Assisted proton transfer

An alternative view of these addition reactions is that the rate-determining step is halide-assisted proton transfer, followed by capture of the carbocation, with or without rearrangement Bromide ion accelerates addition of HBr to 1-, 2-, and 4-octene in 20% trifluoroacetic acid in CH2CI2. In the same system, 3,3-dimethyl-1-butene shows substantial rearrangement Even 1- and 2-octene show some evidence of rearrangement, as detected by hydride shifts. These results can all be accoimted for by a halide-assisted protonation. The key intermediate in this mechanism is an ion sandwich. An estimation of the fate of the 2-octyl cation under these conditions has been made ... 

VII. KINETICS OF ASSISTED PROTON TRANSFER AT LIQUID-LIQUID INTERFACE [14-16]... 

This line is both potential and pH dependent, and it represents the zone where an assisted proton transfer reaction occurs. As a matter of fact, an oblique line in the ionic partition diagram always relates to such a mechanism, the slope indicating the number of protons exchanged at the interface. 

However, this does not preclude the possibility that in a portion of the oxide at least (the outer layer), the OH transport mechanism is operative, with the release of protons at the interface between the two oxide layers. Hence, in such a case, some field-assisted proton transfer is likely to take place through the outer layer while chemical dissolution should be operative at the outer O/S interface. 

Fig, 20 Water assisted proton transfer in an enamine intermediate... 

The pattern in Figure 10.10 contains four energy minima occupied by the initial hydride, the dihydrogen-bonded complex, the intimate ion pair, and the solvent-separated ion pair as a final product of the reaction. For simplicity, the homo-conjugated species [RO- -H- OR] , assisting proton transfer and stabilizing the protonated products, can be ruled out. 

Figure 21. Metal ion effect on the acid hydrolysis of phosphosulfate (a) electrophilic catalysis and (b) metal-ion-assisted, proton-transfer catalysis...

Recently, tautomerism of a hetero-analogue of a carboxylic acid dimer, N,N -diarylamidine dimer [17], in the crystalline state has also been investigated using CP/MAS nmr (Mannle et ah, 1996). A similar tendency concerning phonon-assisted proton transfer was observed. 

For the second step of the cocaine methyl ester hydrolysis involving the water-assisted proton transfer, the calculated energy barrier, 4.8 kcal/mol, associated with transition state TS2dW-Me, is also lower than the corresponding first step. So, with the direct participation of the solvent water molecule in the proton transfer process, the first step of the hydrolysis in aqueous solution should be rate-determining, whether for the cocaine benzoyl ester hydrolysis or for the cocaine methyl ester hydrolysis. This conclusion provides theoretical support for the design of analogs of the first transition state for the cocaine benzoyl ester hydrolysis to elicit anti-cocaine catalytic antibodies [22,25]. 

Mafe S, Ramirez P, and Alcaraz A. Electric field assisted proton transfer and water dissociation at the junction of a fixed-charge bipolar membrane. Chem. Phys. Lett. 1998 294 406 12. 

Metal-assisted proton transfer in the general acid step... 

The classical MD simulations in the previous section suggest that in the ETS, the Mg2+ ion is positioned to shift the pKa of the 2 OH of G8 to act as a general acid, and in the LTS, the Mg2 ion can act as a Lewis acid catalyst to stabilize the leaving group and is poised to assist proton transfer from the 2 OH of G8. The possible roles inferred from purely classical MD simulations are supported by... 

As for pure phosphoric acid, the transport properties of PBI and phosphoric acid also depend on the water activity, this is on the degree of condensation (polyphosphate formation) and hydrolysis. There is even indication that these reactions do not necessarily lead to thermodynamic equilibrium, and hydrated orthophos-phoric acid may coexist with polyphosphates in heterogeneous gel-like microstructures [99]. There is not much known on the mechanism of proton transport in polymer adducts with polyphosphates and/or low hydrates of orthophosphoric acid. Whether the increased conductivity at high water activities is the result of the plasticizing effect of the water on the phosphate dynamics, thereby assisting proton transfer from one phosphate to the other, or whether the water is directly involved in the conduction mechanism has not been elucidated. 

Keywords On-the-lly dynamics simulation Excited-state proton transfer Excited-state tautomerization Water-assisted proton transfer Hydrogen bond rearrangement 7-Azaindole ADC(2)... 

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