Isodesmic reactions definition

One point of debate in defining the magnitude of the captodative effect has been the separation of substituent effects on the radical itself as compared to that on the closed shell reference system. This is, as stated before, a general problem for all definitions of radical stability based on isodesmic reactions such as Eq. 1 [7,74,76], but becomes particularly important in multiply substituted cases. This problem can be approached either through estimating the substituent effects for the closed shell parents separately [77,78], or through the use of isodesmic reactions such as Eq. 5, in which only open shell species are present ... 

A semiempirical procedure based on isodesmic reactions is used to determine unknown heats of formation. The choice of the isodesmic reaction is critical for the validity of the approach. By definition, all the bonds are conserved in number and nature in this type of process. Also, the bond lengths of equivalent bonds in reactants and products should be as similar as possible. The stabihzation energies are based on the type of isodesmic reaction which is given for the influence of amino- and cyano-groups in a captodative radical in equation (3). Only radicals appear as reactants and... 

Some years ago, Pople and co-workers (Hehre et al., 1970) introduced the interesting concept of isodesmic reaction. By definition, this is a reaction in which all the bonds are conserved (in number and nature). The corresponding theoretical heat of reaction is generally reliable due to approximate cancellation of correlation errors. Moreover, in that case, ZPE and thermal corrections are often negligible. Thus, we may write ... 

It is interesting to compare our definition of radical stabilization to previous ones proposed by Szwarc (1948), Benson (1965), and Rodgers et al. (1972). According to Szwarc, the resonance energy of an unsaturated radical is the heat of the following (isodesmic) reaction ... 

By definition, an isodesmic reaction is one in which the number of bonds and the bond types are the same on both sides of the reaction (Chesnut, 1996). An illustration of the use of isodesmic reactions is from the work of Cheng and Li (2003) in which they determined A H29S of n-tert-butyl methanes, where n = 1, 2, 3, and 4. We shall take the simplest example, calculation of AfH29i of er -butylmethane (2,2-dimethylpropane) itself. The isodesmic reaction is... 

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 ... 

It is worth noting that Wheeler et al. °° have delineated a hierarchy of reaction schema that define various levels of conserved components. This includes a more rigorous definition of isodesmic and homodesmotic reactions, along with providing higher order reactions that conserve larger fragments of molecnles. 

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