Structure of Heterocycles

Details of bond lengths and bond angles for all the X-ray structures of heterocyclic compounds through 1970 are listed in Physical Methods in Heterocyclic Chemistry , volume 5. This compilation contains many examples for five-membered rings containing two heteroatoms, particularly pyrazoles, imidazoles, Isoxazoles, oxazoles, isothlazoles, thlazoles, 1,2-dlthloles and 1,3-dlthloles. Further examples of more recent measurements on these heterocyclic compounds can be found in the monograph chapters. 

STRUCTURE OF HETEROCYCLIC RINGS WITH MORE THAN SIX RING ATOMS... 

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

Bertinotti, F., G. Giacomello, and A. M. Liquori. 1956. The Structure of Heterocyclic Compounds Containing Nitrogen. I. Crystal and Molecular Structure of s-Tetrazine. Acta Cryst. 9, 510. 

Figure 28 Structures of heterocycles 349 and 356. Hydrogen atoms have been omitted for clarity.

Chemical properties and structure of heterocycles containing P—C—O and P—C—N fragments are determined by the interaction between the heteroatoms. This interaction is most clearly pronounced in heterocyclic compounds also possessing an acceptor functional group (B(III), O—B(III), N—B(III), etc.). 

As the solid state structures of heterocyclic group 13 amides, phosphides and arsenides have been reviewed in the past,76 they will not be discussed here. Instead, the solid state structures of group 13 stibides and bismuthides will be described in detail. 

Figure 44 Structures of heterocyclic amino-, phosphino- and arsinogallanes 622, 626, and 627.

Fig. 8.1 Chemical structures of heterocyclic N compounds identified in soils. Modified from Schulten and Schnitzer. [17]...

Fig. 2 Electronic configuration and resonance structures of heterocyclic carbenes containing an X7C carbene center...

Figure 7 Chemical structures of heterocycles or carbocycles with vicinal diaryl substitution.

Electronic Structure of Heterocyclic Sulfur Compounds R. Zahradnik, Adv. Heterocycl. Chem., 1965, S, 1-67. 

The Fine Structure of Heterocyclic Oxo Compounds F. Arndt, Angew. Chem., 1949, 61, 397-400. 

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