Nonlinear proton donor

Kiefer PM, Hynes JT (2002) Nonlinear free energy relations for adiabatic proton transfer reactions in a polar environment. I. Fixed proton donor—acceptor separation. J Phys Chem A... 

A similar nonlinear dependence (Figure 8.4) has been reported by Alkorta and co-workers [13]. The authors calculated a set of dihydrogen-bonded complexes where LiH plays the role of a strong proton acceptor while the C-H bond represents a proton-donor component whose strength changes within wide limits. The analytic expression of this dependence is... 

TABLE 8.3. Proton Affinities (kj/moi) Characterizing the CH3OCH3 and BH3N(CH3)3 Molecules at Hydrogen and Dihydrogen Bonding to Linear (a) and Nonlinear (b) Proton Donors... 

Figure 33 depicts a prednisolone calibration curve in dry ACN. The nonlinearity of this profile is obvious. Because there is absolutely no indication of adsorption effects, we hypothesize that there is a following second order chemical reaction. Hydrocortisone gives similar behavior in dry aprotic solvents. Both of these steroids are characterized by an aliphatic alcohol groip which is not associated with the other three ideally behaving steroids. This circumstantial evidence suggests that the anion radical electrode reaction product is being protonated by reactant diffusing toward the electrode. The likelihood that this is the process occurring is further enhanced by the fact that addition of sufficient amounts of a weak proton donor (e.g., H2O) leads to a linear calibration curve with lower sensitivity, as shown in Figure 34. The latter figure illustrates calibration curves for dry and wet ACN and DMF, as well as aqueous base.

The first chemical transformations carried out with Cjq were reductions. After the pronounced electrophilicity of the fullerenes was recognized, electron transfer reactions with electropositive metals, organometallic compounds, strong organic donor molecules as well as electrochemical and photochemical reductions have been used to prepare fulleride salts respectively fulleride anions. Functionalized fulleride anions and salts have been mostly prepared by reactions with carbanions or by removing the proton from hydrofullerenes. Some of these systems, either functionalized or derived from pristine Cjq, exhibit extraordinary solid-state properties such as superconductivity and molecular ferromagnetism. Fullerides are promising candidates for nonlinear optical materials and may be used for enhanced photoluminescence material. 

The first evidence for the protonation of alkanes under highly acidic (superacid) conditions was independently reported by Olah and Lukas, as well as by Hogeveen and coworkers. " Protolytic reactions of hydrocarbons in superacid media were interpreted by Olah as an indication of the general electrophilic reactivity of covalent C-H and C-C single bonds of alkanes and cycloalkanes. The reactivity is due to the donor ability of the a-bond electron pairs via a three-center, two-electron (3c-2e) bond formation and follows the trend tertiary C-H > C-C > secondary C-H primary C-H. The transition state for protolytic ionization of hydrocarbons was presumed to be linear. It was later suggested that such 3c-2e interactions in carbocations generally tend to be nonlinear (even in stericaUy crowded cases) in nature (see also Chapter 1) [Eqs. (6.3) and (6.4)]. That is, such interactions are similar to transition states proposed for frontside Se2 reactions. 

These alkynyl complexes can be protonated to afford vinylidene complexes, which can in turn be deprotonated to give the starting alkynyl complex, reactions that are spectroscopically quantitative. The tabulated data also provide the opportunity to assess the effect of this protonation, in proceeding from alkynyl complex to vinylidene derivative. One would perhaps expect that replacing the electron-rich ruthenium donor in the alkynyl complexes with a (formally) cationic ruthenium centre in the vinylidene complexes would result in a significant decrease in nonlinearity. 

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