IRRAS metallic

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

To achieve surface selectivity in our studies of low area metal catalyzed reactions, we have developed FT-IRRAS techniques for the in situ study of surface adsorbates. With these methods, we are making progress toward examination of complex reactions. 

Preliminary results using fixed polarizer FT-IRRAS to study the melting and hydration of cadmium arachidate on low area metals show that both irreversible and reversible conformational changes can be... 

Two examples of the application of SERS and potential-difference IRRAS methods to the identification of adsorbed intermediates and reaction mechanism elucidation are also described, involving the catalytic electrooxidation of carbon monoxide and small organic molecules on transition-metal surfaces. 

Polyvinylimidazoles are effective anti-oxidants for copper at elevated temperatures. Below 250° C, there is no major degradation of the coated polyvinylimidazole films on copper. Furthermore, degraded polyvinylimidazole films can suppress oxidation even at 400 C. Finally, polyvinylimidazoles are more effective anti-oxidants than benzotriazole and imidazoles at elevated temperatures. It is also demonstrated that FT-IRRAS is a useful technique to study degradation of polymeric coatings and corrosion of metal simultaneously. 

Fig. 37. Schematic comparison between ATR-IR and IR reflection absorption (IRRAS) spectroscopy. In IRRAS, the sample (e.g., an electrode) is typically metallic and reflects the incident IR radiation. The IR beam has to pass through a liquid film twice.

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

The method has been applied, for example, in electrochemical investigations (110) and also for surface catalytic reactions in the presence of a gas phase 111). When PM-IRRAS is used with a thin-layer cell, as depicted in Fig. 37, the contribution from dissolved molecules in the liquid phase can be minimized. Still, the layer thickness has to be small to prevent complete absorption of the IR radiation by the solvent. The combination of polarization modulation and ATR for metal films was demonstrated recently and applied in an investigation of self-assembled octadecylmercaptan monolayers on thin gold films 112). This combination could emerge as a valuable technique for the investigation of model catalysts. 

With infrared reflection absorption spectroscopy (IRRAS), it is possible to obtain information about the orientation of enzyme molecules adsorbed on flat metal surfaces (3,4). Electric dipole-transition moments oriented perpendicular to a flat metal surface show enhanced IR absorbance. IR bands due to vibrations of groups with transition moments oriented parallel to the surface are not observed. The IR-beam component which is polarized perpendicular to the plane of incidence (parallel to the surface) contains no information and can be eliminated by using a polarizer. 

Infrared reflection-absorption spectroscopy (IRRAS) is done at fixed potential. Electric vectors in the incident beam parallel to the metal surface do not interact with adsorbed molecules, whereas those perpendicular to the surface do. The light beam is switched successively between the two directions and the results subtracted. 

For many years, IRRAS has been successfully applied to the study of thin films adsorbates on metal surfaces [36], In the case of monolayers deposited on metal surfaces, an IR external reflection spectrum is obtained by reflecting the incoming radiation from the three-phase ambient-adsorbate-substrate system, measuring the reflected intensity as a function of wavelength, and then ratioing... 

On a metallic substrate, PM increases the surface absorption detectivity of IRRAS by several orders of magnitude and provides high-quality monolayer spectra that can be quantitatively analyzed in terms of orientation and conformation of the surface molecules in a few minutes [85-88]. Moreover, due to the differential nature of the detected signal, these spectra are independent of the isotropic IR absorptions of the sample environment and water vapor interference is diminished. For these particular reasons, it appeared interesting to adapt PM-IRRAS method to the study of a monolayer spread at the air-water interface. 

F. H. Jardine in Chemistry of Platinum Group Metals, F. R. Hartley ed., Elsevier, New York, 1991 J. C. W. Lohrenz and H. Jacobson, Angew. Chem. Int. Ed. Engl. 1996,35, 1305 (cationic Irra complexes C—H activators). 

The first step of the reaction is a single-electron oxidation of the aromatic substrate to form a cation-radical [193,194]. Oxidizing agents for this reaction include Bronsted acids, oxidative Lewis acids, halogens (i.e., Br2), metal salts (e.g., Ti(CF3C02)3), electron-donor-acceptor complexes, irra-... 

In the first case, it has been recognized that if molecules are adsorbed on the surface of a metal, only part of the grazing incident radiation is reflected, part being absorbed by the adsorbed species. This absorption is greatly increased if the incident radiation is polarized perpendicular to the metal surface. This is the basis of so-called Infrared Reflection Absorption Spectroscopy (IRRAS) [27], which is applied widely to surface studies on metal surfaces. In these conditions, an additional selection rule applies ... 

The acid salts of this compd with heavy metals such as Pb and Ag and with alkaline earths such as Ba and Cd are termed 2,4-D irra-resorcimtes. These compds were patented by Briin and Burdett (Ref 2) as ingredients of some priming and ign compns. The salts of Ba, Cd,... 

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