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-ALPY, 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 investigated at pH 3 and at pH 7 in 0.1M KCl electrolyte. SERS spectra show that both the neutral 4-ALPY and a species which is protonated on the aldehyde group (Py-CHOH+) adsorb on the roughened Ag electrode surface with an end-on orientation through the pyridine nitrogen, but the species with a protonated ring (H+Py-CHO) prefers adsorption with a face-on orientation through its pi-system. The electrochemical reduction of ALPY produces a common product, 1,2-bis(4-pyridyl)ethane (BPE), in both acidic and basic media. The major reaction process was studied by time-resolved SERS, TR-SERS, in the time range of 0.1 to 30 ms. A transient intermediate appears at 3.0 ms and is suggested to be the pinicol Py-CH(OH)CH(OH)-Py on the basis of a Gaussian 94 ab initio frequency calculation. It disappears by 10 ms, at which time the BPE product spectrum is established. The reduction of HMPY also undergoes similar reaction pathways in both acidic and neutral pH conditions, and the major product is the same in either condition. A striking fact is that the reduction 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 reactants and the product on the Ag electrode have been identified by SERS, and vibrational assignments of these species have been made.