Tests for Chemical Equivalence

In the NMR spectra presented so far, two or more hydrogens occupying positions that are chemically equivalent give rise to only one NMR absorption. It can be said, in general, that chemically equivalent protons have the same chemical shift. However, we shall see that it is not always easy to identify chemically equivalent nuclei. We shall resort to the symmetry operations presented in Chapter 5 to help us decide on the expected NMR spectrum of a specific compound. 

Molecular symmetry helps establish chemical equivalence 

The best way to approach this kind of problem is to make a model or draw the structure in detail, showing all hydrogens in place. 

Then you need to identify any mirror planes or rotational axes that render groups of hydrogens 

We would therefore anticipate three proton resonance signals arising from the three types of 

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Atoms that cannot be distinguished chemically. The replacement test for chemically equivalent atoms gives identical compounds, (p. 576)... 

We describe in more detail some tests for chemical equivalency in the next section, but for simple molecules such equivalence is obvious. An example is the NMR spectrum of 2,2-dimethyl-1-propanol in Figure 10-11 There are three absorptions—one (most shielded) for the nine equivalent methyl hydrogens of the tertiary butyl group, another for the OH, and a third (most deshielded) for the CH2 hydrogens. 

SAMPLE SOLUTION (a) To test for chemical-shift equivalence, replace the protons at C-1, C-2, C-3, and C-4 of 1-bromobutane by some test group such as chlorine. Four constitutional isomers result ... 

If two protons in the same set (i.e., chemical-shift-equivalent protons in the same multiplet) couple equally to every other proton in the spin system, they are also magnetically equivalent, and the usual Pople notations apply A2, B2, X2 etc. However, if two protons in a set are not magnetically equivalent, the following notations apply AA, BB, XX, etc. To rephrase Two chemical-shift-equivalent protons are magnetically equivalent if they are symmetrically disposed with respect to each proton in the spin system. Obviously magnetic equivalence presupposes chemical-shift equivalence. In other words, do not test for magnetic equivalence unless the two protons in question are chemical-shift equivalent. 

While the use of IgGl as an immunogenicity endpoint to attempt to predict the relative allergenicity of enzymes/proteins is open to criticism as mechanistically irrelevant, the true value of the method will lie in its performance relative to other preclinical test methods and human sensitization patterns. Many newly emerging methods in immunoloxicology rely on surrogate endpoints or endpoints that measure only a component of the response process (e.g. skin equivalent cultures for skin and eye irritation, local lymph node assay for contact sensitization, mouse IgE test for chemical respiratory sensitization). Their value lies in their... 

Chemical luminescence activity of diF-CLs. Di-fluoro-CLs were then tested for chemical luminescence activity. It proved that both diF-DCLs showed the same activities. We therefore concluded that diF-CL-2 has luminescence properties equivalent to those of diF-CL-1. 

The main components of adhesives are polymers such as elastomers and synthetic resins, and monomers and oligomers for reactive adhesives. As shown in O Sect. 40.3.2, properties that should be tested are chemical equivalent, molecular weight, viscosity, etc. In addition, solubility parameters, crystallinity, and presence of functional groups are also indispensable check points for the design of adhesives. 

EIS data is generally interpreted based on defining an appropriate equivalent circuit model that best fits the acquired data. The elements of the circuit model involve a specific arrangement of resistors, capacitors, and inductors that tacitly represent the physicochemical reality of the device under test. Under these circumstances the numerical value for chemical properties of the system can be extracted by fitting the data to the equivalent circuit model. Impedance measurements are typically described by one of two models ... 

Suspend 10 ANRC reference casein (Humko Sheffield Chemical, Teklad Test Diets, or equivalent) in 200 ml water and adjust pH to 8.0 with NaOH. Maintain at pH 8 for at >1 hr. Freeze dry and determine nitrogen using Kjeldahl method (unitb 1.2). 

An additional screening test for TCDD-like (aryl hydrocarbon receptor, AhR, active) chemicals has been developed (Garrison et al. 1996) and is available commercially (Anonymous 1997). Dubbed the CALUX (for chemically activated luciferase gene expression) system, the assay is based on recombinant cell lines into which researchers have inserted a firefly luciferase gene. When exposed to dioxin-like compounds, the recombinant cells luminesce. The method is sensitive to ppt levels of 2,3,7,8-TCDD equivalents in blood, serum, and milk (Anonymous 1997). Samples testing positive can be subjected to more definitive and specific analytical testing. 

We would expect the radical balance reactions (6) through (16) to be equilibrated at all points in these flames. Tests of the equilibration have been made, as for example in Figure 6, by evaluating the equilibrium concentration ratios using experimentally measured concentration values. Since O, H, S, and H2S concentrations were not measured directly we can indirectly evaluate the equilibration of the radical balance process by using reactions that are sums of the above listed processes. Four such reactions are listed below with an indication of a combination of reactions (6) through (16) that is chemically equivalent. 

Here, we report some basic results that are necessary for further developments in this presentation. The merging process of a test particle is based on the concept of cavity function (first adopted to interpret the pair correlation function of a hard-sphere system [75]), and on the potential distribution theorem (PDT) used to determine the excess chemical potential of uniform and nonuniform fluids [73, 74]. The obtaining of the PDT is done with the test-particle method for nonuniform systems assuming that the presence of a test particle is equivalent to placing the fluid in an external field [36]. 

Bonded to a group that withdraws part of the electron density from around the nucleus. The absorptions of deshielded nuclei are moved downfield, resulting in larger chemical shifts, (p. 568) Nuclei that occupy diastereomeric positions. The replacement test for diastereotopic atoms gives diastereomers. Diastereotopic nuclei can be distinguished by NMR, and they can split each other unless they are accidentally equivalent, (p. 592)... 

A shock sensitivity test using a Mklll ballistic mortar (Variable initiator test) has been developed for that purpose 2 0 . One can measure the ease of occurrence of explosions for chemicals with low sensitivity which fall on the boundary of explosives and nonexplosives. The method is place 5g of the test chemical into a 100kg mortar and initiate it with a detonator, which is equivalent to 0.2 - 2.0g of the high explosive PETN. 

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