Resonance structure count

In Fig. 8 the preferred sites for the unpaired electron are estimated from the numbers of resonance structures counted when the electron is fixed at that site. The structural differences between Figs. 6 and 7 can now be understood. In addition the similarity between type uj and aj is explained and also its marginally more stable centre a.i. Also the similar properties of and 63 are rationalised as is the expectancy that is a more likely structure than c. 

The resonance structure count (RSC) in general involves a procedure for calculating the correct number of contributing resonance structures without actually writing them all down. We will not detail this procedure here. Once the procedure is completed, the resonance energy is calculated as the sum of four types of resonance transformations, which relate each resonance structure to another ... 

The theoretical basis of the resonance-structure-count method lies in valence-... 

Experimental log k2 values were correlated with Brown para-localization energies, Dewar reactivity numbers, Herndon structure count ratios, Hess-Schaad resonance energy differences, indices of free valence, and second-order perturbation stabilization energies. The latter are based on Fukui s frontier orbital theory [67] which classifies the Diels-Alder reaction of benzenoid hydrocarbons with maleic anhydride as mainly HOMO (aromatic hydrocarbon)-LUMO (maleic anhydride) controlled. However, the corresponding orbital interaction energy given by... 

Yokono et al. [85] have suggested that the results obtained by Lewis and Edstrom [84] can be understood in terms of the maximum value of the index of free valence as calculated by the HMO method. However, as Herndon [30] has shown, some discrepancies occur when the free valence approach is applied to the experimental findings. He found that the structure count ratio for the single position in each compound that would give rise to the most highly resonance stabilized radical is a reliable reactivity index to correlate and predict the qualitative aspects of the thermal behaviour of benzenoid hydrocarbons. 

Note that the method of counting the number of resonance structures containing a double bond between a given pair of carbon atoms is very crude and cannot distinguish among the last three of the listed bond types, each of which shows a double bond in only one resonance structure. Even within the framework of the limited resonance theory, it would be necessary to know the relative weighting of each of the two equivalent structures (a) and (b) with the nonequivalent structure (c). 

Benzene has two major resonance structures that contribute equally to the resonance hybrid. These are sometimes called Kekule structures because they were originally postulated by Kekule in 1866. You may also encounter benzene written with a circle inside the six-membered ring rather than the three double bonds. This representation is meant to show that the bonds in benzene are neither double nor single. However, the circle structure makes it difficult to count electrons. This text uses a single Kekule structure to represent benzene or its derivatives. You must recognize that this does not represent the true structure and picture the other resonance structure or call upon the MO model presented in Section 16.3 when needed. 

Answer. There are a total of 30 orbitals and 26 electrons to be utilized in bonding. The external B-H bonds utihze 12 orbitals and 12 electrons leaving 18 orbitals and 14 electrons for cluster bonding. Four three-center B-B-B and three two-center B-B bonds utilize 12 + 6=18 orbitals and 8 + 6 = 14 electrons. They may be placed on the framework as shown in the diagram above where the top and the bottom of the octahedron are shown separately. By counting you can find that B(l) and B(2) are associated with three three-center-two-electron bonds (and a B-H bond), B(3) and B(4) with two three-center and one two-center bonds (and a B-H), and B(5) and B(6) with one three-center and two two-center bonds (and a B-H), i.e., eight electrons around each B. Notice that one would need to draw a considerable number of resonance structures to give all the boron atoms the same electronic environment. 

To determine the molecular structure, we must count the electron pairs around the sulfur atom. In each resonance structure the sulfur has one lone pair, one pair in a single bond, and one double bond. Counting the double bond as one pair yields three effective pairs around the sulfur. According to Table 13.8, a trigonal planar arrangement is required, yielding a V-shaped molecule ... 

The analysis of the origins of these flaws showed that two of these, (a) and (d), should be looked upon as myths of uncertain origins, while (h) and (c) are the result of the misuse of simple resonance theory that just counts resonance structures. As Shaik and Hiberty have demonstrated, in each of the four cases the proper use of relatively simple qualitative VB theory leads to correct predictions which are just as convincing as those made by MO theory. 

Each resonance structure must have the same number of tt electrons. Count two for each tt bond only two electrons are counted for a triple bond because only one of the tt bonds of a triple bond can overlap with the conjugated tt system. Also, when a tt system carries a charge, count two for an anion and zero for a positive charge. 

BF3 is often described as a molecule in which boron is electron deficient, with an electron count of six. However, resonance structures can be drawn in which boron has an octet, with delocalized ir electrons. 

ASC represents a structure count exclusive of structures which do not contribute to stabilizing resonance interactions. However, the concept of parity and the derived ASC descriptor do not work in non-altemant systems with three odd-membered rings [Randic and Trinajstic, 1993a]. 

The d electron count of the metal is calculated by subtracting the metal s oxidation state from the number of valence electrons (including the two s electrons) in its elemental state. The d electron count is an inorganic chemistry term for unshared valence electrons. The d electron count of a metal has important ramifications for reactivity. For example, metallocyclopropane resonance structures cannot be drawn for alkene complexes of d° metals. 

The stability of the sigma complex is the guiding principle when determining the site of electrophilic attack on any aromatic ring. The more approximately equal energy resonance structures the carbocation has, generally the more stable it is. Resonance forms that place the positive charge next to an ewg are poor and should not be counted. 

In Worked Problem 4, l it was asserted that the Pli3C anion is more stable than the Ph2CH anion because of the extra stabilization caused by delocalization of the charge over the extra benzene ring. Draw and count the resonance structures for the two anions, with the charge on different carbon atoms, to confirm this. 

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