Back proton transfer

The true acidity of a photoacid is a difficult-to-obtain quantity. Formally, since pKj = -log(kp,/k.p,), where kp, and k p, are overall rates for forward and back proton transfer, respectively, the pfQ obtained from the Forster calculation must be considered approximate. 

Scheme 15.9 Two examples of proton transfer in the excited state. The top (with -naphthol, [73]) is illustrative of an intermolecular proton transfer (to the solvent) jwocess whereas the bottom (with indigo, [22]) is illustrative of an intramolecular proton transfer. In the two cases the kinetic scheme (in the middle) applies with a single ground-state species however in the case of indigo, the back-proton transfer reaction in the excited state is unlikely...

Fig. 7.18. Electronic states and optical transitions in the photochemistry of 3-hydroxyflavone. The excited Si state of the normal molecule decays spontaneously within about less than 30 fs to the state of the tautomer by Excited State Intramolecular Proton Transfer (ESIPT). After radiation to Sq the tautomer decays back to the So state of the normal molecule by Back Proton Transfer (BPT).

Another interesting result coming from these experiments sheds new light on the Back Proton Transfer process. A careful line shape analysis of the vibronic S[—S q transitions made it possible to extract the homogeneous contribution which revealed a rate constant for BPT from the Sq state of 4 10 2 s-i. 

These reactions were further studied at miCTO-ITIES by Quinn et al. who found the reactions to be reversible [204]. In the early days, most electron-transfer reactions were considered heterogeneous, but, as discussed, the locus of the ET step may occur in one of the adjacent phases. More recently, Sugihara et al. showed that the oxidation of ascorbic acid and chlorogenic acid in water by a zinc porphyrin (5,10,15,20-tetraphenylporphirinato zinc(II)) in nitrobenzene occurs on the organic side of the interface, accompanied by a back proton transfer reaction [205]. According to Osakai et al., one case of truly heterogeneous ET reactions was observed with a cadmium tetraphenylporphyrin in nitrobenzene and ferricyanide in water [206]. 

The dienol is unstable, and two separate processes have been identified for ketonization. These are a 1,5-sigmatropic shift of hydrogen leading back to the enone and a base-catalyzed proton transfer which leads to the / ,y-enone. The deconjugated enone is formed because of the kinetic preference for reprotonation of the dienolate at the a carbon. Photochemical deconjugation is a synthetically useful way of effecting isomerization of a,) -unsaturated ketones and esters to the j ,y-isomers. 

DET calculations on the hyperfine coupling constants of ethyl imidazole as a model for histidine support experimental results that the preferred histidine radical is formed by OH addition at the C5 position [00JPC(A)9144]. The reaction mechanism of compound I formation in heme peroxidases has been investigated at the B3-LYP level [99JA10178]. The reaction starts with a proton transfer from the peroxide to the distal histidine and a subsequent proton back donation from the histidine to the second oxygen of the peroxide (Scheme 8). 

In this discussion we shall need to go right back to Chapter 1 and shall need to put together various aspects of ionic processes that have been considered separately in the preceding six chapters. In Sec. 15 we noticed that the relation between equations (26) and (25) was precisely the same as the relation between the equations (19) and (18) that had been obtained in Chapter 1. In Sec. 17 we discussed one type of proton transfer, which involved the formation of two ions at a great distance apart and in the footnote to Sec. 17 it was pointed out that the discussion of equation (27) given in Sec. 15 will apply to the electrostatic part of J. Proton transfers were also considered in Sec. 31 but hitherto we have examined only the type... 

Electrodes and Galvanic Cells. The Silver-Silver Chloride Electrode. The Hydrogen Electrode. Half-cells Containing an Amalgam, Electrode. Two Cells Placed Back to Back. Cells Containing Equimolal Solutions. The Alkali Chlorides as Solutes. HC1 in Methanol or Ethanol Containing a Trace of Water. The Alkali Chlorides in Methanol-Water Mixtures. The Heal of Solution of HC1. Proton Transfer Equilibrium from Measurements of E.M.F. 

Because of their basic properties, aikaioids were among the first naturai substances that eariy chemists extracted and purified. Morphine was isoiated from poppies in 1805 and was the first aikaioid to be characterized. When treated with aqueous strong acid, aikaioids accept protons to produce water-soiubie cations. The protonated aikaioids dissoive, ieaving the rest of the piant materiais behind. Adding strong base to the aqueous extract reverses the proton-transfer reaction, converts the aikaioid back to its neutrai base form, and causes pure aikaioid to precipitate from the soiution ... 

In a related example, the [D, A] complex of hexamethylbenzene and maleic anhydride reaches a photostationary state with no productive reaction (Scheme 17). However, if the photoirradiation is carried out in the presence of an acid, the anion radical in the resulting contact ion pair14 is readily protonated, and the redox equilibrium is driven toward the coupling (in competition with the back electron transfer) to yield the photoadduct.81... 

Such variation in the lifetimes of the ion pairs, which depends on the mode of activation, primarily arises from the difference in the spin multiplicities (see above). None the less, the long-lived ion-radical pair allows the in-cage proton transfer from the cation radical ArMe+ to the CA- anion radical to effectively compete with the back electron transfer,205 i.e.,... 

The second statement has to do with the notion that in the Eigen mechanism for proton transfer there must be intermediate ion pairs. The reference to the unpublished work of Kreevoy and Liang (3) reflects the impact of their studies on some of our own recent work surveyed below. In fact, there is an extensive published literature concerning phenol-amine complexes in which the existence of the intermediates in equation 2 has been established in different organic solvents. One of the oldest such papers is that of Bell and Barrow (11) going back to 1959. Others include Hudson and co-workers (12) in 1972, and Baba and co-workers (13) in 1969. 

Whereas in acetonitrile the rate limiting step is an opening of the solvent shell of a reactant, in benzonitrile the back reaction of (5) between the protonated acridine orange cation (BH ) and the 3-methyl-4-nitrophenolate ion (A ) to form the ion pair is diffusion controlled (although the overall reaction to the neutral molecules is an endothermic process). Because of its lower dielectric constant than acetonitrile, the electrostatic interactions between reactants in benzonitrile outweigh specific solvent effects. In other words, in benzonitrile a rate limiting coupling of proton transfer to the reorientation of solvent dipoles does not occur and the measured rates are very fast. The ion recombination (I) + (II) in benzonitrile has a diffusion controlled specific rate (theoretical) k = 9 -1 -1... 

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