POTENTIAL-DEPENDENT ORIENTATION AND OXIDATIVE DECOMPOSITION OF MERCAPTOALKANENITRILE MONOLAYERS ON GOLD - AN IN-SITU FOURIER-TRANSFORM INFRARED-SPECTROSCOPY STUDY

The potential dependent structural change and irreversible anodic deco mposition of the self-assembled monolayers of 3-mercaptopropanenitrile (HSC2CN) and of 8-mercaptooctanenitrile (HSC7CN) on gold electrode we re investigated by two modes of electrochemically modulated infrared s pectroscopy, namely, the subtractively normalized interfacial Fourier transform infrared reflection-absorption spectroscopy (SNIFTIRS) and t he difference spectra with only one potential alteration. In addition to the bands due to water molecules around 3100-3500 cm(-1) and 1600-1 700 cm(-1), bands were observed around 2930, 2850, 2342, and 2250 cm(- 1) at both the HSC7CN and HSC2CN modified gold electrodes. Except for the band at 2342 cm(-1), all the bands were observed only by using p-p olarized light, suggesting these bands are due to adsorbed species. Th e 2930 and 2850 cm(-1) bands are of the asymmetric and symmetric C-H s tretching modes of methylene groups, respectively. The intensities of these two bands decreased a little as potential became more positive. These results suggest that the alkyl chain stands closer to surface no rmal when potential became more positive. The peak position of the 225 0 cm(-1) band shifted slightly to lower frequency than that of the sam e band in neat liquid of corresponding mercaptoalkanenitrile and was n ot affected by potential so much. On the basis of these results the 22 50 cm(-1) band was assigned as the CN stretching mode of the terminal CN group, which has no direct interaction with gold. The fact that the absorbance decreased as potential became more positive suggests that the orientation of the terminal nitrile group became closer to paralle l to the surface. The 2342 cm(-1) band is due to CO2 in solution gener ated by an irreversible oxidative decomposition of monolayer as this b and was observed even by using s-polarized light and grew in the poten tial region where anodic current flowed. The irreversible anodic decom position of the monolayer in positive potential region was confirmed b y XPS measurements. Effects of alkyl chain length on both the potentia l dependence of peak position and intensity and the nature of anodic o xidation were also discussed.