Adsorption of 1,4-phenylene diisocyanide on silver investigated by infrared and Raman spectroscopy

The adsorption characteristics of 1,4-phenylene diisocyanide (1,4-PDI) on silver have been investigated by means of infrared and Raman spectroscopy. Specifically, in the infrared spectroscopic study, we have employed three different sampling methods, i.e., reflection-absorption infrared (RAIR), surface-enhanced infrared (SEIR), and diffuse reflectance Fourier transform (DRIFT) spectroscopy. The SEIR and DRIFT spectral features of 1,4-PDI assembled, respectively, on vacuum-evaporated thin Ag film and 2-mu m-size Ag powders were both consonant with the RAIR spectral features observed on vacuum-evaporated thick Ag film, implying that the usual infrared surface selection rule was applicable even to SEIR and DRIFT spectroscopy. All three kinds of infrared spectral data clearly dictated that 1,4-PDI should be adsorbed on silver via the carbon lone-pair electrons of one isocyanide group assuming a vertical orientation with respect to the silver substrate. We could also obtain the surface-enhanced Raman (SER) spectrum of 1,4-PDI adsorbed on powdered silver that was used in the DRIFT spectroscopic study. Although an unequivocal SER selection rule has not yet been established, the same conclusion as that from the infrared data could be derived from the peak shift, band broadening, as well as from the presence of the ring C-H stretching band in the SER spectrum.