Proton-transfer process

Viappiani C, Bonetti G, Carcelli M, Ferrari F and Sternieri A 1998 Study of proton transfer processes in... 

Berendsen, H.J.C., Mavri, J. Simulating proton transfer processes Quantum dynamics embedded in a classical environment. In Theoretical Treatments of Hydrogen Bonding, D. Hadzi, ed., Wiley, New York (1997) 119-141. 

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. 

Step 6 Proton transfer processes yielding ammonium ion and the carboxylic acid ... 

Notice that specific acid catalysis describes a situation in which the reactant is in equilibrium with regard to proton transfer, and proton transfer is not rate-determining. On the other hand, each case that leads to general acid catalysis involves proton transfer in the rate-determining step. Because of these differences, the study of rates as a function of pH and buffer concentrations can permit conclusions about the nature of proton-transfer processes and their relationship to the rate-determining step in a reaction. 

Because a relates the sensitivity to structural changes that the proton-transfer process exhibits to that exhibited by dissociation of the acid, it is frequently assumed that the value of a can be used as an indicator of transition-state structure. The closer a approaches unity, the greater is the degree of proton transfer in the transition state. There are limits to the generality of this interpretaton, however. ... 

The details of proton-transfer processes can also be probed by examination of solvent isotope effects, for example, by comparing the rates of a reaction in H2O versus D2O. The solvent isotope effect can be either normal or inverse, depending on the nature of the proton-transfer process in the reaction mechanism. D3O+ is a stronger acid than H3O+. As a result, reactants in D2O solution are somewhat more extensively protonated than in H2O at identical acid concentration. A reaction that involves a rapid equilibrium protonation will proceed faster in D2O than in H2O because of the higher concentration of the protonated reactant. On the other hand, if proton transfer is part of the rate-determining step, the reaction will be faster in H2O than in D2O because of the normal primary kinetic isotope effect of the type considered in Section 4.5. 

The process for initiating radical formation in aromatic amine-vinyl monomer systems have been studied by Feng et al. [80-86] who proposed the formation of an aminium radical as the active state of an exciplex as intimate ion-pair and then a cyclic transition state which then would undergo a proton transfer process of deprotonation leading to the formation of active radical species for initiation as follows ... 

The calibration of a and H12 is straightforward since tp1 and ip2 describe a proton transfer process and the relevant asymptotic points are easily determined using the pKa s of serine and histidine in water (see Chapter 5). The calibration of a 3 and A23 are more involved and require some effort in analyzing the available experimental information about AG 3 and Ag2 3 in water, which are considered below. 

Conway, B. E. Proton Solvation and Proton Transfer Processes in Solution 3... 

This is the first example of a proton transfer process to a hydride complex with a second-order dependence. Theoretical calculations indicate that the role of the HX molecules is the formation of W-H H-Cl- H-Cl adducts that convert into W-Cl, H2 and HCl2 in the rate-determining state through hydrogen complexes as transition states. 

To draw molecular pictures illustrating a proton transfer process, we must visualize the chemical reactions that occur, see what products result, then draw the resulting solution. When a strong base is added to a weak acid, hydroxide ions remove protons from the molecules of weak acid. When more than one acidic species is present, the stronger acid loses protons preferentially. 

When a strong acid is added to a buffer solution, the conjugate base A accepts protons from hydronium ions to form the weak acid HA, preventing a large increase in hydronium ion concentration. (All water molecules except those produced in the proton transfer process are omitted for clarity.)... 

This steep wire-length dependence is in striking contrast with the more rigorous PMF calculations of the same proton transfer process [14, 102], In the PMF calculations, a collective coordinate [103] is used to monitor the progress of the proton transfer without enforcing specific sequence of events involving individual protons along the wire the use of a collective coordinate is important because this allows... 

Sobolewski AL, Domcke W, Hattig C (2005) Tautomeric selectivity of the excited-state lifetime of guanine/cytosine base pairs The role of electron-driven proton-transfer processes. Proc Natl Acad Sci USA 102 17903-17906... 

Sobolewski AL, Domcke W (2003) Ab initio study of the excited-state coupled electron-proton-transfer process in the 2-aminopyridine dimer. Chem Phys 294 2763... 

