P-Proton transfer

Figure 14-6. Two-dimensional free energy surfaces for in-line monoanionic mechanisms for the (A) un-catalytic model reaction in solution, and the catalytic (B) O p and (C) O2p pathways in the hairpin ribozyme. fi is defined as Rp-o5, R02,-P> and 2 is R02,-H2, rOnb-H2, for fl < 0.0 A and r05,-H2, rOnb-H2, f°r 1 > 0.0 A, where Opjg is for the G p proton transfer in (B), and for the O2p proton transfer in (A) and (C), respectively. The units for free energies and distances are kcal/mol and A, respectively...

Skinner, J. L. and Trommsdorf, H. P. Proton transfer in benzoic acid crystals A chemical spin-boson problem. Theoretical analysis of nuclear magnetic resonance, neutron scattering, and optical experiments, J.Chem.Phys., 89 (1988), 897-907... 

Chain transfer of a P-proton to other basic substances in the reaction mixture is also possible. The various P-proton transfer reactions limit polymer molecular weight, but do not terminate the kinetic chain. 

To achieve LCP, one needs to start by choosing the initiator, coinitiator, and other components of a reaction so that there is no nucleophile present that can irreversibly terminate the propagating cationic species. Basic components also need to be avoided to minimize P-proton transfer. However, even with the most judicious choice of reaction system, P-proton transfer is still present because monomer itself is a base. One needs to minimize P-proton transfer to monomer to achieve LCP. 

Burgess conducted a kinetic study of the dehydration of erythro- and f/iraj-2-deuterio-1,2-diphenylethanols (6, 8). The results were consistent with rate-limiting formation of an ion pair, followed by fast. vyw-p-proton transfer. Geometrical constraints require that the abstracted hydrogen is syn with respect to the leaving group. 

FIGURE 7.21 NICI reaction scheme where MH = sample molecule, R = methane, X = F, Cl, Br, I, and the reaction path notation is c = condensation, d = dissociation, s[R] = collisional activation, and p = proton transfer. 

This family of processes includes two kinds of reactions -hydride transfer and p-proton transfer. The reaction of yS-hydride transfer is typical for strongly electronegative high-valent metals such as V(V) (Nabavi, 1991 Kessler, 2000), Mo(VI) (Kessler and Shevelkov, 1998), rhenium (detected even in -i-III oxidation state (Hoffman, 1989)) and... 

More recently this form of cyclopropanol formation was found to occur with P-(p-aminophenyl)pro-piophenones, in which the aniline electron donor promotes a p-proton transfer and 1,3-biradical formation. - This study was particularly interesting in that phosphorescence of the intermediate exciplex was detected. Various structural modifications led to interesting diastereoselectivity in the products. 

Marzocchi M P, Mantini A R, Casu M and Smulevich G 1997 Intramolecular hydrogen bonding and excited state proton transfer in hydroxyanthraquinones as studied by electronic spectra, resonance Raman scattering, and transform analysis J. Chem. Phys. 108 1-16... 

Barbara P F, Walker G C and Smith T P 1992 Vibrational modes and the dynamic solvent effect in electron and proton transfer Science 256 975-81... 

Bala, P., Lesyng, B., McCammon, J.A. Application of quantum-classical and quantum-stochastic molecular dynamics simulations for proton transfer processes. Chem. Phys. 180 (1994) 271-285. 

Reaction (5.N) describes the nylon salt nylon equilibrium. Reactions (5.0) and (5.P) show proton transfer with water between carboxyl and amine groups. Since proton transfer equilibria are involved, the self-ionization of water, reaction (5.Q), must also be included. Especially in the presence of acidic catalysts, reactions (5.R) and (5.S) are the equilibria of the acid-catalyzed intermediate described in general in reaction (5.G). The main point in including all of these equilibria is to indicate that the precise concentration of A and B... 

R. J. Gillespie, in V. Gold, ed.. Proton Transfer Reactions, Chapman and Hall, London, 1975, p. 27. 

The first column corresponds to six-membered transition states where two types can be differentiated situation A, where the proton transfer leads to a neutral tautomer, and situation B (often Tinuvin P or TIN 75), where the proton transfer leads to a zwitterionic tautomer (Scheme 25). 

Different solid-state NMR techniques CPMAS NMR, the second moment of the signal, the spin-lattice relaxation time in the rotating frame T p) were combined to reach the conclusion that in the case of por-phine H2P the double-proton transfer is followed by a 90° rotation within the crystal (see Scheme 2). 

The Variation of J with Temperature. Although each proton transfer has its own characteristic value of 0, the variation of K near the maximum shows a marked degree of uniformity, as already mentioned in Sec. 64. If a parabola of the form p(T — 0)2 is fitted to the experimental results, a single value of p, namely 5 X 10-6, reproduces the variation of log K, not only for proton transfers of class III, but also for those of class II and class IV. If we accept (140) as providing a qualitative theory of the phenomena, we have at once a physical explanation of the observed uniformity. Whether we are concerned with the... 

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