Geminal principle

These sense in which terms like conjugative interactions, nonbonded interactions, etc., are meant will become clear when we discuss each individual type of interaction or effect. Suffice to say that, in many instances, conjugative interactions as well as geminal interactions or bond ionicity effects contain implicitly the idea of nonbonded interactions. Thus, it should be emphasized that the labels of the basic types of interactions proposed here reflect the way in which the problem is formulated rather than different electronic principles. 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

However, Waite s approach has several shortcomings (first discussed by Kotomin and Kuzovkov [14, 15]). First of all, it contradicts a universal principle of statistical description itself the particle distribution functions (in particular, many-particle densities) have to be defined independently of the kinetic process, but it is only the physical process which determines the actual form of kinetic equations which are aimed to describe the system s time development. This means that when considering the diffusion-controlled particle recombination (there is no source), the actual mechanism of how particles were created - whether or not correlated in geminate pairs - is not important these are concentrations and joint densities which uniquely determine the decay kinetics. Moreover, even the knowledge of the coordinates of all the particles involved in the reaction (which permits us to find an infinite hierarchy of correlation functions = 2,...,oo, and thus is... 

In fact, Waite s approach needs a kind of demon to mark (enumerate) pairs of particles at moments of their birth and then following the time development w(r,t) of all such geminate pairs - even when these pairs already completely have mixed at the bimolecular reaction stage What is said above demonstrates quite well how the violation of statistical principles in deriving kinetic equations can lead to unphysical paradoxes (discussed in detail in [14, 15]). 

For geminal nuclei A and X, the Pauli principle favors a parallel precession (Fig. 1.11 (b)). A stabilization-destabilization pattern opposite to the directly bonded and vicinal nuclei arises (Fig. 1.11(a)), and the coupling constant is defined to be negative (Ax < 0). There are experimental techniques to determine the relative sign of coupling constants [5]. 

A related approach is the treatment of geminal dihalides with metal reagents to give an intermediate a-haloalkylmetal species, from which halide may in principle be lost in a net overall a-elimination of the two halide substituents (see Section 4.6.4.2). 

These effects, which have also been called symbiosis, double bond-no bond resonance, clustering, or geminal or anomeric effect [54], can also be explained in terms of the HSAB principle X3C+ will be harder than H3C+ if X is hard. Accordingly, X3C+ will have a higher affinity for the hard X- than H3C+ will have [54]. 

Many experimental data concerning the M.I.R. pertain to the back e.t. between geminate ions, prior to separation in polar solvents [53]. In some cases direct spectroscopic measurements can be made, although these depend on the correctness of the assignment of the absorption spectra. In principle the geminate ion recombination is expected to follow first order kinetics, whereas the diffusional recombination of free ions follows second order kinetics, so there is here another possibility to distinguish between the two processes [54], In some cases, the rate... 

These or similar sets were sometimes used for the rough interpretation of the experimental data [109,187], but in principle EM is much worse than other approximations. The escape from the reaction zone and even more so from the Coulomb well does not proceed by a single jump described as an exponential (rate) process even if /tsep is given a reasonable estimate as in Eq. (3.91). This simplification ignores all subsequent re-contacts and an essential nonexponen-tiality of the whole geminate process [20]. 

In principle, geminal radical pairs in polymeric matrices, A... B, can select among a myriad of processes. The empirically observed major modes of reaction and motion, summarized in Scheme 13.1, include (1) recombination to reform their precursor ... 

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