Two-proton transfer

The mechanism includes two single electron transfers (steps 1 and 3) and two proton transfers (steps 2 and 4) Experimental evidence indicates that step 2 is rate determining and it is believed that the observed trans stereochemistry reflects the dis tribution of the two stereoisomeric alkenyl radical intermediates formed in this step... 

Fuke and Kaya [1989] studied the vibrational selectivity of concerted two-proton transfer in 7-azoindole in the excited electronic state... 

Two-proton transfer has also been observed in naphtazarin crystals in the NMR spectra of... 

On the other hand, it is clear that in the classical regime, T> (T i is the crossover temperature for stepwise transfer), the transition should be step-wise and occur through one of the saddle points. Therefore, there should exist another characteristic temperature. r 2> above which there exist two other two-dimensional tunneling paths with smaller action than that of the one-dimensional instanton. It is these trajectories that collapse to the saddle points atlT = T i. The existence of the second crossover temperature, 7, 2, for two-proton transfer has been noted by Dakhnovskii and Semenov [1989]. 

As another test, we may compare two proton transfers for which 0 happens to fall at almost the same temperature. For phosphoric acid Ki passes through a maximum at 43.1°, as shown in Table 13, while Ka for alanine falls at 44.8°, as mentioned above. According to (143) this implies that the ratio Jnm/Jei has roughly the same value in these two... 

Write the two proton transfer equilibria that demonstrate the amphiprotic character of (a) HC03 (b) HP042. Identify the conjugate acid-hase pairs in each equilibrium. 

Any anion of a weak acid, including the anions of polyprotic acids, is a weak base. The acid-base properties of monoanions of polyprotic acids are complicated, however, because the monoanion is simultaneously the conjugate base of the parent acid and an acid in its own right. For example, hydrogen carbonate anions undergo two proton-transfer reactions with water ... 

The nature of the reaction catalyzed by PLC i( in which phosphatidylcholine is split into diacylglycerol and phosphorylcholine (Fig. 11) requires two proton transfer steps The first is the deprotonation of an active site water to generate the attacking hydroxide nucleophile, and the second is the protonation of the alkoxide leaving group. Although analyses of the X-ray structures of PLCSc and... 

Mechanistically speaking there have been no recent advances. What is known is that, at least for 4-OH formation, the reaction is intermolecular, requires two proton transfers at some stage and that a symmetrical intermediate is involved (often described as... 

In the presence of acetic add (HAc/Ac ) in an aqueous solution containing acidic and basic protons (HsO /H20 and H20/OH ), the following two proton transfer processes of Eqns. 3—44 and 3-45 take place in addition to the ionic dissodation of water molecules of Eqn. 3-36 as shown in Fig. 3-18 ... 

There may be two proton transfers in the carbonic anhydrase II-catalyzed mechanism of CO2 hydration that are important in catalysis, and both of these transfers are affected by the active-site zinc ion. The first (intramolecular) proton transfer may actually be a tautomerization between the intermediate and product forms of the bicarbonate anion (Fig. 28). This is believed to be a necessary step in the carbonic anhydrase II mechanism, due to a consideration of the reverse reaction. The cou-lombic attraction between bicarbonate and zinc is optimal when both oxygens of the delocalized anion face zinc, that is, when the bicarbonate anion is oriented with syn stereochemistry toward zinc (this is analogous to a syn-oriented carboxylate-zinc interaction see Fig. 28a). This energetically favorable interaction probably dominates the initial recognition of bicarbonate, but the tautomerization of zinc-bound bicarbonate is subsequently required for turnover in the reverse reaction (Fig. 28b). 

In the final exploration of the quantum chemistry unit students use a computational chemistry package (eg. Spartan, Gaussian, CaChe, etc.) to calculate the ground state energies, molecular orbitals, and in some cases the excited state energies, of two proton transfer tautomers. Calculations are performed at several different levels of theory, and use both semi-empirical and ab initio methods. Several different basis sets are compared in the ab initio calculations. The students use the results of these calculations to estimate the likelihood of excited state proton transfer. The calculations require CPU time ranging from a couple of minutes to a couple of hours on the PCs available to the students in the laboratory. 

