Resonance in Heterocyclic Molecules

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. 

104 ft. Robinson, Outline di an Electrochemical [Electronic] Theory of the Course of Organic E tactions. Institute of Chemistry of Great Britain and Ireland, London, 1932 Society of Dyets and Colourists, Jubilee Journal, 1934, 65. 

For the five-membered heterocyclic molecules furan, pyrrole, and thiophen, with the conventional structure 

These values indicate that each of the structures with separated charges makes one-quarter the contribution of the principal structure 

Similar resonance is shown by indole. I T J (resonance energy 

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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. 

Resonance integrals of bonds between atoms X and Y, XY, are expressed as defined in Eq. (2), where kXY depends on the bond length. There has been considerable variation in the values taken for the Coulomb and resonance integrals for heterocyclic molecules. One of the best available set of parameters is still that originally suggested by A. Streitwieser (Molecular orbital theory. J. Wiley Sons, Inc., N.Y.-L., 1961) ... 

J. R. Bolton, A. Carrington, and J. dos Santos-Veiga, Electron spin resonance studies of semiquinones and related nitrogen heterocyclic molecules in acid solution, Mol. Phys. 5, 465-473 (1962). 

Molar refractivity is easily derived from the refractive index. Hardness measured in this way correlates with other aromaticity criteria such as resonance energy per electron for a variety of hydrocarbons and heterocyclic molecules (see p. 141) ... 

It was mentioned above (p. 24) that Wheland o had developed a molecular orbital treatment which allowed for overlap between adjacent atomic orbitals. He found that inclusion of overlap made little difference in the calculated resonance energies of hydrocarbons, and this is generally to be expected i for bond orders and charge densities with this class of compounds. However, with heterocyclic molecules some differences do arise. Davies applied the method to a number of heterocyclic compounds and derived values for charge densities, bond orders and free valencies. The coulombic integral for nitrogen is now written + h[y where y = p ha... 

A large class of organic reactions that lead to the formation of five-membered heterocyclic molecules, have been designated as 1,3 dipolar cycloaddition reactions °. Huisgen has defined the 1,3 dipole to be a species which is represented by zwitterionic resonance structures (i.e. the standmd Lewis octet structures) and which undergoes 1,3 cycloadditions to a multiple bond system, the dipolarophile , as in structures (18) and (19). 

The presence in the molecule of imidazoline nitroxides of an additional nitrogen atom or of an N-oxide group in combination with functional groups in position 4 of the heterocycle allows com-plexation, chelation, and cyclometalation without participation of the radical center. This made it possible to investigate for the first time the magnetic resonance spectra of multispin systems. 

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