Proton switching

Electrical properties of liquids and solids are sometimes crucially influenced by H bonding. The ionic mobility and conductance of H30 and OH in aqueous solutions are substantially greater than those of other univalent ions due to a proton-switch mechanism in the H-bonded associated solvent, water. For example, at 25°C the conductance of H3O+ and OH are 350 and 192ohm cm mol , whereas for other (viscosity-controlled) ions the values fall... 

Despite this enormous viscosity, fused H3PO4 (and D3PO4) conduct electricity extremely well and this has been shown to arise from extensive self-ionization (autoprotolysis) coupled with a proton-switch conduction mechanism for the... 

Figure 12.17 Schematic representation of proton-switch conduction mechanism involving [U2PO4I phosphoric acid.

H2SO4.Z2H2O, are known with = 1, 2, 3, 4 (mps 8.5", -39.5". -36.4" and -28.3% respectively). Other compounds in the H2O/SO3 system are H2S2O7 (mp 36") and H2S4O13 (mp 4"). Anhydrous H2SO4 is a remarkable compound with an unusually high dielectric constant, and a very high electrical conductivity which results from the ionic self-dissociation (autoprotolysis) of the compound coupled with a proton-switch mechanism for the rapid... 

In carbonyl addition reactions, a commonly occurring and important mechanistic step is the transfer of a proton from one site to another in a reactive intermediate (proton switch). If the proton switch occurs sufficiently rapidly compared with the rate of collapse of the intermediate to reactants, the overall reaction may be facilitated by trapping of the unstable intermediate by the proton switch (Jencks, 1976). For example, in the formation of oximes from the reaction of benzaldehyde with O-methylhydroxylamine shown in (87H89) (Sayer and Jencks, 1973 Rosenberg et al., 1974), the first unstable intermediate (It) on the reaction pathway is converted by a proton switch (88) to the intermediate (I2) which has less tendency than It to... 

Scheme 11.14 Bifunctional acid-base catalysis of the proton switch which traps the zwitterionic intermediate in methoxyaminolysis of phenyl acetate.

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. 

Fig. 3.6. Displays of the gradient vector fields of the electronic charge density for configurations along the isomerization reaction coordinate for HCN. The label refers to the angle 0 formed between the C-N axis and the vector from the proton to the CN centre of mass. The bond path from the proton switches attractors, from the carbon nucleus to the nitrogen nucleus for some configuration lying between 6 - 72.1° and 72.4°.

Figure 1.47 Proton switching of the quadratic NLO response of pentacyanoiron(II)...

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