Experimental Facts and Their Interpretation

Experimental facts and their interpretation A Stereoselectivity in biologically active compounds... 

Abstract In this chapter, four topics are treated. (1) Fundamental constituents and interactions of matter and the properties of nuclear forces (experimental facts and phenomenological and meson-field theoretical potentials). (2) Properties of nuclei (mass, binding energy, spin, moments, size, parity, isospin, and characteristic level schemes). (3) Nuclear states and excitations and individual and collective motion of the nucleons in the nuclei. Description of basic experimental facts and their interpretation in the framework of shell, collective, interacting boson, and cluster models. The recent developments, few nucleon systems, and ah initio calculations are also shortly discussed. (4) In the final section, the a- and P-decays, as well as the special decay modes observed far off the stability region are treated. 

These experimental facts have been interpreted as follows (77). For the formation of a complex between a hard donor and a hard acceptor, strong bonds between these and the water molecules of their hydration shells have first to be broken. This takes much energy which is not completely regained by formation of the predominantly electrostatic acceptor to donor bond. The net reaction therefore tends to be endothermic. On the other hand, the resulting liberation of several water molecules from the hydration shells implies a large gain of entropy which in fact constitutes the driving force of the reaction between hard particles. 

In the last decade, an intense and successful investigation of this phenomenon has focused on its mechanism. The experimental facts discovered and the debate of their interpretation form large portions of these volumes. The views expressed come both from experimentalists, who have devised clever tests of each new hypothesis, and from theorists, who have applied these findings and refined the powerful theories of electron transfer reactions. Indeed, from a purely scientific view, the cooperative marriage of theory and experiment in this pursuit is a powerful outcome likely to oudast the recent intense interest in this field. 

This is the beauty of this quantity which provides specifically a direct geometrical information (1 /r% ) provided that the dynamical part of Equation (16) can be inferred from appropriate experimental determinations. This cross-relaxation rate, first discovered by Overhau-ser in 1953 about proton-electron dipolar interactions,8 led to the so-called NOE in the case of nucleus-nucleus dipolar interactions, and has found tremendous applications in NMR.2 As a matter of fact, this review is purposely limited to the determination of proton-carbon-13 cross-relaxation rates in small or medium-size molecules and to their interpretation. 

Although the complexity increases rapidly there is no reason that Walsh diagrams cannot be constructed for XY3 pyramidal, XY4 tetrahedral, XYS octahedral, and other molecules. In fact, they have been prepared, but their applications will not be described here. Insofar as these diagrams are amenable to quantitative interpretation, the predictions are in accord with what we know from experimental evidence and valence bond methods. 

The validity of Johnston s interpretation of the experimental facts in terms of the simple unimolecular dissociation (1) has been questioned by Lindars and Hinshelwood120 and by Reuben and Linnett121. These workers maintain that isothermal plots of k versus p are not smooth curves, but consist of a number of straight lines linked by markedly curved portions. To explain such behaviour they incorporate into their mechanism a collision-induced crossover of vibrationally excited N20 (XS) to repulsive 3II and 3E states. While we incline towards the simpler view held by Johnston105 and others106-116, we feel that this feature of the decomposition kinetics merits further investigation. 

Unfortunately, the interpretation of Giletti et al. (1978) does not solve the problem of differential diffusivities of 0 and To do this, their experimental results should be interpreted in terms of interdiffusion of 0 and O. Application of Pick s first law to interdiffusion of the two species would in fact lead to the definition of an interdiffusion coefficient/), so that... 

Quantum mechanics has made important contributions to the development of theoretical chemistry, e.g. the concept of quantum mechanical resonance in the interpretation of the perturbation in the excited states of polyelectronic systems, the concept of exchange in the formation of a covalent bond, the concept of non-localized bonds (though, in my view, unsatisfactory and only arising from a neglect of electronic repulsions), the concept of dispersion forces etc., but it is noteworthy that all these ideas owe their success and justification to their ability to account qualitatively for previously unexplained experimental facts rather than to their quantitative mathematical aspect. 

The problem was that a theory is formed in each discipline, based on their limited experimentation and interpretation. This method is analogous to a group of blind men who are allowed to feel and touch different body parts of an elephant and each comes up with a description of how an elephant as a whole should look like. This is to say that the description offered by each blind man may not be completely untrue but it cannot be used to describe an elephant as a whole. Inasmuch as the experimental observation made by each discipline comes from the same material no theory will be complete unless it can explain at least qualitatively all the experimental facts. These shortcomings led physicists to talk only about the quasi-free electrons and completely ignore the possible existence of covalent... 

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