Thermochemical resonance 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. 

The thermodynamic properties of thiophene,2-methylthiophene, ° and 3-methylthiophene have been computed from careful measurements of the heat capacity of the solid, liquid, and vapor states, the heat of fusion, the heat of vaporization, and the heat of combustion. From the heat of combustion of thiophene and from thermochemical bond energies, the resonance energy of thiophene has been re-estimated to be only 20 kcal/mole. 

In the following paper of this series6 a value of about 1.7 v.e. has been found from thermochemical data for the resonance energy of benzene. Equating the negative of this quantity to 1.1055a, we calculate the value of a to be about —1.5 v.e. This value may not be very reliable, however, since it is based on the assumption that values of bond energies obtained from aliphatic compounds can be applied directly to aromatic compounds. 

V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals," J.Chem.Physics 1 (1933) 362374 Linus Pauling and J. Sherman, "The Nature of the Chemical Bond. VI. Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures," J.Chem.Physics 1 (1933) 606617 and Pauling and Sherman, "The Nature of the Chemical Bond. VH. The Calculation of Resonance Energy in Conjugated Systems," J.Chem.Physics 1 (1933) 679686. 

Although perhaps explicable, or at least precedented, in terms of the greater resonance energy of esters than corresponding amides, nonetheless, a further study is welcomed as this is the only thermochemical information we have on this class of compounds. 

A thermochemical study of quinoxaline was carried out by combustion calorimetry <1998MI93>, and using these experimental values the resonance energy was determined. 

Problem 10.7 Cyclooctatetraene (CgH ), unlike benzene, is not aromatic it decolorizes both dil. aq. KMnO and Brj in CCI4. Its experimentally determined heat of combustion is -4581 kJ/mol. (a) Use the Hiickel rule to account for the differences in chemical properties of CgHg from those of benzene, (b) Use thermochemical data af Problem 10.4 to calculate the resonance energy, (c) Why is this compound not antiaromatic (d) Styrene, CgH5CH==CH2, with heat of combustion —4393 kJ/mol, is an isomer of cyclooctatetraene. Is styrene aromatic ... 

Since this change by the factor 1/1.4 X 10 is due entirely to the complete inhibition of the F, G, H resonance by addition of the proton, the quantity RT In 1.4 X 10 = 8.4 kcal/mole represents the F, G, H resonance energy in aniline. This value is probably more accurate than that given by thermochemical data, 6 kcal/mole (Table 6-2), and the agreement between the two is satisfactory. 

The value 42 kcal/mole foi fhe resonance energy is given by the thermochemical data for the dialkyl carbonates. This value for resonance of the double bond among three positions is not unreasonable when it is compared with the corresponding value of 24 kcal/mole for esters of the fatty acids, in which the bond resonates between two positions. 

Resonance of the carbonate type occurs in urea, CO(NHs), and guanidine, CNH(NH )2. For urea the thermochemical value of the resonance energy is 37 kcal/mole, and for guanidine 47 kcal/mole, the latter being calculated with use of the estimated value 24 kcal/mole for the heat of sublimation. 

We assert in this review that, at this point in time, there are several examples of neutral molecules which have been shown to display either bond or no-bond homoaromaticity. These include, in addition to the boranes mentioned above in Section III. B, cyclohepta-triene, norcaradiene, bridged cycloheptatrienes and norcaradienes, semibullvalenes, bar-baralanes, bridged annulenes, etc. Confirmation of the homoaromatic character of these systems comes from thermochemical and spectroscopic studies, and force field and ab initio calculations. In particular, the work of Roth and coworkers must be mentioned in this connection in that they were the first to provide reliable resonance energies of a large number of these neutral molecules225 226. These authors have also demonstrated that systems such as bicyclo[2.1.0]pentene are homoantiaromatic. 

B. Thermochemical Properties of Delocalized Species Their Relation to Vertical Resonance Energies and jr-Distortivity... 

Another aspect of delocalization, the mixing of Kekule structures and the energetic gain associated with it, will also be quantitatively investigated, and finally our results will be placed into the context of the thermochemical properties of hydrocarbons and the current definitions of resonance energy. 

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