Proton transfer correlation effects

Scheiner, S., Szczesniak, M. M., and Bigham, L. D., Ab initio study of proton transfers including effects of electron correlation, Int. J. Quantum Chem. 23, 739-751 (1983). 

Since the rate was independent of acidity even over the range where H0 and pH differ, and the concentration of free amine is inversely proportional to the acidity function it follows that the rate of substitution is proportional to h0. If the substitution rate was proportional to [H30+] then a decrease in rate by a factor of 17 should be observed on changing [H+] from 0.05 to 6.0. This was not observed and the discrepancy is not a salt effect since chloride ion had no effect. Thus the rate of proton transfer from the medium depends on the acidity function, yet the mechanism of the reaction (confirmed by the isotope effect studies) is A-SE2, so that again correlation of rate with acidity function is not a satisfactory criterion of the A-l mechanism. 

Deuterium isotope effects on 1SN chemical shifts Deuterium isotope effects on 15N chemical shifts AN(D), similarly as for AC(D), can be employed in determination of mole fraction of the proton transferred form Schiff bases.44 A similar correlation between the mole fraction of NFI-form and AN(D) values was found (Figure 2). For the Schiff bases in which proton transfer takes place, the AN(D) values varied from —2 to + 5 ppm and depend on solvent and temperature. The AN(D) values of... 

By correlating the gas-phase and solution data derived from proton transfer reaction, the deviations were accounted for by solvation effects on the reactive centre and/or the substituent. 

The new treatment had its origins partly in ab initio molecular orbital calculations of substituent effects and partly in extensive studies of gas-phase proton transfer reactions from about 1980 (Section V.A). Various aspects of this work essentially drew attention to the importance of substituent polarizability. In 1986 Taft, Topsom and their colleagues252 developed a scale of directional substituent polarizability parameters , oa, by ab initio calculations of directional electrostatic polarization potentials at the 3-21G//3-31G level for a large set of CH3X molecules. The oa values were shown to be useful in the correlation analysis of gas-phase acidities of several series of substrates252, and such work has subsequently been extended by Taft and Topsom151. 

For a spin-1/2 nucleus, such as carbon-13, the relaxation is often dominated by the dipole-dipole interaction with directly bonded proton(s). As mentioned in the theory section, the longitudinal relaxation in such a system deviates in general from the simple description based on Bloch equations. The complication - the transfer of magnetization from one spin to another - is usually referred to as cross-relaxation. The cross-relaxation process is conveniently described within the framework of the extended Solomon equations. If cross-correlation effects can be neglected or suitably eliminated, the longitudinal dipole-dipole relaxation of two coupled spins, such... 

Kinetic Acidities in the Condensed Phase. For very weak acids, it is not always possible to establish proton-transfer equilibria in solution because the carbanions are too basic to be stable in the solvent system or the rate of establishing the equilibrium is too slow. In these cases, workers have turned to kinetic methods that rely on the assumption of a Brpnsted correlation between the rate of proton transfer and the acidity of the hydrocarbon. In other words, log k for isotope exchange is linearly related to the pK of the hydrocarbon (Eq. 13). The a value takes into account the fact that factors that stabilize a carbanion generally are only partially realized at the transition state for proton transfer (there is only partial charge development at that point) so the rate is less sensitive to structural effects than the pAT. As a result, a values are expected to be between zero and one. Once the correlation in Eq. 13 is established for species of known pK, the relationship can be used with kinetic data to extrapolate to values for species of unknown pAT. 

The rate coefficient of a reactive process is a transport coefficient of interest in chemical physics. It has been shown from linear response theory that this coefficient can be obtained from the reactive flux correlation function of the system of interest. This quantity has been computed extensively in the literature for systems such as proton and electron transfer in solvents as well as clusters [29,32,33,56,71-76], where the use of the QCL formalism has allowed one to consider quantum phenomena such as the kinetic isotope effect in proton transfer [31], Here, we will consider the problem of formulating an expression for a reactive rate coefficient in the framework of the QCL theory. Results from a model calculation will be presented including a comparison to the approximate methods described in Sec. 4. 

In some cases radical cations may undergo cycloadditions with an acceptor derived intermediate without prior proton transfer. This is observed especially for radical cations without sufficiently acidic protons, although it is not limited to such species. For example, the photoreaction of chloranil with 3,3-dimethylindene results in two types of cycloadducts [141]. In the early stages of the reaction a primary adduct is identified, in which the carbonyl oxygen is connected to the p-position of the indene (type B) in the later stages this adduct is consumed and replaced by an adduct of type A, in which the carbonyl oxygen is connected to the a-position. CIDNP effects observed during the photoreaction indicate that the type B adduct is formed from free indene radical cations, which have lost their spin correlation with the semiquinone anions. 

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