Electrode molecular orientation

It is intriguing that upon emersion the value of A0 changes up to about 0.3 V compared with the immersed state.41 This has been attributed42,51 to the different structure of the liquid interfacial layer in the two conditions. In particular, the air/solvent interface is missing at an emersed electrode because of the thinness of the solvent layer, across which the molecular orientation is probably dominated by the interaction with the metal surface. 

Com, R. M., In situ second harmonic generation studies of molecular orientation at electrode surfaces, in Adsorption of Molecules at Metal Electrodes, J. Lipkowski and P. N. Ross, Eds., VCH, New York, 1992, p. 391. 

Adsorbate Molecular Orientation at Electrode Surface. Adsorption of some molecules from solution produces an oriented adsorbed layer. For example, nicotinic acid (NA, or 3-pyridinecarboxylic acid, niacin, or vitamin B3) is attached to a Pt(lll) surface primarily or even exclusively through the N atom with the ring in a (nearly) vertical orientation (12) (Fig. 10.5a). 

Figure 10.5. Adsorbate molecular orientation at the electrode surface d) nicotinic acid (Z ) benzoic acid (c) 2,6-pyridinedicarboxylic acid. (From Ref. 12, with permission from the American Chemical Society.)...

When the symmetry factor was introduced by Volmer and Erdey-Gruz in 1930, it was thought to be a simple matter of the fraction of the potential that helps or hinders the transfer of an ion to or from the electrode (Section 7.2). A more molecularly oriented version of the effect of P upon reaction rate was introduced by Butler, who was the first to apply Morse-curve-type thinking to the dependence of theenergy-dis -tance relation in respect to nonsolvent and metal—hydrogen bonds. 

Structural information regarding the molecular orientation of adsorbates on electrodes can be obtained using a number of Raman spectroscopic probes. While these experiments are not routine to conduct, they are beginning to approach the simplicity of the FTIRRAS experiments just described. Figure 9.13... 

In the EFISH method, the molecule of interest is dissolved in an appropriate solvent and put into a cell of the type shown in Figure 9. Electrodes above and below the cell provide the means for a D.C. electric field, which orients the solute (and solvent) molecules through its interaction with the molecular dipoles. Similar to the poled polymer approach, the average molecular orientation is increased along the field direction and an oriented gas model used to extract p. 

This paper will survey the current status of surface analysis in the examination of chemically modified electrode surfaces. In doing so, we shall take selected examples from our laboratory and the literature to illustrate some of the methods that have been employed to answer questions about surface topography, atomic and molecular speciation, and molecular orientation and bonding. 

Electrochemical studies on SAMs have proven invaluable in elucidating the impact of various molecular parameters such as bridge structure, molecular orientation or the distance between the electroactive species and electrode surface. As described above in Section 5.2.1, the kinetics of heterogeneous electron transfer have been studied as a function of bond length for many systems. Similarly, the impact of bridge structure and inter-site distances have been studied for various supramolecu-lar donor-acceptor systems undergoing photoinduced electron transfer in solution. In both types of study, electron transfer is observed to increase as the distance between the donor and acceptor decreases. As discussed earlier in Chapter 2, the functional relationship between the donor-acceptor distance and the electron transfer rate depends on the mechanism of electron transfer, which in turn depends on the electronic nature of the bridge. 

Most studies that aim to improve the characterization of the electrode surface have been carried out using high-vacuum techniques such as Auger [1, 2], XPS [3, 4], SIMS [5, 6], etc. However, these techniques involve the removal of the electrode from the electrolyte and the information derived from them may not reflect the state of the electrode in-situ. In addition, many of these techniques lack the molecular specificity afforded by vibrational spectroscopy and it has long been realised that IR spectroscopy would be an ideal method if it could be applied to the in-situ study of the electrode surface. Information from IR would include, potentially, molecular composition and symmetry, bond lengths and force constants (perhaps allowing us to estimate the strength of a chemisorption bond), and molecular orientation. 

HCl, which has a dipole moment. When the electric field is applied, there again will be a tendency for the electron cloud to move towards the positive electrode this effect is referred to as atom and molecular polarization. In addition, because the hydrogen chloride molecule has a permanent dipole moment, there will be some orientation polarization that is, the molecules will tend to orient in such a way that the negative chlorine ends will be towards the positive electrode and the positive hydrogen ends will be towards the negative electrode. This orientation polarization makes an additional contribution to the capacitance. 

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