Proton rates

Gutman M 1986 Application of the laser-induced proton pulse for measuring the protonation rate constants of specific sites on proteins and membranes Methods Enzymol. 127 522-38... 

In the case of a slow protonation rate (with inefficient proton donors and/or low concentrations of the proton source), the alternative could be an EECC mechanism through a disproportionation process, still at the potential of the first step. 

The slow protonation rate of the conjugated anion of the sulphone (1st step) leads to the obtainment of a pseudo one-electron process. However, no self-protonatiori process exists in the presence of an excess of a proton donor of lower pKa than that of the electroactive substrate and Figure 6a, curve 2 shows evidence for a two-electron step. Full substitution on the a carbon, as in the case of phenyl 2-phenylbut-2-yl sulphone, does not allow one to observe any deactivation (Figure 6b, curve 1). It is worth mentioning that cathodic deactivations of acidic substrates in aprotic solvents are rather general in electrochemistry, e.g. aromatic ketones behave rather similarly, showing deprotonation of the substrate by the dianion of the carbonyl compound39. 

A typical example is the protonation of tetraphenylporphirin (TPP) at the dodecane-acid solution interface. The interfacial protonation rate was measured by a two-phase stop flow method [6] and a CLM method [9]. In the former method, the stagnant layer of 1.4 jxm still existed under the highly dispersed system. In the CLM method, the liquid membrane phase of 50-100 /am thickness behaved as a stagnant layer where the TPP molecule has to migrate according to its self-diffusion rate. 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

Under selected conditions, the observed rate of oxygen exchange in these complexes can primarily be a function of the protonation rate, i.e., the inversion of the metal center, since the exchange on an oxo at any M = O site must proceed though the bottleneck of the dioxo species which is further discussed in Section V. 

Statement number 6 has to do with carbon acids and is supported by reference (7). There are, in fact, other references that suggest solvent plays a much more direct role in the kinetics of protonating carbanions than statement number 6 would imply. For example, there is evidence that nuclear reorganization and rehybridization of the carbon atom are too rapid to have much kinetic importance when compared with solvent reorientation. The strong dependence of carbanion protonation rates on the solvent supports this view. These rates are typically much faster in organic solvents, such as DMSO, than in water. A particular reaction that was studied in different solvents (17) is... 

The protonation rate constant of the triplet state of thionine has been measured as a function of temperature and hydrogen ion concentration using flash photolysis with a frequency-doubled neodynium laser 1614)... 

Rate constants for the protonation of radical-anions in dimethylformamide by added phenol can be determined by electrochemical techniques [8], Pulse radiolysis methods have been used to measure the rate constants in an alcohol solvent. This technique generates the radical-anion on a very short time scale and uv-spectroscopy is then be used to follow the protonation of this species to give the neutral radical with different uv-absorption characteristics [9]. In the case of anthracene, the protonation rate is 5 x 10 M" s with phenol in dimethylformamide and 5 x 10 s in neat isopropanol. Protonation by hydrogen ions approaches the diflusion-controlled limit with a rate constant of 10 M s in ethanol [9]. 

In fact, if free H+ attacks a hydride, proton transfer should be suppressed by the addition of X ions. However, NBn4Bp4, for example, does not affect the protonation rates of cw-[FeH2PP3] by the action of HBF4 in THE solutions. This is good evidence for the parallel attack of the molecnlar form HX and the ion pair nnder conditions when ion pairs react more slowly than HX [5]. 

A comparative study of gas-phase and liquid-phase CH acidity of a-substituted cyclopropanes has verified the correctness of an electrostatic model for the effect of solvation on equilibrium acidity 127 among variants of the LCAO MO method, only the semiempirical AMI method accurately predicted the proton affinity of the conjugate carbanions effects of solvation on protonation rates have been desegregated. 

Chemical Shift Measurements. Under slow-exchange conditions, the ionization ratio cannot be measured. In fact, one of the major advantages of the superacidic media is the ease with which weak bases can be fully protonated and directly observed by NMR. Because it is known that the protonation rates are practically diffusion-controlled ( 409 liter mol-1 s-1), under these conditions (< 10 2 s 1) the indicator is totally in the acidic form described by the NMR spectmm and no variable is available to measure the ionization ratio. 

The ESPT of naphthylammonium [190] and phenanthrylammonium [191] ions in their 18-crown-6 ether complexes in MeOH-H20 (9 1) solvent shows that the excited-state proton-transfer rate decreases markedly on complexing. The back-protonation rate in the excited state is negligibly small compared with those of the other decay processes, which essentially means that there is no excited-state protropic equilibrium in the crown complexes. The one-way proton-transfer reaction is elucidated by the presence of the excited neutral amine-crown complex (RNH2-crown) produced by deprotonation of (RN+H3-crown). There is a large steric effect on protonation to the amino group of the excited neutral complex. 

Deprotonation rate constant (sec-1) kp (Protonation rate constant) (M-1/sec) Tp tdp ... 

The rate constants for protonation of the excited singlet states of several compounds were determined by Weller (1961). Although the measurement of excited state equilibrium constants has become more common, there have been relatively few determinations of the rate constants involved. Trieff and Sundheim (1965) investigated the effects of solvent changes on the rates of protonation and deprotonation of 2-naphthol in the S) state. The dissociation rate constant decreased progressively with the addition of methanol or glycerol to the aqueous solution but the protonation rate constant varied in a more complex manner. As mentioned above, Stryer (1966) found both rate constants smaller in D20 than in H20. 

There is another series, the 1-X-l-phenylethylenes (equation 49, R2 = Ph), which also gives a very good correlation between protonation rates and [Pg.1100]

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