Atomic nitrogen within

The third chapter of this volume is concerned with mono- and diazaquinones. S. R dl (Prague, Czech Republic) reviews compounds that contain one nitrogen or two nitrogen atoms directly within the quinonoid ring, a subject that has not been previously summarized in a comprehensive manner. This is in distinction to the several specialized reviews of quinones themselves. Heterocyclic quinones, in the sense of compounds in which the quinonoid ring is fused to a heterocyclic ring, were covered in Volume 45 of our series in 1989. 

In addition to the compounds listed above, a number of triazolium [36], thiazolium [37], and benzimidazolin-2-thione [32] derivatives as well as five-membered heterocycles with only one nitrogen atom [38, 39] or two phosphorus atoms [40] within the heterocycle, all of which are suitable for the generation of heterocyclic carbenes, have been described. 

Figure 2.8. Diagram of a dipeptide. The diagram shows a dipeptide of glycine in the standard conformation 180°, 180°. To a first approximation, all atoms seen in the dipeptide backbone are found within the plane of the paper. For a dipeptide of glycine, the only atoms not within the plane are the side chain hydrogens. The unit begins at the first Ca and goes to the second Ca. Both c > and xp are defined as positive for clockwise rotation when viewed from the nitrogen and carbonyl carbon positions toward the Ca.

The least squares refinement of this model using SHELXL-97 yields Rp = 42 % as shown in Table 6.39. All oxygen atoms and the isolated nitrogen atom look reasonable because their distances to the vanadium atoms are within acceptable limits. What is not shown in Table 6.39 is that the coordination polyhedra of V atoms make both chemical and physical sense vanadium oxide forms a layer, composed of VO5 pyramids and tetrahedral VO4, as illustrated in Figure 6.28. This layer is similar to layers known for... 

According to X-ray diffraction, the molecular structure of different substituted 1,2,4,6-thi-atriazines possesses a nonplanar ring system with a sulfur atom out of the best plane25 and with pseudoaxial orientation27- 29 of the sulfur-bonded substituent. The distances between endo-cyclic carbon and nitrogen atoms lie within the range of 128-138 pm. The endocyclic N—S distances indicate typical ylidic bonds of 152-169 pm. One structure was assigned by electron diffraction.26... 

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