Heterocycles, aromaticity of

The relative aromaticity of common heterocycles is shown below  

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Various methods to quantify the aromaticity of heterocycles and the interrelationship between different scales are controversial and have been covered in a recent article.6b... 

B. Aromaticity and Anti aromaticity of Heterocyclic Compounds That Are 7t-Isoelectronic with the Parent Hydrocarbon... 

Microwave spectroscopy indicates that aromaticity diminishes in the order 1,2,5-thia-diazole > thiophene > l,3,4-thiadiazole> l,2,5-oxadiazole> 1,2,4-oxadiazole <84CHEC-1(4)545, B-85MI 410-01>. The aromaticity of heterocycles has been discussed by Katritzky and Barczynski (90JPR885) and by Bird <94H(37)249>. The thermal stability of 2,5-substituted thiadiazoles (23) was studied by differential scanning calorimetry and shown to increase as the rt-contribution of the substituents becomes greater <89MI410-01>. 

Heterocyclic These are compounds having at least one hetero atom (any other atom but carbon, e.g. O, N and S) within the ring, and conforming to Hiickel s rule. The aromaticity of heterocyclic compounds, e.g. pyridine and pyrrole, can be explained as follows. 

For reviews of aromaticity of heterocycles, see Katritzky Karelson Malhotra Heterocycles 1991, 32, 127-161 Cook Katritzky Linda Adv. Heterocycl. Chem. 1974, 17, 255-356. 

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. 

Magnetic criteria have received wide application mainly as a qualitative test for aromaticity and antiaromaticity. The values of the exaltation of diamagnetic susceptibility (in 10-6A cm-3 mol-1), and therefore aromaticity, decrease in the sequence thiazole (17.0) > pyrazole (15.5) > sydnone (14.1). The relative aromaticity of heterocycles with a similar type of heteroatom can be judged from values of the chemical shifts of ring protons. The latter reveals paramagnetic shifts when Tr-electron delocalization is weakened. For example, in the series of isomeric naphthoimidazoles aromaticity decreases in the sequence naphthof 1,2-djimidazole (8 = 7.7-8.7 ppm) > naphtho[2,3- perimidine (8 = 6.1-7.2 ppm). This sequence agrees with other estimates, in particular with energetic criteria. 

The Aromaticity of Heterocycles and Tautomeric Equilibriums A. R. Katritzky, Lect. Heterocycl. Chem., 1972,1, 39-49. 

Thakkar and his group have completed many careful studies of intracules, extracules, Coulomb holes, and related topics.145 These position-space results are complemented by Thakkar s many studies of momentum densities and related quantities. He also has a long-standing interest in electron and X-ray scattering.146 His current interests include the relationship147 between the aromaticity of heterocyclic compounds and their polarizabilities, and the prediction of push-pull molecules that have a large nonlinear optical response and are thus candidates for materials to be used in optical computers. 

Many papers (see, for instance [16, 17]) and some books [18] have been devoted to discussion of the aromaticity of heterocycles. However, a simple problem such as knowing where the imidazolium cation (1) and imidazolate anion (2) are more or less aromatic than imidazole itself (3) is not solved, and probably never will be because it depends on the chosen aromaticity criteria. The NICS(l) values (NICS at 1 A above the ring plane, calculated at the GIAO//B3LYP/6-311++G level) are... 

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