Positive ions proton transfer

The positive mode reagent ions consist of protons in various states of hydration H (H20)n, while the negative mode reagent species are primarily 0",02" and CO3. In the positive mode proton transfer (1) dominates the Cl chemistry i.e. 

The gas and aqueous-phase basicities of pyrimidine are distinctly smaller by 3.4 and 1.1 pAi units, respectively, than those of pyridazine. A theoretical explanation has been proposed by considering important NH and lone pair electrostatic interactions that act from the 2-position. The proton-transfer equilibrium for the basicity comparison shows that the position of equilibrium is in the direction expected for the dominant effect to be the relief of the destabilization imparted by electrostatic repulsion between lone pair electrons, i.e., to the left in the equation (Equation (1)). Another significant contribution to AG° values is the field inductive effect of the electronegative aza substituent, which is less at the 3- than at the 2-position. This aza substituent effect destabilizes cations. Second-order attractive interactions between the lone pair electrons and the adjacent NH in the conjugate acid of pyridazine are also invoked. The attractive electrostatic interaction in the conjugate acid of pyridazine, and the predominant lone pair repulsion in pyridazine, are opposed by a favorable aza substituent effect. This accounts for the positive AG°(g) value. Solvation by water is expected to preferentially stabilize the neutral species or ion which is internally destabilized. The result is a smaller equilibrium shift in solution compared with the gas phase <86JA3237>. 

The pJCj values are now available for many hydride complexes. Extensive tables have been compiled recently by Bullock and by Tilset. The rate of proton transfer to and from transition metals is rather slow (see below), so it is often possible to detect separate NMR signals for M-H and M , and tiius to determine the position of proton transfer equilibria between hydride complexes (M-H) and bases (B), or metal bases (M") and organic acids (HA). The pX values in Table 3.1 have been obtained in acetonitrile, an excellent solvent for acid-base chemistry because it solvates cations well enough to minimize ion pair formation it is both a weak acid and a weak base, with a very low autoprotolysis constant (ion product). ... 

In less polar solvents it is impractical to determine pK values, either because the solvent is not readily protonated (CH Cy or because the solvent permits extensive ion pair formation and its protonated form is unstable (THF). However, the positions of proton transfer equilibria in such solvents (e.g.. Equation 3.125) have been used to estimate relative aqueous pfC/s for hydride complexes that are insoluble in (or unstable in) water. CyjPH, whose aqueous pfC is 9.7, has often been used as an anchor—that is, the acidities of other RjPH and hydride complexes have been measured relative to CyjPH, leading to the "pseudo-aqueous" pJacidity scales in CHjClj and in THF. However, complications due to ion pairing make such pK values less reliable than direct measurements in CHjCN (or water). Morris has linked his THF acidity scale with acidities in DMSO, a solvent that minimizes ion pair formation, but is incompatible with many organometallic complexes. ... 

Mechanistic insights into the rhenium-catalysed alcohol-to-olefin dehydration reaction have been reported. The experimental studies showed the dependence on water and the intolerance to base, and the involvement of proton transfer in the catalytic cycle. A car-benium ion intermediate has been suggested. Kinetic isotope effect studies, furthermore, ruled out proton loss from the / -position of the alcohol as the rate-determining step. The DFT calculations indicated that the lowest energy pathway most likely proceeds through coordination of the alcohol to the rhenium centre with a subsequent carbon-oxygen breakage that yields a carbenium ion. Proton transfer from the carbenium ion to water. 

Several processes are unique to ions. A common reaction type in which no chemical rearrangement occurs but rather an electron is transferred to a positive ion or from a negative ion is tenued charge transfer or electron transfer. Proton transfer is also conunon in both positive and negative ion reactions. Many proton- and electron-transfer reactions occur at or near the collision rate [72]. A reaction pertaining only to negative ions is associative detaclunent [73, 74],... 

Up to the present the principal interest in heteroaromatic tautomeric systems has been the determination of the position of equilibrium, although methods for studying fast proton-transfer reactions (e.g., fluorescence spectroscopy and proton resonance ) are now becoming available, and more interest is being shown in reactions of this type (see, e.g., references 21 and 22 and the references therein). Thus, the reactions of the imidazolium cation and imidazole with hydroxyl and hydrogen ions, respectively, have recently been demonstrated to be diffusion controlled. ... 

