Electrophilicity energy change

We call the carbocation, which exists only transiently during the course of the multistep reaction, a reaction intermediate. As soon as the intermediate is formed in the first step by reaction of ethylene with H+, it reacts further with Br in a second step to give the final product, bromoethane. This second step has its own activation energy (AG ), its own transition state, and its own energy change (AG°). We can picture the second transition state as an activated complex between the electrophilic carbocation intermediate and the nucleophilic bromide anion, in which Br- donates a pair of electrons to the positively charged carbon atom as the new C-Br bond starts to form. 

Also, it is interesting to note that in the smooth quadratic interpolation, the curve of the total energy as a function of the number of electrons shows a minimum for some value of N beyond N0 (see Figure 2.1). This point has been associated by Parr et al. [49] with the electrophilicity index that measures the energy change of an electrophile when it becomes saturated with electrons. Together with this global quantity, the philicity concept of Chattaraj et al. [50,51] has been extensively used to study a wide variety of different chemical reactivity problems. 

When two molecules approach each other, the initial interaction is dominated by (long-range) Coulomb forces. With decreasing intermolecular distance, the mutual polarization of the electronic structures of the compounds will increase, and at a reactive distance between the atomic centers A and B of the two molecules a partial delocalization of electronic charge between the two reaction sites takes place. Following a perturbational MO treatment as introduced by Klopman (1968), the energy change upon interaction between the atomic site A of a donor molecule (the nucleophile) and the atomic site B of an acceptor molecule (the electrophile), separated by the distance RAB, can be written as ... 

So that this approach can be adapted to the rate of analogous process, that is, the reaction between N- and E-X, the bond is assumed to extend to the critical distance characteristic of the transition state. The nucleophile then interacts with the electrophilic center with transfer of charge Ze. This reaction leads to partial bond formation and partial desolvation involving free energy changes yDN E and — aAGNs, respectively. 

FIGURE 12.1 Energy changes associated with the two steps of electrophilic aromatic substitution. 

Finally, consider the addition of an experimentally ambiphilic carbene (MeOCCl) to a simple alkene (e.g., propene). Now the carbene and aUcene FMO s are roughly in balance both differential FMO energies will be comparable. This is depicted in the A Case of Fig. 6. Now changing the alkene s Me substituent to BuO will raise both the K and 7t orbital energies of the alkene, and convert the A Case to the N Case i.e., the addition reaction will become electrophilic. Alternatively, changing propene s Me substituent to CN will convert the A Case to the N Case the addition reaction will become nucleophilic. Of course, these imaginary manipulations describe the behavior of an ambiphilic carbene, and the expression of these thought experiments is represented by the relative reactivities of MeOCCl collected in Table 2. 

The interaction energy (A ), Gibbs free-energy change (AG), reaction enthalpy (AH), and reaction electrophilicity (Am) of all the complexation reactions between the metalloboranes and the nucleotides, provided in Table 2, are negative, which ensures thermodynamic feasibility. The given metalloboranes thus effectively bind with nucleotide moieties and can be conceived as suitable anticancer drugs. 

Table 2 Interaction energy (A , kcal/mol), Gibbs free-energy change (AG, kcal/mol), reaction enthalpy (A//, kcal/mol), hardness (rj, eV), electrophilicity (co, eV), and reaction eleclrophilicity (Aco, eV) of (B3H3)2Ti, (B3H3)2V, and their corresponding adducts with the four nucleotides at B3LYP/6-311 + G(d) level of theory...

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