Qualitative theory

Although sophisticated electronic structure methods may be able to accurately predict a molecular structure or the outcome of a chemical reaction, the results are often hard to rationalize. It therefore becomes difficult to apply the findings to other similar systems. Qualitative theories, on the other hand, are unable to provide accurate results, but they may be useful for gaining insight, e.g. why a certain reaction is favoured over another. They also provide a link to many concepts used by experimentalists. 

FIGURE 8-4 Effect of contact angle on emulsion type. 

Studies on the coalescence of oil and water droplets at the oil-water interface confirm the conclusion that the dispersed phase will be the one that makes the 

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This qualitative theory still provides the most widely used means for describing reactions in organic chemistry. Two principal modes of electronic interaction in organic molecules are recognised the inductive and mesomeric effects. 

The Variation of J with Temperature. Although each proton transfer has its own characteristic value of 0, the variation of K near the maximum shows a marked degree of uniformity, as already mentioned in Sec. 64. If a parabola of the form p(T — 0)2 is fitted to the experimental results, a single value of p, namely 5 X 10-6, reproduces the variation of log K, not only for proton transfers of class III, but also for those of class II and class IV. If we accept (140) as providing a qualitative theory of the phenomena, we have at once a physical explanation of the observed uniformity. Whether we are concerned with the... 

The activated complex can be described as involving resonance of the fourth bond of carbon between the hydroxyl and iodine ions. Some very interesting rough quantum-mechanical calculations bearing on the theory of chemical reactions have been made of Eyring and Polanyi and their collaborators. It is to be hoped that the quantitative treatments can be made more precise and more-reliable but before this can be done effectively there must take place the extensive development of the qualitative theory of chemical reactions, probably in terms of resonance. 

However, there is much to be harvested by seeking the underlying rules governing molecular properties of a similar family and distinguishing those rules from the sophisticated numerical results for individual molecules. For this purpose, qualitative theories are still desirable to provide useful concepts for elucidating intriguing molecular structures and chemical reactions, and more importantly, to predict the observable properties of new molecules before we carry out resource-consuming computations or experiments. 

The period 1930-1980s may be the golden age for the growth of qualitative theories and conceptual models. As is well known, the frontier molecular orbital theory [1-3], Woodward-Hoffmann rules [4, 5], and the resonance theory [6] have equipped chemists well for rationalizing and predicting pericyclic reaction mechanisms or molecular properties with fundamental concepts such as orbital symmetry and hybridization. Remarkable advances in aeative synthesis and fine characterization during recent years appeal for new conceptual models. 

J. Cronin, Differential Equations Introduction and Qualitative Theory, Second Edition (1994)... 

A. N. Michel and K. Wang, Qualitative Theory of Dynamical Systems (1995)... 

Table 5.6 Values for the angular integrals used in the qualitative theory...

The mobility of a species ut is a parameter connecting diffusion and migration processes. This section will be concerned primarily with the qualitative theory of mobility, which has characteristic properties for... 

The most accepted among the qualitative theories of mass spectral fragmentation are the conception of charge and unpaired electron localization and the estimation of ions and neutral particles stability. Despite their qualitative character these approaches are quite useful to work with mass spectra. Both theories use the principle of the minimal structural changes at each stage of fragmentation, while the structure of the molecular ion is considered to be the same as that of the initial molecule. Certain isomerization processes of M+ before the fragmentation are usually a matter of special study. 

The Diels Alder reaction has been an important testing ground for qualitative theories for many years256,257K In this section we consider the importance of nonbonded attractive interactions on Diels Alder reactivity. Our model reaction system is butadiene and the three isomers of dicyanoethylene ... 

While the theoretician might be interested in the subtle features of a problem which can only be revealed by explicit computation, the organic chemist is more interested in a general qualitative theory which can be directly applicable to problems of interest without the need of computer assistance. Hence, we should reduce the quantitative scheme to a qualitative scheme which can still retain the important features of the theory. The various steps which one has to follow in attempting a qualitative configuration interaction analysis are given below ... 

It is true that the idea of resonance energy was then provided by quantum mechanics. . . but the theory of resonance in chemistry has gone far beyond the region of application in which any precise quantum mechanical calculations have been made, and its great extension has been almost entirely empirical.. . . The theory of resonance in chemistry is an essentially qualitative theory, which, like the classical structure theory, depends for its successful application largely upon a chemical feeling that is developed through practice. 46... 

F. Perrin Discussion on Jean Perriris diagram for the explanation of the delayed fluorescence by the intermediate passage through a metastable state First qualitative theory of fluorescence depolarization by resonance energy transfer... 

Carbocyclic molecules occupy a special position in stereochemistry. While Barton s proposal (80) triggered explosive developments in the conformational analysis of six-membered ring compounds (80a), the inability of the qualitative theory to interpret the behavior of rings other than six membered prompted Hendrickson to utilize the MM method (9b). Further developments in this area have been well summarized (10, 81). Some of the more recent topics are mentioned below. 

L.P. Shilnikov, A.L. Shilnikov, P.V. Turaev, and O.L. Chua. Part II. World Scientific, chapter Methods of qualitative theory in nonhnear dynamics. Series in Nonlinear Science, 2001. 

Ref 5, p 123) and the "Geometrical Model Theory of Cook (described under Detonation, Geometrical Theory of Cook) (Ref 15, pp 125-26). There is also "Qualitative Theory described by Taylor (Ref 3, pp 145-48)... 

Detonation, Qualitative Theory of. A brief description of a qualitative theory of the variation of detonation vel with chge diameter and the effect of confinement, as described by Taylor (1952), pp 145-48 is as follows ... 

This qualitative theory gives a picture of why, for any expl, there is a critical min diam below which the expl fails to detonate, another less clearly defined diam above which the expl detonates at its max and, in between, a range of diam over which the deton vel increases as the max vel is approached. It also explains why encasing the expl in a rigid tube increases the vel of deton... 

Harkins proposed tile mechanism of emulsion polymerization in 1943 (2). His qualitative theory needs a few corrections but its basis has been and will be accepted. The theory is explained using Figure 11.1.2. 

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