Sobolewski AL, Domcke W (2006) Role of electron-driven proton-transfer processes in the excited-state deactivation adenine-thymine base pair. J Phys Chem A 110 9031-9038... 

These experiments demonstrate the importance of proton transfer processes during hole transfer through DNA. S. Steenken has already remarked that a proton shift between the G C bases stabilizes the positive charge [23]. If such a proton shift is coupled with the hole shift, a deuterium isotope effect should arise. Actually, H/D isotope effects are described by V. Shafiro-vich, M.D. Sevilla as well as H.H. Thorp in their articles of this volume. Experiments with our assay [22] also demonstrate (Fig. 16) that hole transfer in protonated DNA (H20 as solvent) is three times more efficient than in deuterated DNA (D20 as solvent). If this reflects a primary isotope effect, it shows that the charge transfer is coupled with a proton transfer. 

Figure 6. Jablonski diagram for the excited-state proton transfer and energy dissipation in TIN kSo s0> ks,s,-, kT,Tl- rate constants of proton-transfer processes in the ground state, first excited singlet state, and triplet state, respectively, and k,j rate constants of radiationless deactivations and k,- rate constants of intersyslem...

The relative ease with which proton transfer is accomplished is responsible for the importance of the generalized acid-base concept in solution chemistry. The Br0nsted concept of acidity is most useful in this respect. Br0nsted defined an acid as a species that tends to give up a proton and a base as a species that tends to accept a proton. In this sense any proton transfer process having the general form... 

The second group of intermolecular reactions (2) includes [1, 2, 9, 10, 13, 14] electron transfer, exciplex and excimer formations, and proton transfer processes (Table 1). Photoinduced electron transfer (PET) is often responsible for fluorescence quenching. PET is involved in many photochemical reactions and plays... 

The fundamental approach to a proton transfer process, which is crucial to mimic many chemical and biological reactions, has relied deeply on studies of excited-state intramolecular proton transfer (ESIPT) reactions in the condensed phase. 

Elsaesser TH, Bakker HJ (2002) Ultrafast hydrogen bonding dynamics and proton transfer processes in the condensed phase. Springer, Heidelberg... 

The values of 3/(NH,H) coupling constant observed for imine proton can be helpful in detection of the proton transfer processes and determination of mole fractions of tautomers in equilibrium. For NH-form, this value is close to 13 Hz, lower values usually indicate the presence of tautomeric equilibrium. It should be mentioned that the values below 2.4 Hz have not been reported. The chemical shift of C—OH (C-2 for imines, derivatives of aromatic ortho-hydroxyaldehydes or C-7 for gossypol derivatives) carbon to some extent can be informative, however, this value depends on type of substituents and should be interpreted with caution. 

Concerning the mechanism of O/H insertion, direct carbenoid insertion, oxonium ylide and proton transfer processes have been discussed 7). A recent contribution to this issue is furnished by the Cu(acac)2- or Rh2(OAc)4-catalyzed reaction of benz-hydryl 6-diazopenicillanate 237) with various alcohols, from which 6a-alkoxypenicil-lanates 339 and tetrahydro-l,4-thiazepines 340 resulted324. Formation of 340 is rationalized best by assuming an oxonium ylide intermediate 338 which then rearranges as shown in the formula scheme. Such an assumption is justified by the observation of thiazepine derivatives in reactions which involved deprotonation at C-6 of 6p-aminopenicillanates 325,326). It is possible that the oxonium ylide is the common intermediate for both 339 and 340. 

The assumption that 339 arises from the oxonium ylide by a proton transfer process is supported by the reversed product ratio obtained in the reaction with ethanol in the presence of diazabicyclo[4.3.0]non-5-ene (DBN)-... 

Large numbers of reactions of interest to chemists only take place in strongly acidic or strongly basic media. Many, if not most, of these reactions involve proton transfer processes, and for a complete description of the reaction the acidities or basicities of the proton transfer sites have to be determined or estimated. These quantities are also of interest in their own right, for the information available from the numbers via linear free energy relationships (LFERs), and for other reasons. 

Furthermore, all highly conserved amino acid positions, that are essential for the formation of the H-cluster containing hydrophobic niche or essentially involved in the proton transfer process can be found at their expected position in the sequences of both HydA fragments. 

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