Chemical trends, approximate

Whereas JAB involves the nuclear magnetogyric ratios, the reduced coupling constant KAB represents only the electronic contribution and is thus approximately isotope independent and may exhibit chemical trends. 

Some Chemical Considerations Relevant to the Mouse Bioassay. Net toxicity, determined by mouse bioassay, has served as a traditional measure of toxin quantity and, despite the development of HPLC and other detection methods for the saxi-toxins, continues to be used. In this assay, as in most others, the molar specific potencies of the various saxitoxins differ, thus, net toxicity of a toxin sample with an undefined mixture of the saxitoxins can provide only a rough approximation of the net molar concentration. Still, to the extent that limits can be placed on variation in toxin composition, the mouse assay can in principle provide useful data on trends in net toxin concentration. However, the somewhat protean chemistry of the saxitoxins makes it difficult to define conditions under which the composition of a mixture of toxins will remain constant thus, attaining a reproducible level of mouse bioassay toxicity is difficult. It is therefore useful to review briefly some of the chemical factors that should be considered when employing the mouse bioassay for the saxitoxins or when interpreting results. Similar concepts will apply to other assays. 

The fluorines of CF2H groups, attached at the 2- or 3-position of a pyridine ring, appear at approximately -116 ppm, whereas a CF2H substituent at the 4-position appears at -113 ppm. A secondary CF2 substituent exhibits a similar trend in chemical shift (Scheme 4.55). 

To test if dilution of the products of CNO burning may explain the difference in abundance pattern with evolved giants and a possible excess in 12 C visible in N-rich stars (see left panel of Fig. 4), we use simple models in the plane [C/N] vs [O/N] (right panel of Fig. 4). Starting from the approximate composition of N-poor stars, the trend for different fractions of gas processed in the complete CNO-cycle (solid line) reproduces fairly well the data, albeit it predict too low C abundances for N-rich dwarfs. Pollution from RGB stars with composition N-rich from very deep mixing (complete CNO and Na enrichment involved, dotted line) reproduces also rather well the data, apart for N-rich dwarfs. On the other hand, the N-poor case, typical of the chemical composition of field RGB stars, is a very poor match (dashed line). Moreover, in this case, the model would predict C-poor, Na-poor stars, whereas no one is observed among over 40 dwarfs/subgiants in the 3 clusters. 

While the enthalpy of formation is the property of interest in chemical thermodynamics of materials, many books focus on the lattice enthalpy when considering trends in stability. The static non-vibrational part of the lattice enthalpy can be deconvoluted into contributions of electrostatic nature, due to electron-electron repulsion, dispersion or van der Waals attraction, polarization and crystal field effects. The lattice enthalpy is in the 0 K approximation given as a sum of the potential energies of the different contributions ... 

Though some more traditional thermodynamicists will be dismayed by the concept of solution phase bond dissociation enthalpy, the fact is that the database involving these quantities is growing fast. When used judiciously, they may provide important chemical insights—as is indeed the case for the stability of the O-H bond in phenolic compounds. Although solution phase bond dissociation enthalpies are not true bond dissociation enthalpies, because they include some contribution from intermolecular forces, a series of solution values like those in table 5.2 may be (and often is) taken as a good approximation of the trend in the gas-phase. 

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