Resonance limit structures

The positive charge in 28 is stabilized by /j-cr-C-C-hyperconj ligation with the C-C-ring bonds of the two cyclopropyl moieties. In the parlance of VB theory this is described by resonance of 28 with non-bonding resonance limiting structures, the homoallenyl cation type structure 28a, the homopropargyl cation type structure 28b and the Dewar-type limiting resonance structure 28c. 

The isocyanate group reactivity is based on the polarisation of these groups represented by the following resonance limit structures [1] (equation 1.1) ... 

Hall112,329 has proposed a unifying concept based on tetramethylen.es (resonance hybrids of 1,4-diradical and zwitterionic limiting structures - Scheme 3.67) to rationalize all donor-acceptor polymerizations. The predominant character of the tetramethylenes (zwitterionic or diradical) depends on the nature of the substituents. 12" 40 However, more evidence is required to prove the more global application of the mechanism. 

The linear and nonlinear optical properties of one-dimensional conjugated polymers contain a wealth of information closely related to the structure and dynamics of the ir-electron distribution and to their interaction with the lattice distorsions. The existing values of the nonlinear susceptibilities indicate that these materials are strong candidates for nonlinear optical devices in different applications. However their time response may be limited by the diffusion time of intrinsic conjugation defects and the electron-phonon coupling. Since these defects arise from competition of resonant chemical structures the possible remedy is to control this competition without affecting the delocalization. The understanding of the polymerisation process is consequently essential. 

This redistribution of electrons provides us with one or more new resonance structures (also called canonical structures, limiting structures, or... 

Protonation on C-2 gives an unfavourable primary carbocation. On the other hand, protonation on C-1 gives a favourable resonance-stabilized allylic carbocation. The two products are then formed by captnre of chloride in a ratio that reflects the relative contribution of the limiting structures for the allylic carbocation one is tertiary and the other is primary. 

You may think that is the end of the problem but, since we have an unsymmetrical diene, it is also necessary to consider protonation of the other double bond. Protonation on C-4 also gives a favourable resonance-stabilized allylic carbocation, this time with primary and secondary limiting structures. Protonation on C-3 gives an unfavourable primary carbocation with no resonance stabilization. Since the products formed are related to initial protonation at C-1, it is apparent that, despite the stability associated with an allylic cation, a tertiary limiting structure is formed in preference to that with a secondary limiting structure. 

On the other hand, Searles and co-workers, arguing on the basis of the low electron density on oxygon indicated by nuclear magnetic resonance and basicity data, favored a hybrid composed of limiting structures (IVe), (IVb), and (IVe). 

Dithiolenes are best considered to be a resonance hybrid of the limiting structures (1)—(3). In both bis- and tris-dithiolenes the electron delocalization is not limited to the ligand, but includes the metals to give rise to cyclic delocalization ( aromaticity ). To symbolize this electron delocalization in dithiolenes, they can be represented, in a manner similar to that used for benzene, by formulas containing a ring inside the framework given by the metal, sulfur and carbon atoms. We will use this notation, shown in (4), throughout this chapter. 

The para-methoxy group is involved in the p-rr-resonance delocalization of the positive charge as shown in the resonance structure 323B of the limiting structures 323A-323D. Hence, the C4-oxygen bond in 323 has a partial double bond character. 

The tetramethylene is a resonance hybrid of 1,4-diradical (, = y in Eq. (22)) and zwitterionic (, = +,—) limiting structures. The character of the tetramethylene is determined by the nature of the terminal substituents. A very strong donor substituent at one of the terminal carbons and a very strong acceptor substituent at the other leads to zwitterionic intermediates. Otherwise, for instance, phenyl or vinyl group at the donor terminal and diester, cyano-ester or anhydride at the acceptor terminal, will favor the diradical form. 

Heat resistance of organic polymers is far lower than that of metals, ceramics, and glass. There have been major improvements, based on aromatic and heterocyclic resonance, ladder structures, and other mechanisms, and we may see further improvement in the future. Perhaps more serious limitations are the high cost of synthesis and the difficulty of processing these polymers into the desired final products. This is an area where the polymer chemist could use more help from the plastics engineer. 

The intense colors of compounds 3-6 (Table XV) are attributed to the pronounced charge separation due to resonance between the dithiolium and quaternary ammonium-dithiole limiting structures.23 Corresponding to this interpretation, the band observed at 515-540 min weakly acidic media suffers a blue shift to 375-390 mp, in 70% perchloric acid (the dimethylamino group is fully protonated). 

A limiting ionic structure with a doubly charged positive chromium ion and six CN ions is not in agreement with the magnetic data, since the Gr + ion has four unpaired electrons. Furthermore, such a structure cannot resonate with the form XXVII since resonance can only occur when the value of the total spin is identical in both forms. The ionic limiting structure must, therefore, be given by XXVIII, An example of an intermediate resonance structure is given by XXIX. 

The magnetic moment of Gr(NH3)3Gl3 indicates the existence of three unpaired electrons in agreement with the resonance between structures VIII and IX and other types which are intermediate between the limiting ionic and covalent forms. 

The high reactivity of chelated lithium alkyl compounds severely limits structural study of pure compounds, particularly in aromatic solvents. Most of our more recent work on chelated lithium alkyl systems used H and 7Li NMR to observe various metalation reactions like the self-metalation or aging reaction of TMED LiBu in heptane (I, 2). Much of our current insight into the structural features of chelated alkali metal systems comes from careful quantitative study of systems with relatively stable anions like resonance stabilized carbanions (5) and the systems described in this paper. We discuss magnetic resonance experiments on two systems (a) chelated lithium halides Chel LiX, examples of the recently discovered inorganic salt chelates (6), and (b)... 

The N-N bond has single bond character. Therefore, pyridazine can be described as a resonance hybrid with limiting structures a and b, the canonical form a making the major contribution ... 

---