Pseudo A-term

Near-degenerate states lead to oppositely signed terms that occur at nearly the same energy leading to a overall spectral feature that looks like an A term and is called a pseudo-A term (11). In many cases it may be that a quick SOS calculation may be sufficient to describe a pseudo-A term. We have had some success in describing the MCD spectra of furan, pyrrole, and related compounds through this approach (98). 

B-terms arise from field induced mixing of excited and ground states to other states with Zeeman interaction [10]. States with similar energy are prone to mix more. These transitions have absorption-band shape and are temperature independent. However, if the two mixed states are degenerate in energy, an A-term is observed. When the states are similar in energy then a derivative shaped signal is observed (pseudo A-term) [10]. 

Earlier studies have attributed this new signal to a three-electron reduced oxyl or hydroxyl species with the T2 Cu center still reduced, based on EPR studies using isotopically enriched O2 [106]. However, all four Cu centers in the NI have been determined to be oxidized by XAS edge studies [22], In addition, a pseudo-A term is observed in the CT region of flic MCD spectrum of the Nl that is very different from what is observed in the MCD spectrum of a hydroxyl radical [22,105]. Thus, XAS and MCD studies have provided definitive evidence that the Nl form is not a radical species produced by a 3e reduction of O2, but rather a 4e" reduced product of O2 reduction with all four Cu centers oxidized. 

In some applications like newborn screening and filter paper blood spots, the internal standard that is labeled cannot be mixed with blood. It can only be present in the extraction solvents. Therefore, only the extracted metabolites can be quantitatively measured. I have denoted a term called pseudo-isotope dilution to account for the differences between traditional isotope dilution and the technique commonly used in newborn screening by MS/MS. A special analysis is capable using this technique, however, in terms of an extraction efficiency experiment. With isotope-labeled standards you can perform an experiment whereby a traditional isotope-dilution technique (internal standard added to liquid blood and spotted) is compared to pseudo-isotope dilution techniques (internal standard is added to the extraction matrix). The ratio of the results of these two analysis (pseudo/traditional) is the extraction efficiency. 

If both of B and C are present, a measurement of the temperature dependence of the MCD spectrum leads us to separate them. Two partially overlapping B terms of opposite signs cannot be readily distinguished from an A term, and then a couple of the B terms is called an apparent or a pseudo-T term 295). 

For a surface that has a nonuniform density distribution of sites vs. heat of adsorption, the dependency of Ah on 0 would be more complicated and the residual contribution of pseudo-configurational terms would be difficult to evaluate. One might, however, nevertheless determine whether a unique representation of the experimental data in a form... 

The use of both Eyring and Arrhenius equations requires the use of appropriate rate constants. For a second-order reaction, for example, second-order rate constants should be used. Fitting conditional pseudo-first-order rate constants, as is sometimes incorrectly done, introduces an additional temperature-independent term. As a result, what may be reported as AS is in fact the sum (AS + ln[excess reagent]), as can be easily shown by substituting /c excess reagent] for k in Equation 8.117. The calculated A// term, on the other hand, is the same regardless of which rate constant, second order or pseudo-first order, is used. 

The parameters that are extricated from a line shape analysis are the reciprocal mean lifetimes, l/rsp(sp = species), between successive exchanges via each particular step. In chemical usage l/rsp is preferably written as kj, the pseudo-first-order rate constant. The results of NMR line shape analysis provide kinetic information via the well-known relationship given in equation 57. A term such as in equation 5 is used for each separate exchange step. 

As described in the Introduction, it is usually possible to consider the modeling of experimental data separately from the scheme actually used to move atoms about. Ideally, the different models should be able to be used in the different minimization or dynamics schemes. Thus, the subsequent sections describe the kind of data offered by NMR and the kinds of penalty functions or pseudo-energy terms that can be used to represent them. For convenience, we use nomenclature common to force field-based approaches where one refers to a distance constraint potential Vdc r) as a function of intemudear distance. 

Rather than simply adjusting a power term, Scarsdale et al.47 noted that the pseudo-energy term was being based on a distance r, but what was measured was more directly related to r 6. With this reasoning, they used a term of the form... 

A more difficult question is the relative balance between the conventional force field and the pseudo-energy terms representing experimental data.81 If... 

When making such a decision, some considerations should be borne in mind. First, if one is using a refinement scheme that produces a known distribution of structures, then one can calculate the likely deviation that the pseudo-energy terms will permit. For example, if an MD refinement is used, structures will be able to cross barriers of about BT. Then, if one is using a quadratic form to enforce distance restraints, one could recast Eq. [9] as follows to get an idea of the violations that would be permitted for a given force constant and temperature ... 

Measured enthalpies of solution of tetra-n-butylammonium bromide (Bu NBr) in mixtures of water (W) with acetonitrile (ACN) and with ethylene carbonate (EC) are compared with those in mixtures of water with some other aprotic solvents which were reported earlier. The results can be fairly well described by an equation which can be derived either from a cooperative hydration model or from a chemical, pseudo equilibrium model. This equation is tested by varying systematically the nature of the solute and of the cosolvent. In order to describe the experimental results in W-ACN and in W-EC as such, it will be necessary to extend the equation with a term which comprises any specific (nonhydrophobic) interactions of the solute with the (inert) cosolvent. 

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