Structures and Spectroscopic Properties of Aromatic Heterocycles

This chapter describes the structures of aromatic heterocycles and gives a brief summary of some physical properties. The treatment we use is the valence-bond description, which we believe is sufficient for the understanding of all heterocyclic reactivity, perhaps save some very subtle effects, and is certainly sufficient for a general text-book on the subject. The more fundamental, molecular-orbital description of aromatic systems, is still not so relevant to the day-to-day interpretation of heterocyclic reactivity, though it is necessary in some cases to utilise frontier orbital considerations, however such situations do not fall within the scope of this book. 

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In the discussion of structural and spectroscopic aspects of thiophenes to follow, the reader does well to bear in mind that in fact aromaticity is a hallmark of thiophenes that dominates their properties and governs their reactions. Moreover, thiophene stands as uniquely aromatic among the five-membered heterocycles. In concrete terms this means that not only does thiophene undergo the reactions and have the physical and spectral properties associated with the concept of aromaticity, but that this aromatic character is steadfastly maintained through all manners of transformations including fusion with other aromatic rings. This attachment of substituents or fusion with other rings spawns thousands of aromatic derivatives. 

No systematic experimental studies on thermodynamic properties of five-membered heterocycles with three or more heteroatoms and at least one tetracoordinated silicon atom were reported. The aromaticity of the fully conjugated germadisiloleanion 21, a heavy congener of the cyclopentadienyl anion, was deduced mainly from its NMR spectroscopic and structural parameters <2005JA13143>. 

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