Classical mechanisms, hydrogen atom transfer

Complications that arise with this simple reaction are twofold. First, because of the low mass of the hydrogen atom its movement frequently exhibits non-classical behavior, in particular quantum-mechanical tunneling, which contributes significantly to the observed kinetic isotope effect, and in fact dominates at low temperature (Section 6.3). Secondly, in reaction 10.2 protium rather than deuterium transfer may occur ... 

In this subsection, we study the dynamical electron mechanism of the elementary process (in gas phase) of double proton transfer in FAD with an electron wavepacket method. To begin with, we ask ourselves a question what is the basic mechanism of this double proton transfer (Fig. 7.18(a)), hydrogen-atom migration (Fig. 7.18(b)), hydride transfer (Fig. 7.18(c)), or else (We term the event of Fig. 7.18(a) proton transfer tentatively imtil we can determine the mechanism.) At a glance none of the above three mechanisms seems totally satisfactory. What is wrong with these classic mechanisms ... 

The second example is the hydrogen atom tunneling in the triatomic molecule HO2 for zero total angular momentum (/ = 0). In this case an analytical DMBE (double many-body expansion) potential function is available [126]. Since the quantum mechanically exact calculation can be carried out, the numerical results based on the present semi-classical theory are compared with the exact results. The exact numerical calculations are carried out in the hyperspherical elliptic coordinates (p, f, r]) [127,128], which are convenient for describing a light atom transfer between two heavy atoms. The coordinate p is called hyperradius, which represents the mass scaled radius of the hypersphere and is defined as... 

Westheimer s interpretation of KIE is based on identical pre-exponential factors for H or D transfers, neglects the contribution of the bending vibrations, and assumes that classical mechanics can describe the movement along the reaction coordinate. It is, nevertheless, a useful approximation for mechanistic interpretations of KIE. For example, the observation of a sizeable KIE when a hydrogen atom is replaced by deuterium in a given compound indicates that the bond where the isotopic substitution was made is practically broken in the rate-determining step of the reaction. 

It is also remarkable that M- H-X systems show proton transfer which is mechanistically close to that of classical hydrogen bonds. Protonation of the metal atom occurs via M- H bonds and ion pairs (Scheme 2.4) lying as intermediates on the reaction coordinate. As we will see below, a very similar mechanism operates in the case of dihydrogen-bonded complexes. 

Reactions involving four electrons and three centres can include the formation of a chemical bond at the expenses of another bond which is consequently broken. A large variety of reactions can be explained by such a mechanism, by way of example attention here will be focused on bimolecular nucleophilic substitutions (Sn2) and proton transfers. Typically a four electron - three centre unit AXB, in which the central atom X could be a hydrogen or a carbon atom, is mainly described by the resonance of the following three classical VB structures... 

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