Hydrogen and proton transfer

In contrast to inorganic molten salts, the fluidity of ionic hquids at room temperature permits their use as solvents for chemical reactions. Electrostatic properties and charge mobility in ionic hquids can play a distinctive role in chemical reactivity, as compared with neutral solvents. In particular, hydrogen and proton transfer reactions are likely to be sensitive to an ionic environment due to the hydrogen-bond acceptor ability of the anions. Such type of reactions are fundamental in acid-based chemistry and proton transport in solution. 

Chemical reactions such as hydrogen and proton transfer occur inside the plume and have certain exothermicities associated with them. 

To test and train the ReaxFF parameters for reactive events we considered a number of hydrogen and proton transfer reactions for neutral, cation, and anion systems. Figure 6.9 and Table 6.1 compare the QM and ReaxFF barriers... 

One after the other, step through (or animate) the sequence of structures depicting the SN2 and proton transfer reactions shown above. Compare the two. From what direction does cyanide approach the hydrogen in HCl From the same side as Cl ( frontside ), or from the other side ( backside ) Does the Sn2 reaction follow a similar trajectory ... 

Three kinds of equilibrium potentials are distinguishable. A metal-ion potential exists if a metal and its ions are present in balanced phases, e.g., zinc and zinc ions at the anode of the Daniell element. A redox potential can be found if both phases exchange electrons and the electron exchange is in equilibrium for example, the normal hydrogen half-cell with an electron transfer between hydrogen and protons at the platinum electrode. In the case where a couple of different ions are present, of which only one can cross the phase boundary — a situation which may exist at a semiperme-able membrane — one obtains a so called membrane potential. Well-known examples are the sodium/potassium ion pumps in human cells. 

Using either of the above approaches we have measured the thermal rate constants for some 40 hydrogen atom and proton transfer reactions. The results are tabulated in Table II where the thermal rate constants are compared with the rate constants obtained at 10.5 volt cm.-1 (3.7 e.v. exit energy) either by the usual method of pressure variation or for concurrent reactions by the ratio-plot technique outlined in previous publications (14, 17, 36). The ion source temperature during these measurements was about 310°K. Table II also includes the thermal rate constants measured by others (12, 13, 33, 39) using similar pulsing techniques. 

Marcus, R. A., Similarities and differences between electron and proton transfers at electrodes and in solution, Theory of hydrogen evolution reaction, Proc. Electrochem. Soc., 80-3, 1 (1979). 

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

Noyori and coworkers reported well-defined ruthenium(II) catalyst systems of the type RuH( 76-arene)(NH2CHPhCHPhNTs) for the asymmetric transfer hydrogenation of ketones and imines [94]. These also act via an outer-sphere hydride transfer mechanism shown in Scheme 3.12. The hydride transfer from ruthenium and proton transfer from the amino group to the C=0 bond of a ketone or C=N bond of an imine produces the alcohol or amine product, respectively. The amido complex that is produced is unreactive to H2 (except at high pressures), but readily reacts with iPrOH or formate to regenerate the hydride catalyst. 

Figure 13.14 Structural basis for electron and proton transfer at centre P part a shows ubiqunol hydrogen bonded to Hisl81 of ISP and Glu272 of cytochrome b. Part b shows the ubiquinol-binding pocket after movement of Glu272. (From Hunte et al., 2000. Copyright 2003, with permission from Elsevier.)...

C. A. Hasselbacher, E. Waxman, L. T. Galati, P. B. Contino, J. B. A. Ross, and W. R. Laws, Investigation of hydrogen bonding and proton transfer of aromatic alcohols in non-aqueous solvents by steady-state and time-resolved fluorescence, J. Phys. Chem. 95, 2995-3005 (1991). 

State for proton removal from the open intermediate. At the lower buffer concentration, the proton-transfer step occurs more slowly than closing of the hydrogen bond, and proton transfer is rate limiting. At the higher... 

Other reviews on proton transfer deal with the following topics hydrogen bonding and H+ transfer " , general acid/base catalysis " and proton transfers in biological systems involving proteins . 

The related elements, proton hydrogen (H ), and hydride (H ) change their physical properties (including their size) drastically by the change of the number of electrons. Hydrogen-bond and proton-transfer interactions are the key to understanding many chemical reactions, biological activities, structure of molecular assemblies and supramolecules, functionalities in the solid state, etc. 

S. Bratos, J.-C. Leicknam, G. Gallot, and H. Ratajczak, Ultrafast Hydrogen Bond Dynamics and Proton Transfer Processes in the Condensed Phase, T. Elsaesser and H.J. Bakker (eds.), Kluwer Academic, Dordrecht, 2002, p. 5. 

---