Proton- and ion-transfer reactions

Contrary to outer sphere electron transfer reactions, the validity of the Butler-Volmer law for ion transfer reactions is doubtful. Conway and coworkers [225] have collected data for a number of proton and ion transfer reactions and find a pronounced dependence of the transfer coefficient on temperature in all cases. These findings were supported by experiments conducted in liquid and frozen aqueous electrolytes over a large temperature range [226, 227]. On the other hand, Tsionskii et al. [228] have claimed that any apparent dependence of the transfer coefficient on temperature is caused by double layer effects, a statement which is difficult to validate because double layer corrections, in particular their temperature dependence, depend on an exact knowledge of the distribution of the electrostatic potential at the interface, which is not available experimentally. Here, computer simulations may be helpful in the future. Theoretical treatments of ion transfer reactions are few they are generally based on variants of electron transfer theory, which is surprising in view of the different nature of the elementary act [229]. 

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],... 

Figure 5 Glow discharge mass spectra of brass acquired after different trapping periods. Proton and charge transfer reactions with adventitious water result in the removal of argon related ions.

The substrate water molecules are prepared in a stepwise fashion for 0-0 bond formation by binding to the M OjCa cluster and by (partial) deprotonation. The concerted oxidation of the activated substrate occurs then either in two 2e steps or in one concerted 4e reaction step, thus avoiding high-energy intermediates. It is the matrix (protein) and the Ca2+/CF ions that allow for the coupling of proton- and electron-transfer reactions to occur. These features are... 

A variety of thermochemical data were obtained for negatively charged metal complexes. For example, appearance energy measurements photodissociation, energy-resolved, collision-induced dissociation, electron-, proton- and anion-transfer reactions to and from negatively charged ions, were all used for obtaining a variety of thermochemical parameters for ions, molecules, and radicals. 

The major problem in method (a) is that in ion-molecule interchange, considerable momentum in the direction of travel of the incident ion is imparted to both final products. Hence, in a perpendicular type apparatus only transfer of low weight particles can be observed at all and only at very low velocities of the incident ions (1, 9, 10, 11, 12, 13, 19, 20, 23, 27). Cross-sections cannot be measured. The value of these investigations is that some ion-molecule reactions—e.g., proton transfer and hydride ion transfer—can be identified. The energetics and the competition between charge exchange and ion-molecule reactions can be discussed, and by using partially deuterated compounds, one can obtain a detailed picture of the reaction. 

However, a closer inspection of the experimental data reveals several differences. For ion-transfer reactions the transfer coefficient a can take on any value between zero and one, and varies with temperature in many cases. For outer-sphere electron-transfer reactions the transfer coefficient is always close to 1/2, and is independent of temperature. The behavior of electron-transfer reactions could be explained by the theory presented in Chapter 6, but this theory - at least in the form we have presented it - does not apply to ion transfer. It can, in fact, be extended into a model that encompasses both types of reactions [7], though not proton-transfer reactions, which are special because of the strong interaction of the proton with water and because of its small mass. 

In a recent upsurge of studies on electron transfer kinetics, importance was placed on the outer shell solvent continuum, and the solvent was replaced by an effective model potential or a continuum medium with an effective dielectric constant. Studies in which the electronic and molecular structure of the solvent molecules are explicitly considered are still very rare. No further modem quantum mechanical studies were made to advance the original molecular and quantum mechanical approach of Gurney on electron and proton (ion) transfer reactions at an electrode. 

If the electron-transfer step in an electrode reaction is preceded by a chemical reaction that involves proton transfer, the polarographic current often will be a complex function of the concentration of the electroactive species, the hy-dronium ion concentration, and the rate constants for proton and electron transfer. Currents controlled by the rate of a chemical reaction are called kinetic currents and often are observed in the reduction of electroactive acids (e.g., pyruvic acid), in which the protonated form of the acid is more easily reduced than the anion. A polarogram of pyruvic acid in unbuffered solution exhibits two waves whose relative wave heights depend on the concentration of pyruvic acid and the solution pH.59... 

Oxide dissolution in aqueous electrolytes involves transfer of metal and oxygen ions to the solution. Since O2- ions cannot be transferred into the solution, protonation must precede the ion transfer reaction, which leads to strongly pH-dependent dissolution rates [37], The cation and oxygen transfer reactions may be regarded as statistically independent with... 

Despite the different nature of the protonating agents, the different exothermicity of their reactions, and the large differences in the composition and the pressure of the gaseous system, the mass-spectrometric results of Field and Munson appear in excellent agreement with those obtained from the study of the reactions the HeT+ ion, as illustrated in Table 22 for the case of c-CeHjg. The formation of protonated cyclohexane, and the occurrence of hydride-ion transfer reactions from c-CgHig to propyl and pentenyl ions (equations 77 and 78 of Fig. 9) were observed by Abramson and Futrell (1967) in their study of the protonation of cyclohexane with CHO+ ions. It should be mentioned, however,... 

On the other hand, Kihara s group reported interesting ET systems for biological molecules including L-ascorbic acid [13], flavin mononucleotide (FMN) [14] and 3-nicotinamide adenine dinucleotide (NADH) [15]. While these ET systems are very important from a biological viewpoint, their reaction mechanisms are often complicated by the coupling of ET and proton or ion transfer. 

Carbenium ions react with neutral nucleophiles to produce onium ions. A favorable equilibrium between active carbenium ions and temporarily inactive onium ions can be used to produce well-defined polymers (Chapter 4). However, rather than reacting directly with carbenium ions, nucleophiles may also react with Lewis acids to form strong complexes, thereby reducing their activity and ability to ionize covalent compounds. A third reaction that basic nucleophiles may be involved in is /3-proton elimination this transfer reaction may subsequently result in termination if it involves a strong base [Eq. (131)]. 

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