And Dewar structures

The Rumer diagrams for benzene actually mirror the real spatial symmetry, and thus the Kekule and Dewar structures emerge. 

FIGURE 7.4 (a) Kekule and Dewar structures of benzene, and their symmetry... 

With the advent of the computer era, it is now possible to reexamine and rethink the resonance theory at the ab initio level. For example, throughout Pauling and Wheland s books, benzene is supposed to be a hybrid of two Kekule structures, by noting that Dewar and other ionic structures make little contribution to the resonance in benzene. However, classical ab initio VB calculations with all possible 175 resonance structures by Norbeck et al. [51] and Tantardini et al. [3], where strictly atomic orbitals are used to construct VB functions, manifested that the five covalent Kekule and Dewar structures make even less contribution to the ground state of benzene than the other 170 ionic structures. This prompts us to reconsider the mathematical formulations for resonance structures [52]. 

Naphthacene XV) has five Kekul6 structures and Dewar structures with... 

In addition to the Kekule and Dewar structures in the separate benzene rings, diplienyl possesses additional structures with n bonds between the rings... 

Van Tamelen proposed another mechanism for this isomerization involving an equilibrium among the cyclopropenyl thioketone and Dewar structures (Scheme 1).3... 

The coefiidents of the characteristic, matching and sextet polyno mials are related to the number of Kekul and Dewar structures of the pertinent conjugated molecule. Hence all these polynomials are of some relevance in resonance theory. 

The benzene molecule is a hybrid of Kekule and Dewar structures shown in B and C, Fig. 91. Since the molecule is planar, Ladenburg s prism formula is excluded. The Claus and the centric formulae (see p. 802) can be formed from the Kekule and Dewar structures and are not what are called canonical structures . ... 

The Dewar-type structure for the central ring of the anthracene system in 19 is possible only because the 9,10 substituents prevent the system from being planar. From 19 is the actual structure of the molecule and is not in resonance with forms like 20, although in anthracene itself, Dewar structures and structures like 20 both contribute. This is a consequence of rule 2 (p. 41). In order for a 20-like stracture to contribute to resonance in 19, the nuclei would have to be in the same positions in both forms. 

From 1958, when Craig et al. (17) and Dewar et al. (18) introduced their intuitive and controversial ideas on the structure of cyclophosphazenes, until recently (79) the extent of the electronic delocalization in inorganic rings has constituted a challenge to both theoreticians and chemists. 

The beauty of the Dewar structure, Ingold remarked, lay in the fact that it explains quinoid transformations and aromatic tautomerism as well-established principles of aliphatic and alicyclic chemistry, that is, it provided a rationale for the unification of aliphatic and aromatic chemistry. 

Dewar structure org chem A structural formula for benzene that contains a bond between opposite atoms. dii-or, str3k-ch3r dew point chem The temperature and pressure at which a gas begins to condense to a liquid. du, point ... 

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