .e., 1,2-bis(4-pyridyl)ethane (BPE). The surface species for the react ants and the product on the Ag electrode have been identified by SERS, and vibrational assignments of these species have been made.