Resonance structures. See

Fig. 41. Proposed mechanisms for the reactions (a) Y + propene, (b) Y + cis-2-butene, (c) Y + 1-butene, (d) Y + isobutene. Note that the mechanism for Y + trans-2-butene is similar to that for Y + cis-2-butene and so is not shown. Double-sided arrows indicate resonance structures. See text for details.

These simple examples illustrate the basic rules for mechanism and the use of curly arrows. The concepts are no different from those we have elaborated for drawing resonance structures (see Section 2.10) ... 

Figure 5.5.6-1 Conversion of the retinenes into chromophores. (a) Retinaldehyde as conventionally displayed, (b) presentation rotated to stress length of conjugated carbon chain, (c) presentation modified to stress total conjugation, (d) presentation modified to illustrate combined conjugation and electronic resonance over the maximum length of the molecule, (e) presentation modified to show the potential separation of the conjugated structure from the electronically resonant structure. See text for details...

Ozone is known as a very reactive agent in both water and air. The high reactivity of the ozone molecule is due to its electronic configuration. Ozone can be represented as a hybrid of four molecular resonance structures (see Fig. 2). As can be seen, these structures present negative and positively charged oxygen atoms, which in theory imparts to the ozone molecule the characteristics of an electrophilic, dipolar and, even, nucleophilic agent. 

It would seem that the ideal bonding model would be one with the simplicity of the LE model but with the delocalization characteristics of the MO model. We can achieve this by combining the two models to describe molecules that require resonance. Note that for species such as 03 and NO3- the double bond changes position in the resonance structures (see Fig. 14.47). Since a double bond involves one <7 and one tt bond, there is a a bond between all bound atoms in each resonance structure. It is really the tt bond that has different locations in the various resonance structures. 

As mentioned in the introduction, the wave function of CO may be approximated as a superposition of three resonance structures (see Fig. la). In the example of Cr(CO)6, we discussed the bonding on the basis of structure I. The only orbitals which seem to incorporate structures II and III are the delocalized double bond resonance structures may be important in describing the bonding in such molecules. In our recent study of the PtCO molecule we have found that besides the Pt-CO dative bond VBO structure, the Pt=CO structures shown schematically in Fig. 5a contribute quite significantly. The SOPP orbitals for one of the double bond resonance structures are shown in Fig. 5. The bonds between the d,s,p-hybrids on Pt and the hybrids on the carbon atom are seen to be bent. 

A description of both the allenyl and propargylic tautomers is included here because there is convincing evidence that the former often contains a significant contribution from a propargylic resonance structure (see Section V). 

Analysis of this data reveals that the parameters are very similar to those of the monocyclic model compound A -carboxamidopyrazole, but there is a small shielding effect due to contribution of the resonance structures (see Scheme 1), which may be indicative of a small magnetic ring current. The H coupling constants are of similar magnitude to those of simple pyrazoles. The chemical shift of the carbonyl carbon appears in the amide rather than in the ketone region. 

A novel disodium salt of bisdithiocarbamate of urea 69 and its Cu(ll) complex have been prepared and characterized. Thermal decomposition of 70 at 360 °C afforded 69 as a white crystalline residue, which was hygroscopic, water soluble, and still had good complexing ability for various metal ions. This material was found to be stable up to 800 °C, with the stability being attributed to the existence of several possible resonating structures (see Scheme 7) <2004MI139>. 

The resulting carbocation can be stabilized by resonance in fact, it is a hybrid of two contributing resonance structures (see Sec. 1.12). 

Allyl and benzyl radical are substantially stabilized, as anticipated from the resonance structures (see Section 1.3.6). Comparing the BDEs of propene and toluene to an appropriate reference such as ethane suggests resonance stabilization energies of 12.4 and 14.1 kcal / mol, respectively. An alternative way to estimate allyl stabilization is to consider allyl rotation barriers (Eq. 2.12). Rotating a terminal CH2 90° out-of-plane completely destroys allyl resonance, and so the transition state for rotation is a good model for an allylic structure lacking resonance. For allyl radical the rotation barrier has been determined to be 15.7 kcal / mol, in acceptable agreement with the direct thermochemical number. 

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