The Zwitterions of Amino Acids. Now that we have decided what aspects are important, we may mention some other types of proton transfers. Consider, for example, the positive ion NHJ CHjCOOH of glycine. If we transfer the proton from the carboxyl group to a distant water molecule, we have... 

In any pure liquid, the transfer of a proton from one molecule to another (distant) molecule has been named autoprololysis. I11 any solvent this process creates a positive and a negative ion and must clearly belong to class II it will not differ from other proton transfers of class II except for the fact that the relation between Kx and K will be different. On the left-hand side of (127) and (128) there is no solute particle hence the increase in the cratic term is greater than in (119) or (121). In (128) we have Aq — +2, and... 

Substituted Ammonium Ions. Like NH4C1 the substance NH3-(CH3JCI, where a CH3 group has been substituted for one hydrogen, forms a crystalline solid and so do the substances NH2(CH3)2C1 and NH(CH3)3C1. When one of these substances is dissolved in water, it is completely dissociated into Cl- ions and molecular positive ions corresponding to (NH4)+. Suppose now that such a solution contains an NH3 molecule, and consider the following proton transfer... 

Furthermore, since in Sec. 121 we found the value J = 0.36 electron-volt for the proton transfer (211), this gives the occupied proton level of the (HCOOII2)+ ion a position at (0.52 — 0.36) = 0.16 electron-volt above that of the (H30)+ ion in formic acid as solvent. This is shown in Fig. 65, where, for comparison, a diagram for proton levels in aqueous solution has been included, the level of the (H30)+ ion in aqueous solution being drawn opposite to the level of the same ion in formic acid solution. This choice is quite arbitrary, but was made in order to show more clearly that we may expect that one or more acids that are strong... 

There are, in fact, two reasons why we should prefer to discuss proton transfers of class I. In concentrated solutions the average electrostatic forces between the ions will be intense. Only in proton transfers of class I does the number of positive and negative charges in the solution remain unaltered when the proton is transferred only here do we find the possibility that the contribution from the interionic forces will remain almost unchanged in a proton transfer. At the same time, although the number... 

It was pointed out in Sec. 126 that in any proton transfer of class I the number of negative ions remains unchanged and the number of positive ions remains unchanged and consequently there is the possibility that the contribution from the interionic forces shall remain unchanged. Whether this is so or not can be decided only by experiment. Consider what result should be obtained if (218) and (219) are applicable. In this case, if experimental values of the left-hand side of (218) are plotted... 

Chemical ionization (Cl) The formation of new ionized species when gaseous molecules interact with ions. This process may involve the transfer of an electron, proton, or other charged species between the reactants in an ion-molecule reaction. Cl refers to positive ions, and negative Cl is used for negative ions. 

This proton transfer step produces an intermediate that is very similar to a bromo-nium ion (a three-membered ring with a positive charge on an electronegative atom). Just as a bromonium ion can be attacked by water, similarly, a protonated epoxide can also be attacked by water ... 

Much effort has been expanded in drawing mechanistic inferences from the observation that cofacial bismetalloporphyrins containing a non-redox-active metal ion are fairly selective catalysts (e.g., (DPA)CoM, where M = Lu, Sc, Al, Ag, Pd, 2H, i.e., monometallic porphyrins Fig. 18.15). At least two hypotheses have been proposed (i) polarization of the 0-0 bond in catalytic intermediates by the second ion (on an N-H moiety) acting as a Lewis acid [CoUman et al., 1987, 1994] and (ii) spatial positioning of H+ donors especially favorable for proton transfer to the terminal O atoms of coordinated O2 [Ni et al., 1987 Rosenthal and Nocera, 2007]. To the best of my knowledge, neither hypothesis has yet been convincingly proven nor resulted in improved ORR catalysts. When seeking stereoelectronic rational of the observed av values, it is useful to be mindful that a fair number of simple Co porphyrins are also relatively selective ORR catalysts (Section 18.4.2). 

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