Proton transfer internal

The presence of water molecules near the Qb site suggests that water plays an important role in proton transfer. The positions of these waters are not established with certainty by the computational analysis. However, a strong indication of their existence is the presence in the X-ray structure of significant voids near Qb that are bordered by polar groups in the protein. Because the distances between protonatable amino acid groups are, in some cases, significantly larger than the distances for proton transfer, internal water molecules are likely to play an important role as proton donors and acceptors. 

However, not all such proton transfers are diffusion controlled. For example, if an internal hydrogen bond exists in a molecule, reaction with an external acid or base is often much slower. In the following case ... 

C17-0120. In aqueous solution, amino acids exist as zwitterions (German for double ions ), compounds in which internal proton transfer gives a molecule with two charged functional groups. Use Lewis structures to illustrate the proton transfer equilibrium between the uncharged form of glycine (NH2 CH2 CO2 H) and its zwitterion form. 

Examples illustrating the reactions 21-23 are given in Figures 10-12. Shown in Figure 10 is the CID mass spectrum for the desolvation of Ni2+(H2O)10. The sequence of product ions Ni2+(H20) where n = 9 to n = 4 illustrates the sequential solvent loss represented by equation 21. The CID spectra in Figure 11 demonstrate that for the n = r = 4, charge reduction via internal proton transfer (see equation 23)... 

The activation energy for the charge reduction reaction is due to two factors the bond stretching and distortions of the originally near linear complex, so as to achieve the internal proton transfer and the increase of energy due to the Coulombic repulsion between the two charged products, a repulsion that leads to a release of kinetic energy on their separation. 

However, one of the most common mechanisms is undoubtedly proton transfer but whereas in alkene polymerizations this reaction leaves a terminal double bond, in arylene polymerizations these are generally not found. Instead the terminal group is usually a substituted indane formed by an internal Friedel-Crafts alkylation [8, 21, 23], e.g., for a-methyl styrene ... 

This chapter is restricted to intermolecular photophysical processes2). Intramolecular excited-state processes will not be considered here, but it should be noted that they can also affect the fluorescence characteristics intramolecular charge transfer, internal rotation (e.g. formation of twisted charge transfer states), intramolecular proton transfer, etc. 

Class C Fluorophores that undergo no photoinduced proton transfer but only photoinduced electron transfer. The fluorescence quantum yield of these fluorophores is very low when they are in the non-protonated form because of internal quenching by electron transfer. Protonation (which suppresses electron transfer) induces a very large enhancement of fluorescence (see Section 10.2.2.5). The bandshapes of the excitation and fluorescence spectra are independent of pH. 

The elimination of the amino moiety requires that it first is protonated. Because an internal proton transfer can occur only when the nonbonded electron pair of the leaving N-atom is oriented toward His57, the leaving N-atom must undergo inversion of configuration (Fig. 3.6, B). Inversion of a pyramidal N-atom is normally a fast process. Here, the equilibrium is likely to favor the ( -configured N-center (Fig. 3.6, B), since the (i )-configured N-center (Fig. 3.6, A) is destabilized by an unfavorable dipole-dipole interaction... 

It is assumed that an excited state charge transfer complex is formed between the nitroaromatic in its first triplet state and the respective substrate. Internal proton transfer is immediately followed by hberation of carbon dioxide. Finally hydrolysis of the hemiacetal Ar —X—CH2OH (X = NH or S) leads to 2-chloro-aniline or thiophenol, respectively. In the decarboxylation of a-phenylthio-acetic acid, some methyl-phenylsulfide is also formed. (7t,7r )-nitroaromatics are more reactive than nitro compounds with lowest (n,7t )-triplets iso). 

Why is no addition product observed in the gas phase, in contrast to solution This is not a case of no endothermic reactions both the proton transfer reaction (6b) and the alkoxide addition reaction (6a) are exothermic pathways. When an exothermic reaction occurs in solution, the excess energy is passed to the solvent. In the gas phase, with no solvent available, the excess energy remains in the intermediate. This can result in an effective internal temperature for that intermediate of hundreds to thousands of degrees. If there is some other bond that can be broken to yield a product ion plus a neutral in a pathway that is exothermic with respect to the reactants, the intermediate will fragment by that method, and the observed product will be that fragment ion. This internal temperature is the reason for the very short lifetime of the intermediates mentioned above. 

However, if there is no other exothermic pathway available, all the intermediate can do is revert to reactants. In such a situation, the more favorable the addition process is, the more internal energy is in the intermediate and the faster the reverse dissociation will occur. The better the addition is thermochemically, the worse it is kinetically. For the proton transfer pathway (6b), the neutral methanol product can carry off the excess energy as translational energy (and capture some of it in the newly formed OH bond) and the reaction proceeds. 

In view of the above, several workers, notably Zoltewicz, have stressed the likely importance of the internal return mechanism (Scheme 12) for proton-transfer reactions from heterocyclic compounds. 

The carbonyl ylide 1 can undergo an internal cyclization reaction to generate the corresponding epoxide 2, which is in fact an equilibrium process, and epoxides themselves have frequently served as precursors to carbonyl ylides. Other pathways such as concerted rearrangements and internal proton transfers have also been observed to neutralize the charged ylide intermediate and give substituted ethers as represented by 3. Perhaps the best known studies and most synthetically useful... 

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