Barrier, intrinsic

IS BARRIER- INTRINSIC SAFETY BARRIER VB - VOLTAGE BONDING (EQUIPOTENTIAL)... 

Fig. 5 Reaction barriers (intrinsic activation energies) of proton exchange reactions of C2-C4 alkanes in ZSM-5 calculated using PBE/DNP as a function of cluster size...

The [AS siJagx values were obtained from A (38T, PBE/CBS) - A (38T, PBE/DNP). The proposed extrapolation scheme given by Eq. (7) is in the same light as it has been previously suggested by Sauer et al. [41] Extrapolated proton exchange barriers (intrinsic) are ranging from 20.9 (ethane) to 30.5 (i-butane/3) kcal/mol, and their values as well as the decomposition energies are shown in Table 8. 

Saltiel J and Sun Y-P 1989 Intrinsic potential energy barrier for twisting in the f/ a/rs-stilbene SI State in hydrocarbon solvents J. Phys. Chem. 93 6246-50... 

In the above discussion it was assumed that the barriers are low for transitions between the different confonnations of the fluxional molecule, as depicted in figure A3.12.5 and therefore the transitions occur on a timescale much shorter than the RRKM lifetime. This is the rapid IVR assumption of RRKM theory discussed in section A3.12.2. Accordingly, an initial microcanonical ensemble over all the confonnations decays exponentially. However, for some fluxional molecules, transitions between the different confonnations may be slower than the RRKM rate, giving rise to bottlenecks in the unimolecular dissociation [4, ]. The ensuing lifetime distribution, equation (A3.12.7), will be non-exponential, as is the case for intrinsic non-RRKM dynamics, for an mitial microcanonical ensemble of molecular states. 

The mere fact that voltage, current, or even both, are at low levels does not guarantee a circuit to be intrinsically safe, even though intrinsically safe circuits do utilize relatively low voltage and current levels. Intrinsically safe systems employ electrical barriers to assure that the system remains intrinsically safe. The barriers limit the voltage and current combinations so as not to present an ignition hazard should a malfunction develop. Typically, devices upstream of barriers are not intrinsically safe and are installed in control rooms or other unclassified locations. All devices and wiring on the downstream side of the barriers are intrinsically safe and can be installed in classified areas. 

In the previous chapter, a comprehensive description was provided, from four complementary perspectives, of the process of how human errors arise during the tasks typically carried out in the chemical process industry (CPI). In other words, the primary concern was with the process of error causation. In this chapter the emphasis will be on the why of error causation. In terms of the system-induced error model presented in Chapter 1, errors can be seen as arising from the conjunction of an error inducing environment, the intrinsic error tendencies of the human and some initiating event which triggers the error sequence from this imstable situation (see Figure 1.5, Chapter 1). This error sequence may then go on to lead to an accident if no barrier or recovery process intervenes. Chapter 2 describes in detail the characteristics of the basic human error tendencies. Chapter 3 describes factors which combine with these tendencies to create the error-likely situation. These factors are called performance-influencing factors or PIFs. 

Figure S-16. Schematic reaction coordinate diagrams of a reaction series, showing ACj, the intrinsic barrier, and ACma, the maximum standard free energy difference.

AGo is called the intrinsic barrier for the reaction series. (Strictly speaking, we should write AGj = AGS,ax /4, because AG° can be negative.)... 

Equation (5-69) describes rate-equilibrium relationships in terms of a single parameter, the intrinsic barrier AGo, which therefore assumes great importance in interpretations of such data. It is usually assumed that AGo is essentially constant within the reaction series then it can be estimated from a plot of AG vs. AG° as the value of AG when AG = 0. Another method is to fit the data to a quadratic in AG and to find AGq from the coefficient of the quadratic term. ... 

If the intrinsic barrier AGq could be independently estimated, the Marcus equation (5-69) provides a route to the calculation of rate constants. An additivity property has frequently been invoked for this purpose.For the cross-reaction... 

