Aromaticity potential energy

If the Lewis base ( Y ) had acted as a nucleophile and bonded to carbon the prod uct would have been a nonaromatic cyclohexadiene derivative Addition and substitution products arise by alternative reaction paths of a cyclohexadienyl cation Substitution occurs preferentially because there is a substantial driving force favoring rearomatization Figure 12 1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution For electrophilic aromatic substitution reactions to... 

FIGURE 12 1 Potential energy diagram for elec trophilic aromatic substitu tion... 

Divide the heats of combustion by the number of carbons The two aromatic hydrocarbons (benzene and [18]annulene) have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons (cyclooctatetraene and [16]annulene) On a per carbon basis the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons... 

Figure 12.1 is a potential energy diagram describing the general mechanism of electrophilic aromatic substitution. For electrophilic aromatic substitution reactions to... 

Fig. 12-7. Potential energy contour diagram showing the course of an aromatic substitution X+ + ArH - ArX + H+ (after Zollinger, 1956 a).

The formation of such excimers, which only exist in the excited state, is commonplace among polynuclear aromatic hydrocarbons, the simple potential energy diagram for which is shown in Figure 6.4. 

As the final example of the simplified treatment of an aromatic ring, a novel potential energy calculation of a naphthoquinone derivative (36) with a program called EENY will be briefly mentioned (155). This program seems to resemble WMIN in that it calculates only van der Waals energy. The rotation around the cyclopropyl-quinonoid bonds is calculated to have a barrier of about 10 kcal/mol. In this and another case (156), the results could be considerably improved by full relaxation MM calculations. 

Several descriptions of the process of addition of the electrophile X" " to aromatic substrates, based on kinetic and other evidence have been given and most versions agree that the potential energy surface does not consist of a simple barrier, but involves details relating to metastable intermediates. 

The planar form of phosphole is a first-order saddle point on the potential energy surface, 16—24 kcal/ mol above the minimum (at different levels of the theory). ° (The calculated barriers are the highest at the HF level, which underestimates aromatic stabilization of the planar saddle point, while the MP2 results are at the low end.) It has been demonstrated by calculation of the NMR properties, structural parameters, ° and geometric aromaticity indices as the Bird index ° and the BDSHRT, ° as well as the stabilization energies (with planarized phosphorus in the reference structures) ° and NIGS values ° that the planar form of phosphole has an even larger aromaticity than pyrrole or thiophene. 

Figure 4.5, Potential energy diagrams for the homogeneous electron transfer reaction between an aromatic radical-anion and a second aromatic with a frangible R-X bond, (a) The situation where back electron transfer and bond cleavage have similar free energy of activation, (b) The situation where the RX radical-anicm has high energy and the R-X bond has low dissociation ertergy.

The formulation of structural criteria rests essentially on the idea that the 7r-delocalization is the factor that causes the aromatic stabilization. The following manifestations of 7r-delocalization are considered in the connection the planar geometry of the ring as a factor dictated by the requirement for better overlap of the p -orbitals, equalization of the lengths of the bonds in the ring, and the correspondence of the most symmetrical structure to a minimum on the potential energy surface (PES). 

The dicarboxonium ions would be useful intermediates for the diacylation of aromatics. The 1,2-dicarboxonium ion (oxalyl dication, 17) has yet to be experimentally obtained. The ionization of the oxalyl fluoride in SbFs presumably forms the donor-acceptor complex, 18, which spontaneously decomposes to CO and COF2. The expected oxalyl dication (OCCO), 17, was not observed although theoretical calculations at MP2/6-31G level indicate 17 to be a minimum on the potential energy surface. 

The adsorption and diffusion properties of benzene are of immense interest in zeolite research aromatics play important roles in a number of zeolite-catalyzed processes. Theoretical simulations of benzene diffusion first began to be published in the late 1980s. The first studies evaluated and minimized the potential energy of a molecule such as benzene within the channels, a method less computationally demanding than the MD simulations that followed. Most recent studies have used the TST formalism. 

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