Carbonic acid is an important component of the environment because it is formed whenever carbon dioxide dissolves in lake water or seawater. In fact, the oceans provide one of the delicate mechanisms for helping to maintain a constant concentration of carbon dioxide in the atmosphere. Carbonic acid takes part in two proton transfer equilibria ... 

Let s use the carbonic acid system as an example. Consider the following two proton transfer equilibria ... 

Step 2. Because we have two acids (HCN and H20) and just one base (H20), two proton-transfer reactions are possible ... 

As for the peroxidases, Compound I and water are formed in the first step from one equivalent of hydrogen peroxide and the resting state of the catalase. The back-reaction, however, does not proceed via Compound II but rather via a two-electron-two-proton transfer cascade, in which both hydrogen atoms of a second molecule of hydrogen peroxide are transferred to the ferryl subunit of the porphyrin cofactor. Due to the similarity of catalases and peroxidases, it is not too surprising that this reaction is also catalyzed by most peroxidases. Alternatively, catalases and some peroxidases react with alkyl hydroperoxides via the respective alkanol to an aldehyde or ketone (Scheme 2.17). A requirement for this reaction is an easily accessible active site for the hydroperoxide, so that only those peroxidases with open access such as CPO or CiP are able to promote this reaction. 

As was the case with 271, the treatment of palladacycle 277 with water also results in the liberation of H2, the mechanistic features of which were studied theoretically, using the somewhat simplified model complex PdMe2 (H2C=N-NH)3BH, 105 and proceeding from the assumption that an unstable palladium-hydride species is involved. It was thus concluded that reduction of H20 to H2 involves two proton-transfers to palladium, each proceeding with protonation of the pendant pyrazolyl donor, which then behaves as an intramolecular nucleophile (Scheme 20). 

Push-pull acid-base catalysis has been proposed to account for the proton switch mechanism which occurs in the methoxyaminolysis of phenyl acetate (Scheme 11.14) where a bifunctional catalyst traps the zwitterionic intermediate. A requirement of efficient bi-functional catalysis is that the reaction should proceed through an unstable intermediate which has p values permitting conversion to the stable intermediate or product by two proton transfers after encounter with the bifunctional catalyst the proton transfer with monofunctional catalysts should also be weak. 

Fuke and Kaya [1989] studied the tautomerization of dimer (6.15) formed in a supersonic jet to investigate the vibrational selectivity of concerted two-proton transfer in excited electronic state. Tunneling in the NH N fragments leads to broadening of certain vibrational bands in the fluorescence excitation spectrum (Figure 6.16). Tunneling is promoted by the symmetric intermolecular vibration with frequency 120 cm-1. The widths of bands with n = 0, 1, and 2 are 5, 10, and... 

Two-proton transfer in crystals of carboxylic acids has been studied thoroughly by the 7 -NMR and IINS methods. The proton spin-lattice relaxation time, measured by T,-NMR, is associated with the potential asymmetry A, induced by the crystalline field. The rate constant of thermally activated hopping between the acid monomers can be found from Tj using the theory of spin exchange [Look and Lowe, 1966] ... 

Two-Proton Transfer in Carboxylic Acid Dimer Crystals... 

Generally speaking, the PES of two-proton transfer in two coupled XH Y fragments is at least four dimensional. As shown by Shida et al. [1991a,b], it is possible to choose the coordinates... 

Figure 6.20. (a) Projection of a three-dimensional PES K(p,p2,p3) for two-proton transfer in formic acid dimer onto the (p, p,) and (p, p3) planes. In contrast with points A and B, in points C and D the potential along the p3 coordinate is a double well resulting in bifurcation of the reaction path [from Shida et al., 1991b]. (b) The contour lines correspond to equilibrium value of p3 and potential (6.37) when V(Q) = V0(Q4 - 2Q2), V0 = 21 kcal/ mol, C = 5.()9V0, A = 5.351/, Qn = 0.5. When Q > Qc, two-dimensional tunneling trajectories exist in the shaded region between curves 1 and 2. Curve 3 corresponds to synchronous transfer. 

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