JOURNAL OF RAMAN SPECTROSCOPY, zyxwvutsrqp VOL. 22, zyxwvut 197-203 (1991) Raman and Surface-Enhanced Raman Scattering of Violuric Acid and Violurate Ion Adsorbed on a Silver Electrode L. F. C. de Oliveira, P. S. Santos and J. C. Rubim Jnstituto de Quimica da Universidade de zyxwvutsrq Sgo Paulo, C.P. 20780,01498 Slo Paulo, SP, Brazil The Raman spectra of violuric acid zyxwvuts [ 2,4,6(1H,3H)-pyrimidinetrione-5-oxime, H,V I and sodium violurate (NaH,V) were investigated as solids and in aqueous solution at different pH. A tentative vibrational assignment is presented. From the Raman spectra obtained in solution it was possible to identify some characteristic Raman shifts for H,V, H,V- and HVI- (violurate dianion). The surface-enhnnced Raman (SER) spectra of the H,V- ion adsorbed on a silver electrode in borate buffer solution (pH zyxwvu 8.4) was ;also investigated. For potentials less negative than -0.5 V the H,V- anion adsorbs with the nitrogen atom of the oxime group and the oxygen atom of a neighbouring carbonyl group towards the surface. For potentials more negative the surface species undergoes reduction, with the oximate group (=N-O-) being reduced to form :an NH, group. The SER spectra of this new species are enhanced for potentials more negative than -0.7 V. INTRODUCTION Violuric acid [2,4,6(1H,3H)-pyrimidinetrione-5-oxime, H3V] (I) and its derivatives have been used in the spec- trophotometric determination of transition metal ions owing to their strong complexing character.' This char- acteristic has motivated the investigation of the struc- ture of several transition metal-violurate complexe~.~-~ The acid-base equilibrium of H3V in water is also well k n o ~ n . ' * ~ . ~ * ~ * ~ The first ionization of H,V gives the violurate anion (H2V-) (11): vibrational assignments of parent molecules such as barbituric acid [2,4,6( 1 H,3H)-pyrimidinetrione] ''*' ' and alloxan [2,4,5,6(1H,3H)-pyrimidinetetr0ne].'~ Since in aqueous solution the predominant species is the H2V - anion, the surface-enhanced Raman scat- tering (SERS) of this anion in borate buffer solution was also investigated. A study of the dependence of the SERS intensities on the applied potential is made in order to obtain information about the adsorption geometry and on the reduction products, as it is well known that some ketoxime or nitroso compounds can be reduced in a potential region where the silver elec- trode is still SERS active.', EXPERIMENTAL II I The further deprotonation of H2V- leads to the forma- tion of the violurate dianion, HV2- (pKa, 9.6). The pKa, corresponding to the third deprotonation is zyxwv ca 13.1, Of all these species, the most important from the point of view of coordination chemistry is the H2V- anion. It can be coordinated to transition metal ions through the carbonyl zyxwvutsrq 0-6 atom (monocoordinated)6 or as bidentate ligand through the lone pair of N-5 of the oxime group and the 0-6 Most of the papers dealing with the coordination chemistry of violurate present very little information about the vibrational spectrum of the ligand. Usually only a list of some IR frequencies for NaH,V and for the metal complexes are presented with a tentative as~ignment.~?~.~ The main purpose of this paper is to present for the first time the Raman spectra of violuric acid and its anions in the solid state and in aqueous solution. A ten- tative vibrational assignment is made based on the 03774486/91/040197-07 $05.00 zyxwvutsrqpo 0 1991 by John Wiley & Sons, Ltd. The Raman instruments used to obtain the Raman and SER spectra were described el~ewhere.'~.'~ All the SER spectra wlere obtained with an NaH,V concentration of z 0.002 M. The working solutions were prepared with doubly distilled water and chemicals of analytical purity. The working electrode was a silver rod (99.99% purity) of geometrical area 0.20 cm2 inserted in poly- tetrafluorethylene. A platinum foil of large area was used as an auxiliary electrode and a saturated calomel electrode (SCE) as the reference electrode. Before each experiment the working electrode was optically polished with alumina papers and the working solution was deaerated for 15 min with nitrogen. Infrared spectra were obtained in a Perkin Elmer Fourier transform IR 1750 instrument, using KBr pellets and1 Nujol mulls. NaH2V was prepared from 1 : 1 aqueous solutions of H,V (Merck) and NaOH. Received 4 October 1990 Accepted 18 October 1990