Dewar configurations

If we attempt, in accordance with condition 3 (p. 200), to construct all possible Lewis-Langmuir (non-polar) configurations for benzene with electron pair bonds, it appears that there are five of them. Besides the two Kekule configurations there are also three Dewar configurations. 

In various reactions it reacts with the formation of a six-memberedring this can be understood as a consequence of an important contribution of the Dewar configuration below to the active state (compare anthracene, p. 209). 

Besides the Kekule and Dewar configurations also polar configurations are possible (Craig) ... 

In addition one can also speak of a free valence (index) or indice de liaison fibre of 0.07 for each corner of the benzene molecule since the para-para bond in the DEWAR-configurations is not a real bond but rather represents two free electrons (with opposite spins) at opposite corners. 

In spite of early claims and counterclaims, there are no disadvantages to the demountable capsule and other manufacturers have followed suit and now also supply demountable detectors. For most users, this offers no advantage other than a flexibility that they may never need. Certainly, for the manufacturers there are significant advantages in that a detector need not be committed to a particular Dewar configuration until just before shipping. Because of this, it is a possibility that the demountable detector will become the norm for aU manufacturers. 

Certain equipment configurations allow for the use of Dewar flask testing at elevated pressures. Several arrangements have proved successful such as a sealed glass ampoule in the Dewar flask, a steel pressure vessel in the flask, a Dewar flask in an autoclave under inert gas pressure, and a stainless steel Dewar flask. Dewar flasks provided with an addition line can also be used to study chemical reactions. In Figure 2.21, typical temperature-time curves of Dewar flask experiments are shown. 

There are a number of different types of adiabatic calorimeters. Dewar calorimetry is one of the simplest calorimetric techniques. Although simple, it produces accurate data on the rate and quantity of heat evolved in an essentially adiabatic process. Dewar calorimeters use a vacuum-jacketed vessel. The apparatus is readily adaptable to simulate plant configurations. They are useful for investigating isothermal semi-batch and batch reactions, and they can be used to study ... 

The concept is in many ways attractive, and is based upon less speculative applications of quantum mechanical ideas than those described earlier in this section, but it does not result in any form of chemical bonding in the usual sense either by partial or complete a bonds or tr bonds. It would seem to be, in fact, more appropriate in describing stable Tt complexes as visualized by Mulliken (1950) and Dewar (1951). The role of the frontier orbitals in providing the configuration that... 

Duncanson (i) of the molecular orbital bonding concepts of Dewar (2), which he developed to explain the structure of Ag+-olefin complexes, led to the suggestion that ethylene is symmetrically coordinated to the metal. Platinum, atomic number 78, has the electronic configuration of the xenon core (Is 2s 2p 35 3p 3d Z = 54), then... 

However, despite their proven explanatory and predictive capabilities, all well-known MO models for the mechanisms of pericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theory [3] and the Dewar-Zimmerman treatment [4-6] share an inherent limitation They are based on nothing more than the simplest MO wavefunction, in the form of a single Slater determinant, often under the additional oversimplifying assumptions characteristic of the Hiickel molecular orbital (HMO) approach. It is now well established that the accurate description of the potential surface for a pericyclic reaction requires a much more complicated ab initio wavefunction, of a quality comparable to, or even better than, that of an appropriate complete-active-space self-consistent field (CASSCF) expansion. A wavefunction of this type typically involves a large number of configurations built from orthogonal orbitals, the most important of which i.e. those in the active space) have fractional occupation numbers. Its complexity renders the re-introduction of qualitative ideas similar to the Woodward-Hoffmann rules virtually impossible. 

At this point, it is appropriate to draw a parallel with the straightforward MO explanations for the aromaticity of benzene using approaches based on a single closed-shell Slater determinant, such as HMO and restricted Hartree-Fock (RWF), which also have no equivalent within more advanced multi-configuration MO constructions. The relevance of this comparison follows from the fact that aromaticity is a primary factor in at least one of the popular treatments of pericyclic reactions Within the Dewar-Zimmerman approach [4-6], allowed reactions are shown to pass through aromatic transition structures, and forbidden reactions have to overcome high-energy antiaromatic transition structures. 

For a reaction adequately described by just two configurations, reactant and product, the analysis of substituents effects is straightforward and was first treated by Horiuti and Polanyi (1935) almost 50 years ago. Subsequent contributions by Bell (1936) and Evans and Polanyi (1938) have led to these general ideas being jointly termed the Bell-Evans-Polanyi principle (Dewar, 1969). The treatment of multiconfiguration reactions is analogous and is illustrated in Fig. 12. Let us discuss this in detail. 

CISD configuration interaction with singly and doubly excited determinants only DRE Dewar resonance energy... 

Cycloadditions are easier to treat than unimolecular reactions they only require an evaluation of the best FO overlap (rule 4). Let us look at the cyclodimerization of butadiene. Woodward and Hoffmann suggested that the experimentally observed endo compound is due to secondary interactions (shown by the double arrows above), which increase the stabilizing the FO s interaction.23 Cisoid configurations are often adopted by the dienophile in Diels-Alder reactions,24 as first suggested by Dewar.20 For... 

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