Reactions (5-70), (5-71), and (5-72) have intrinsic barriers AGq, Ag1a> and AGcc. respectively. Then AGo is estimated with Eq. (5-73). 

For the identity reactions, the intrinsic barriers are their free energies of activation, which can be determined by tracer studies or less directly by rate-equilibrium correlations. ... 

Figure 5-25. Sections through a hyperbolic paraboloid energy surface constructed over an RIP diagram. The intrinsic barrier AG of the main reaction and the intrinsic well AC of the disparity reaction are shown.

In Eq. (7-21), AGo is the intrinsic barrier, the free energy of activation of the (hypothetical) member of the reaction series having AG" = 0. It is evident that the Marcus equation predicts a nonlinear free energy relationship, although if a limited... 

Other measures of nucleophilicity have been proposed. Brauman et al. studied Sn2 reactions in the gas phase and applied Marcus theory to obtain the intrinsic barriers of identity reactions. These quantities were interpreted as intrinsic nucleo-philicities. Streitwieser has shown that the reactivity of anionic nucleophiles toward methyl iodide in dimethylformamide (DMF) is correlated with the overall heat of reaction in the gas phase he concludes that bond strength and electron affinity are the important factors controlling nucleophilicity. The dominant role of the solvent in controlling nucleophilicity was shown by Parker, who found solvent effects on nucleophilic reactivity of many orders of magnitude. For example, most anions are more nucleophilic in DMF than in methanol by factors as large as 10, because they are less effectively shielded by solvation in the aprotic solvent. Liotta et al. have measured rates of substitution by anionic nucleophiles in acetonitrile solution containing a crown ether, which forms an inclusion complex with the cation (K ) of the nucleophile. These rates correlate with gas phase rates of the same nucleophiles, which, in this crown ether-acetonitrile system, are considered to be naked anions. The solvation of anionic nucleophiles is treated in Section 8.3. 

For an adequate barrier against moisture, sufficient thickness of paint must be applied. The modern trend is to apply high-build coatings based on media having high intrinsic water resistance. Such paints may be pigmented with corrosion inhibitors or minerals which impede the flow of moisture through the film. 

Parker [55] studied the IN properties of MEH-PPV sandwiched between various low-and high work-function materials. He proposed a model for such photodiodes, where the charge carriers are transported in a rigid band model. Electrons and holes can tunnel into or leave the polymer when the applied field tilts the polymer bands so that the tunnel barriers can be overcome. It must be noted that a rigid band model is only appropriate for very low intrinsic carrier concentrations in MEH-PPV. Capacitance-voltage measurements for these devices indicated an upper limit for the dark carrier concentration of 1014 cm"3. Further measurements of the built in fields of MEH-PPV sandwiched between metal electrodes are in agreement with the results found by Parker. Electro absorption measurements [56, 57] showed that various metals did not introduce interface states in the single-particle gap of the polymer that pins the Schottky contact. Of course this does not imply that the metal and the polymer do not interact [58, 59] but these interactions do not pin the Schottky barrier. 

Another approach was used some years ago by Dewar and Storch (1989). They called attention to solvent effects in ion-molecule reactions which do not yield an activation energy in theoretical calculations related to gas-phase conditions, but which are known to proceed with measureable activation energy in solution. Dewar and Storch therefore make a distinction between intrinsic barriers due to chemical processes and desolvation barriers due to chemical processes. 

Further improvements can be achieved by replacing the oxygen with a non-physiological (synthetic) electron acceptor, which is able to shuttle electrons from the flavin redox center of the enzyme to the surface of the working electrode. Glucose oxidase (and other oxidoreductase enzymes) do not directly transfer electrons to conventional electrodes because their redox center is surroimded by a thick protein layer. This insulating shell introduces a spatial separation of the electron donor-acceptor pair, and hence an intrinsic barrier to direct electron transfer, in accordance with the distance dependence of the electron transfer rate (11) ... 

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