Reactivity probing

It is often said that the property of acidity is manifest only in the presence of a base, and NMR studies of probe molecules became common following studies of amines by Ellis [4] and Maciel [5, 6] and phosphines by Lunsford [7] in the early to mid 80s. More recently, the maturation of variable temperature MAS NMR has permitted the study of reactive probe molecules which are revealing not only in themselves but also in the intermediates and products that they form on the solid acid. We carried out detailed studies of aldol reactions in zeolites beginning with the early 1993 report of the synthesis of crotonaldehyde from acetaldehyde in HZSM-5 [8] and continuing through investigations of acetone, cyclopentanone [9] and propanal [10], The formation of mesityl oxide 1, from dimerization and dehydration of... 

REACTIVE PROBING OF THE POTENTIAL ENERGY SURFAC ISOMERIC IONS... 

In the following sections, studies of isomeric ions are reported in which the ions are reactively probed. Where calculations are available, information on potential energy surfaces is given. This is usually the structure of the stable isomeric forms and transition states and their relative energies thus only points on the potential surface are known. The detailed form of the potential surface is almost never available nor is the connectivity between the various states usually established theoretically (chemical intuition is often used to connect the states). Pertinent experimental data on CID and metastable ions, isomers produced in binary reactions, and potential surfaces probed by binary reactions (with the excited isomeric ion as the reaction intermediate) are also given. 

DNA and RNA may be modified with hydrazide-reactive probes by reacting their cytosine residues with bisulfite to form reactive sulfone intermediates. These derivatives undergo transamination to couple hydrazide- or amine-containing probes (Draper and Gold, 1980) (Chapter 27, Section 2.1). 

Note At a level of 50-pl probe addition, polyclonal human IgG will be modified at a level that gives an F/P ratio of about 0.113. Since the labeling occurs only at the oxidized carbohydrate sites, the fluorophore incorporation typically is less than that observed when using amine-reactive probes. 

The ethylene diamine-dextran derivative may be used for the coupling of carboxylate-contain-ing molecules by the carbodiimide reaction, for the coupling of amine-reactive probes, or to modify further using heterobifunctional crosslinkers. The hydrazide-dextran derivative may be used to crosslink aldehyde-containing molecules, such as oxidized carbohydrates or glycoproteins. 

In a second approach of the reactivity, one fragment A is represented by its electronic density and the other, B, by some reactivity probe of A. In the usual approach, which permits to define chemical hardness, softness, Fukui functions, etc., the probe is simply a change in the total number of electrons of A. [5,6,8] More realistic probes are an electrostatic potential cf>, a pseudopotential (as in Equation 24.102), or an electric field E. For instance, let us consider a homogeneous electric field E applied to a fragment A. How does this field modify the intermolecular forces in A Again, the Hellman-Feynman theorem [22,23] tells us that for an instantaneous nuclear configuration, the force on each atom changes by... 

Relative reactivities of molecules can also shed light upon the validity of our general theoretical approach. In this section we shall examine the following reactivity probes of nonbonded interaction ... 

In Part II we examined in detail various experimental tests of nonbonded interactions. Here, we again focus on specific physical and reactivity probes in order to test the importance of n—a interactions in organic problems. Specifically, we shall examine the following two areas ... 

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