Isodesmic reactions intermediates

In order to obtain some insight concerning the effect of fluorine on metabolic activation of fluorinated PAHs, isodesmic reactions (Figure 12) were calculated as a measure of the relative ease of formation of the fluorinated epoxides. Almost no energy differences were observed, consequently formation of these intermediates could be assumed not to be as relevant as carbocation formation in determining the relative reactivity of the ultimate metabolites. 

The 1,2,3-tri-f-butylcyclopropenium cation is so stable that the perchlorate salt can be recrystallized from water. An X-ray study of triphenylcyclopropenium perchlorate has verified the existence of the carbocation as a discrete ion. ° Quantitative estimation of the stability of the unsubstituted cyclopropenium ion can be made in terms of its pXjj+ value of —7.4, which is intermediate between such highly stabilized ions as triphenylmethyl cation and the M-(4-methoxyphenyl)methyl cation. (See Section 4.4.1 for the definition of pXr+). An HF/6-31G MO calculation on the following isodesmic reaction ... 

Enthalpies are evaluated using ab initio (G3MP2B3 and G3) and Density Functional Theory (DFT) methods, group additivity (GA) and literature data. The ab initio and DFT methods are combined with isodesmic reaction analysis to improve the calculated enthalpy values of stable species (reactants, intermediate adducts and products). 

Isodesmic reaction analysis is used to calculate the enthalpies of the intermediates and the well depths of adducts. The new Group Additivity parameters developed in a previous chapter, aided to evaluate the enthalpies of formation of large molecules. 

Computational methods have also been used frequently to estimate the thermodynamic stabilities of superelectrophiles. These calculations have often involved the estimation of barriers to gas phase dissociation or deprotonation, and the proton affinities of conventional electrophilic intermediates. Other useful studies have calculated the heats of reactions for isodesmic processes. An interesting example of these calculations comes from a study of the protoacetyl dication (Cf COH2"1- ).42 In calculations at the 6-31G //4-31G level of theoiy, the protoacetyl dication (83) is estimated to react with methane by hydride abstraction with a very favorable... 

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