Dewar resonence energy

A modified definition of resonance energy has been introduced by Dewar (66T(S8)75, 69JA6321) in which the reference point is the corresponding open-chain polyene. In principle this overcomes the difficulties inherent in comparing observed stability with that of an idealized molecule with pure single and double bonds, as thermochemical data for the reference acyclic polyenes are capable of direct experimental determination. In practice, as the required data were not available, recourse was made to theoretical calculations using a semiempirical SCF-MO method. The pertinent Dewar Resonance Energies are listed in Table 30. 

By contrast, the Dewar resonance energy represents solely the contribution coming from the cyclic electron (bond) delocalization since the model reference structure is represented not by a system of isolated 7r-bonds, but by a hypothetical cyclic polyene with the number of tr- and tr-bonds equal to that in a given molecule. Making use of the additivity of bond energies in acyclic polyenes (65JA692), one may calculate the total energy... 

The Dewar resonance energy (DRE) is found as the difference between the heats of atomization of a given conjugated molecule and the classical Kekule reference structure... 

Since the Dewar resonance energy differs from REs derived for a hypothetical reference system with regard to the bond energy ascribed to a C—C single bond, RE values can be normalized by using the RE value of 1,3-butadiene (or appropriate butadiene derivatives) according to equation 987 ... 

In addition, there is also the question of how to use suitable reference molecules to obtain normalized RE values which correspond to Dewar resonance energies. In the case of cyclopropyl homoconjugation, butadiene is clearly the wrong reference molecule to consider the two C—C single bonds a adjacent to the fusion bond/(see Scheme 10). The... 

CM) is shown in Scheme 22, and it is seen to involve cyclic conjugation, but all the interactions between the "T-bond units have a polyenic character. The relative energy of the two structures, as in eq 7, is defined as the Dewar resonance energy (DRE) of the cyclic molecule. 

Figure 6 shows a Hess cycle which links benzene (b) to its reference molecule (R), such that the energies of the steps sum up to the Dewar resonance energy of benzene, DRE(b). At the VBSCF/6-31G level,111 the DRE(b) value is 20.4 kcal/mol, which is within 1 kcal/mol of the experimentally derived value27 based on heats of formation of benzene and cyclohexatriene (the latter value is determined from an additivity scheme). Our older value in the original literature,5 DRE(b) = 31 kcal/mol, was determined with the Kollmar method,157 which is less accurate than the VBSCF method111 used here. Since, our VB-derived DRE(b) value matches the unanimously... 

Figure 6. Hess cycle used to relate the Dewar resonance energy of benzene, DRE(b), to the vertical resonance energy, Bq. Vertical resonance energy values for benzene and distorted benzene (calculated in this study111) are shown below the cycle. All energies are given in kcal/mol.

CISD configuration interaction with singly and doubly excited determinants only DRE Dewar resonance energy... 

The decreasing order of aromaticity based on various criteria is (Dewar resonance energy (DRE) values in kcal mol ) benzene (22.6) > thiophene (6.5) > selenophene > pyrrole (5.3) > tellurophene > furan (4.3) (see Figure 25). 

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