Potential Surface Scans

The reaction coordinate is one specific path along the complete potential energy surface associated with the nuclear positions. It is possible to do a series of calculations representing a grid of points on the potential energy surface. The saddle point can then be found by inspection or more accurately by using mathematical techniques to interpolate between the grid points. 

This type of calculation does reliably find a transition structure. However, it requires far more computer time than any of the other techniques. As such, this is generally only done when the research requires obtaining a potential energy surface for reasons other than just finding the transition structure. 

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FIGURE 22.3 The bond order of the C-C bond, the total and the free valence of the C atoms as a function of the C-C distance (obtained in a relaxed potential surface scan using the UHF method). 

In adsorptive stripping voltammetry the deposition step occurs without electrolysis. Instead, the analyte adsorbs to the electrode s surface. During deposition the electrode is maintained at a potential that enhances adsorption. For example, adsorption of a neutral molecule on a Hg drop is enhanced if the electrode is held at -0.4 V versus the SCE, a potential at which the surface charge of mercury is approximately zero. When deposition is complete the potential is scanned in an anodic or cathodic direction depending on whether we wish to oxidize or reduce the analyte. Examples of compounds that have been analyzed by absorptive stripping voltammetry also are listed in Table 11.11. 

In Gaussian, potential energy surface scans are automated. Here is a sample input file fora simple potential energy surface scan ... 

This input file requests a potential energy surface scan for CH by including the Scan keyword in the route section. The variables section of the molecule specification uses an expanded format ... 

The results of a potential energy surface scan appear following this heading within Gaussian output ... 

We ll look at examples of potential energy surface scan calculations in Exercise 8.2. 

In this exercise, you will explore the bond rupture process by performing a potential energy surface scan. Run potential energy surface scans for these molecules, gradually increasing one of the C-H bond lengths, using the specified model chemistries ... 

This particular potential energy surface seems very clean-cut, because there is a single minimum in the range of variables scanned. The chances are that this minimum is a local one, and a more careful scan of the potential surface with a wider range of variables would reveal many other potential minima. 

BB-SFG, we have investigated CO adsorption on smooth polycrystaHine and singlecrystal electrodes that could be considered model surfaces to those apphed in fuel cell research and development. Representative data are shown in Fig. 12.16 the Pt nanoparticles were about 7 nm of Pt black, and were immobilized on a smooth Au disk. The electrolyte was CO-saturated 0.1 M H2SO4, and the potential was scanned from 0.19 V up to 0.64 V at 1 mV/s. The BB-SFG spectra (Fig. 12.16a) at about 2085 cm at 0.19 V correspond to atop CO [Arenz et al., 2005], with a Stark tuning slope of about 24 cm / V (Fig. 12.16b). Note that the Stark slope is lower than that obtained with Pt(l 11) (Fig. 12.9), for reasons to be further investigated. The shoulder near 2120 cm is associated with CO adsorbed on the Au sites [Bhzanac et al., 2004], and the broad background (seen clearly at 0.64 V) is from nomesonant SFG. The data shown in Figs. 12.4, 12.1 la, and 12.16 represent a hnk between smooth and nanostructure catalyst surfaces, and will be of use in our further studies of fuel cell catalysts in the BB-SFG IR perspective. 

The symmetric pair of voltammetric peaks in the Ru(OOOl) base CV in the range 0.1-0.3 V (peaks B and B ), which is best seen for a lower potential limit of min = 0.1V, is tentatively assigned to (14.5), which can run reversibly in both directions. This assignment is based on the assumption that the surface is covered by 0.5 ML Oad at 0.3 V. Only for more negative potential limits, when OHad is further reduced to H2O and replaced by Hupd according to (14.1) and (14.2), does the re-oxidation of the adlayer require H2O dissociation according to (14.3) and (14.4). This provides a simple explanation why the pronounced hysteresis between OHad removal (peak A ) and reformation of OHad/Oad (peak A) is only observed when the potential is scanned to < 0.1 V. 

An unusually extensive battery of experimental techniques was brought to bear on these comparisons of enantiomers with their racemic mixtures and of diastereomers with each other. A very sensitive Langmuir trough was constructed for the project, with temperature control from 15 to 40°C. In addition to the familiar force/area isotherms, which were used to compare all systems, measurements of surface potentials, surface shear viscosities, and dynamic suface tensions (for hysteresis only) were made on several systems with specially designed apparatus. Several microscopic techniques, epi-fluorescence optical microscopy, scanning tunneling microscopy, and electron microscopy, were applied to films of stearoylserine methyl ester, the most extensively investigated surfactant. 

Anodic limit, potential referred to Li+/Li, cutoff current density in parentheses. Scan rate 5 mV s. Activated carbon as working surface. Scan rate 10 mV s. Supporting electrolyte 0.1 M BU4NBF4. Scan rate 100 mV s . The solvent-free condition was realized by using an ionic liquid based on Imldazolium cation, at 80 °C. Scan rate 20 mV s . ... 

Fig. 11 Cyclic current-potential curve forAu(lOO) in 0.1 M H2SO4 solution beginning of polarization —0.2 V (versus SCE) using a freshly prepared reconstructed surface. Scan rate ...

In the Figure 3.18 example, after imposing Ej across the electrode-solution interface, the potential is scanned negatively toward the standard redox potential of the O/R couple. The ratios of O and R that must exist at the electrode surface (Cr/Cq) at several potentials during the scan are given in each figure. These values are dictated by the Nernst equation for a reversible system (see Table 3.1). Since the solution initially contained only O, the R required to satisfy the Nernst equation is obtained from O by reduction, causing cathodic current. 

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