Electronic barrier

Calculating the electronic barrier with an accuracy of 0.1 kcal/mol is only possible for very simple systems. An accuracy of 1 kcal/mol is usually considered a good, but hard to get, level of accuracy. The situation is slightly better for relative energies of stable species, but a 1 kcal/mol accuracy still requires a significant computational effort. Thermodynamic corrections beyond the rigid rotor/harmonic vibrations approximation are therefore rarely performed. 

Weisz, P. B. Electronic barrier layer fenomina in chemisorption and catalyse. 

The role of PyO in the second of the proposals seems eminently reasonable. It assists oxidation of Re(V) to Re(VII) and reduction of PyO to Py. To test this hypothesis, three other nucleophiles were independently added to a system in which 2-Me,4-N02C5H3N0 was the substrate. They were 4-MeC5H4N, C5H4N, and [Bu4 N]Br. This substrate, which suffers from added steric and electronic barriers to reactivity, was selected so as to lower all of the rates into a more readily measured time frame. 

Macroscopic n-type materials in contact with metals normally develop a Schottky barrier (depletion layer) at the junction of the two materials, which reduces the kinetics of electron injection from semiconductor conduction band to the metal. However, when nanoparticles are significantly smaller than the depletion layer, there is no significant barrier layer within the semiconductor nanoparticle to obstruct electron transfer [62]. An accumulation layer may in fact be created, with a consequent increase in the electron transfer from the nanoparticle to the metal island [63], It is not clear if and what type of electronic barrier exists between semiconductor nanoparticles and metal islands, as well as the role played by the properties of the metal. A direct correlation between the work function of the metal and the photocatalytic activity for the generation of NH3 from azide ions has been made for metallized Ti02 systems [64]. 

At present, there are two contrasting theories serving to describe the dynamics of proton transfer when an electronic barrier exists in the transfer coordinate. The... 

There is one experimental parameter that does serve to distinguish between the semiclassical model and the quantum model for nonadiabatic proton transfer. In the semiclassical model, if one assumes that the magnitude of the electronic barrier directly correlates with the thermodynamic driving force, a statement of the Hammond postulate, then as the driving force increases the rate of reaction increases, eventually reaching a maximum rate. The quantum model has a... 

On a cautionary note, if there is strong electronic coupling between the reactant and product states that serves to reduce the electronic barrier in the proton-transfer coordinate below the zero-point energy of the transferring vibration, then... 

Additional explanations consider the influence of adsorbed water molecules on the oxygen chemisorption, which affects the electronic barrier heights between the grains [85]. 

The activation polarization loss is dominant at low current density. At this point, electronic barriers have to be overcome prior to current and ion flow. Activation losses show some increase as current increases. Ohmic polarization (loss) varies directly with current, increasing over the whole range of current because cell resistance remains essentially constant. Gas transport losses occur over the entire range of current density, but these losses become prominent at high limiting currents where it becomes difficult to provide enough reactant flow to the cell reaction sites. 

Finally, in case 3 (Figure 3), Estel and E are both of similar and substantial magnitude. The energy minima fall in the neighborhood of 0 = 45°, 135°, 225° and 315°, and in systems with suitable barriers and substituents the passages past both the steric and the 7r-electronic barriers can be followed by NMR spectroscopy. 

The free carrier optical reflection of test modules before and after damp heat indicates that the effective carrier density is not much affected [58]. Hence, the degradation of the ZnO sheet resistance is probably more of a carrier transport problem. It is, at present, unclear where electron barriers are located. They may be present at the grain boundaries in general [59]. In this case, the disturbances of the ZnO microstructure (induced by the substrate but also depending on preparation parameters) are only harmful because they allow a faster penetration of the humidity into the film. On the other hand, the disturbed regions may themselves be highly resistive after damp heat exposure, which forces the current to percolate around these... 

Steric factors probably prohibit simultaneous rotation of the olefin and alkyne C2 units which would crowd all four metal-bound carbons into the same plane. Separate rotation of each unsaturated ligand was explored theoretically using the EHMO method. Rotation of the olefin destroys the one-to-one correspondence of metal-ligand tt interactions. Overlap of the filled dxz orbital with olefin n is turned off as the alkene rotates 90°, creating a large calculated barrier for olefin rotation (75 kcal/mol). Alkyne rotation quickly reveals an important point the absence of three-center bonds involving dir orbitals allows the alkyne to effectively define the linear combinations of dxy and dyz which serve as dn donor and dir acceptor orbitals for 7T and ttx, respectively. Thus there should be a small electronic barrier to alkyne rotation (the Huckel calculation with fixed metal... 

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