Adsorbate orientation

If values of AG° i as a function of electrode potential are reported only maximum values are fisted except for data reported for specific adsorbate orientations. 

The EMIRS and SNIFTIRS methods provide the IR vibrational spectra (really the difference spectra - see later) of all species whose population changes either on the electrode surface or in the electrical double layer or in the diffusion layer in response to changing the electrode potential. Spectra will also be obtained for adsorbed species whose population does not change but which undergo a change in orientation or for which the electrode potential alters the Intensity, the position or shape of IR absorption bands. Shifts in band maxima with potential at constant coverage (d nax 6 very common for adsorbed species and they provide valuable information on the nature of adsorbate/absorbent bonding and hence also additional data on adsorbate orientation. 

Examination of the azide bending-mode region (600-700 cm 1) in the SER spectra (Figure 2) is also instructive with regard to adsorbate orientation. Thus, the pair of bands (at ca. 610 and 670 cm"1) seen at the least negative potentials are characteristic of end-on coordinated azide (15) the loss of the lower-frequency partner for E < -0.15 V is therefore also indicative of the removal of azide bound in this adsorbate geometry, again in harmony with the interpretation of the infrared spectra (7). 

Chemisorption Adsorbate Orientation Competitive Chemisorption Adsorbate Exchange Adsorbate Reactivity Electrocatalvsis Synthesis Mode of Coordination Ligand Substitution Ligand Exchange Ligand Reactivity Homoqeneous Catalysis... 

This is, however, an illustration of the importance of taking adsorbate deformations into account, and the usefulness of theoretical modelling for doing so. A computationally optimized 1 shifted adsorption geometry is shown in figure 5 (right), and this has an adsorbate orientation very close to the desired 45°, and not the simplistically expected 60°. Instead, the deformed 2 shift structure does not have a 45° orientation, but rather one of 35°. 

Compound 22, Fig. 27(a), and related compounds exhibit three adsorbed orientations... 

The enantioselectivities of reactions on chiral surfaces are of interest from a practical standpoint and are the result of enantiospecific differences in reaction energetics and reaction barriers. Another manifestation of the enantiospecific interaction between a chiral adsorbate and a chiral surface is adsorbate orientation. Enantiospecific orientations of chiral adsorbates on naturally chiral metal surfaces have been predicted by molecular simulation studies. The first studies using Monte Carlo methods to study chiral cycloalkanes adsorbed on chiral surfaces pre-... 

Although infrared absorption intensities are very sensitive to molecular orientation, deriving quantitative information about molecular orientation is not easy [33,34], On the other hand, photoelectron diffraction is relatively easily interpreted to yield adsorbate orientations on surfaces. X-ray photoelectron diffraction and density functional theory calculations have been used in tandem to study the orientations of D- and L-cysteine adsorbed on the Au(17, 11, 9) surface (Fig. 4.9) [35]. Cysteine is an amino acid with the functional group R = CH SH. On the gold surface the S-H bond dissociates to give a thiolate bond to the surface. X-ray photoelectron diffraction of the N Is level indicates that the N-C bond in o-cysteine is oriented roughly parallel to the step edge on the Au(17, 11, 9) surface while the N-C bond in... 

Street SC, GeUman AJ (1996) Quantitative adsorbate orientation from vibrational spectra Ethoxides on Cu(lll). J Chem Phys 105 7158... 

Hines M. A., Harris T. D., Harris A. L. and Chabal Y. J. (1993), Looking up the down staircase surface Raman spectroscopy as a probe of adsorbate orientation , J. Electron Spectrosc. Relat. Phenom. 64—65, 183-191. 

Allen and Van Duyne" " have correlated the Raman intensity for the cyano group in 2-, 3-, and 4-cyanopyridine to its direction with respect to the surface. They found that the molecule for which this group is parallel to the surface exhibits the weakest signal. Creighton" " discussed the possibility of determining adsorbate orientation from SERS relative intensities. The conclusion is that it is feasible, unless chemical changes are involved. 

Whilst UV-visible techniques have proved extremely useful, they unfortunately lack molecular specificity. This means that they are not suitable for identifying adsorbates or studying adsorbate orientation and local environment. This type of information is best provided by vibrational spectroscopy, and therefore a considerable effort has been expended in developing vibrational spectroscopies that can be used in situ in an electrochemical cell. This effort is beginning to pay off, and techniques based on both infrared spectroscopy and Raman scattering spectroscopy are being increasingly used. 

With increasing sensitivity of detectors, the detection of unenhanced surface Raman scattering becomes a possibility, and if this is achieved a wide range of effects can be investigated. In particular the determination of adsorbate orientation from depolarisation ratios and the angular dependence of the Raman signal will be possible. Currently, such measurements only serve as a means of investigating the enhancement mechanism. 

Creighton JA (1983) Surface raman electromagnetic enhancement factors for molecules at the surface of small isolated metal spheres the determination of adsorbate orientation from sets relative intensities. SurfSci 124 209... 

---