Resonance energy heterocycles

Hess Jr., B.A. and Schaad, L.J. (1973). Htickel Molecular Orbital n-Resonance Energies. Heterocycles Containing Divalent Sulfur. J.Am.Chem.Soc., 95,3907-3912. 

Hess Jr. B. A. and Schaad, L. J. 1973. Hueckel molecular orbital pi.-resonance energies. Heterocycles containing divalent snlphnr. J. Am. Chem. Soc. 95 3907-3912. 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

The tautomeric equilibria of these heterocycles always involve one or more non-aromatic tautomers. An important factor in determining the extent to which such non-aromatic tautomers are involved is the magnitude of the potential loss of resonance energy. 

Resonance energies and tautomerism of substituted aromatic heterocycles and their benzo derivatives Reaction-field-supermolecule approach to calculation of solvent effects... 

Data are given in Table IV for heterocyclic compounds. For piperidine there is no difference between E and E, showing that the bond energies used are applicable to saturated heterocyclic molecules. Pyridine and quinoline differ from benzene and naphthalene only by the presence of one N in place of CH and, as expected, the values 1.87 v.e. and 3.01 v.e., respectively, of the resonance energy are equal to within 10 percent to the values for the corresponding hydrocarbons. 

A new index of aromaticity has been proposed and applied to neutral and mesoionic five-membered heterocycles. The aromaticity index is based on data from experimentally determined bond lengths which yield statistically evaluated bond orders. A reasonable parallel exists between the aromaticity index and resonance energies <85T1409>. 

For pyridine, pyrazine, and related six-membered heterocyclic molecules Kekul6 resonance occurs as in benzene, causing the molecules to be planar and stabilizing them by about 40 kcal/mole. The interatomic distances observed in these molecules,106 C—C = 1.40 A, C—N = 1.33 A, and N—N 1.32 A, are compatible with this structure. The resonance energy found for quinoline, 69 kcal/mole, is about the same as that of naphthalene. 

A linear relationship has been shown to exist between structural aromaticity indices and resonance energies (92T335). From this so-called unified aromaticity index, IA, has been proposed. It makes for a more appropriate comparison the aromaticity of heterocycles of different size. 

Calculated resonance energies of some seven-membered heterocycles are given in Table 7. 

Results of MNDO calculation of lA-azonine (35 X=NH) are in agreement with experimental evidence that this is a planar, aromatic molecule. The calculated geometry of oxonin (35 X = 0), as a buckled, unsymmetrical polyenic heterocycle, is also in agreement with its known properties. The MNDO calculations on thionin (35 X = S) indicate that this molecule is planar, which should allow effective tt delocalization, and at least some aromatic character (86MI927-OI). The topological resonance energy model also predicts lA-azonine and thionin to be aromatic, and oxonin nonaromatic (84JHC273). 

The variety of aromaticity indices based upon structural or magnetic molecular properties along with their values for pyrrole, the other five-membered heterocycles, benzene and cyclopentadiene are summarized in Table 31 in Section 3.01.5.2. Of these indices, the majority predict the aromaticity order benzene >thiophen>pyrrole >furan, in agreement with resonance energy calculations (Section 3.01.5.2). 

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