Nitrogen atomic arrangement

The only ring system with a 0 3 nitrogen atom arrangement that has been studied in the past period is pyrrolo[2,l- /] [l,2,3,5]tetrazine, which was not synthesized earlier. The first synthesis was presented by Cirrincione et al. in two successive publications <1999S2082, 2003BMC2371>. The results are depicted in Scheme 36. 

Based on symmetry and steric arguments, the meso ligand is most likely to coordinate only with its four nitrogen atoms arranged in a single plane, while the racemic ligand may coordinate in either this fashion or in a folded form. 

Figure 9.19. A. The 2 possible oxygen-nitrogen atomic arrangements in the Si20sN2 unit of Y4Si207N2. B. N MAS NMR spectrum of Y4Si207N2 showing 2 distinct nitrogen environments, consistent with N in the bridging site and 1 of the terminal sites (A, lower). From Hauck et al. (1993), by permission of the Royal Society of Chemistry.

Sauter (2000) [266] investigated the structure of amorphous boron carbonitride of composition B31C37N32 by X-ray and neutron diffraction as well as by NMR spectroscopy. It was shown that annealed precursor-derived B-C-N ceramics (1200 K < T < 1600 K) contain predominantly tricoordinated boron, carbon and nitrogen atoms arranged in hexagonal rings, or fragments of them, as structural units. No phase separation occurred. 

Chiral Center. The chiral center, which is the chiral element most commonly met, is exemplified by an asymmetric carbon with a tetrahedral arrangement of ligands about the carbon. The ligands comprise four different atoms or groups. One ligand may be a lone pair of electrons another, a phantom atom of atomic number zero. This situation is encountered in sulfoxides or with a nitrogen atom. Lactic acid is an example of a molecule with an asymmetric (chiral) carbon. (See Fig. 1.13b.)... 

This method of ring closure is the most convenient for the synthesis of ring-fused 1,2,4-triazoles with this arrangement of nitrogen atoms. It has been used extensively to form the analogous [5,6] ring-fused systems, and structural ambiguity is only encountered... 

Mesomerism involving polarized and nonpolarized contributing enamine forms influences the enamine s spectral properties and chemical reactivity. For mesomerism to be present, a planar arrangement is required for the three atoms of enamine grouping and the five atoms immediately bound to this system. If this condition is not fulfilled, full interaction of the tt electrons of the double bond with the free electron pair on the nitrogen atom is impossible. Enamines in which mesomerism is inhibited do not show the properties characteristic of enamines, and only the mutual electrostatic interaction of the double bond and lone electron pair of the nitrogen atom can be observed. Such steric hindrance of mesomerism occurs mainly in polycyclic systems. 

Tliis chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH, for example, types 1-3 starting from compounds 1-6. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as in 15 or 16, as well as derivatives in which the central double bond contains heteroatoms as in 17, are not included. For azoniafulvalenes of type 17 and related heterocyclic betaines see (94AFIC197). 

Nitrogen-containing fulvalenes have not been systematically studied by mass spectroscopy. Only isolated data for several examples of compounds have been reported. Most of the data consist of electron impact (El) mass spectra recorded for analytical purposes. Only a minor fraction dealt with the characterization of ion structures or focused on the effects of substituents, the ring size of fulvalenes, or the number and arrangement of nitrogen atoms and the fragmentation pathways. 

Confirmation of the linear arrangement came by physical techniques, especially electron diffraction and infrared spectroscopy. Later the nonequivalence of the nitrogen atoms in diazoaeetic ester was shown by means of labeling. ... 

If it is assumed that 2,2 -bipyridine is bonded to the catalyst by both nitrogen atoms, then the position of the chemisorbed molecule on the metal is rigidly fixed. Unless two molecules of this base can be adsorbed at the required distance from each other and in an arrangement which is close to linear, overlap of the uncoupled electrons at the a-position cannot occur. The failure to detect any quaterpyridine would then indicate that nickel atoms of the required orientation are rarely, if ever, available. Clearly the probability of carbon-carbon bond formation is greater between one chemisorbed molecule of 2,2 -bipyridine and one of pyridine, as the latter can correct its orientation relative to the fixed 2,2 -bipyridine by rotation around the nitrogen-nickel bond, at least within certain limits. 

Quinoxalines, because of the 1 4 arrangement of their ring nitrogen atoms, are only weakly basic. The effect of substituents on basic strength is illustrated in Table II thus -Me, -NH2, a-NHMe, and... 

Cyclazines are conjugated cyclic molecules in which planarity is secured by three covalent bonds to a central nitrogen atom. They differ in the size of the individual ring units. In the nomenclature of cyclazines, the name is preceded by the number of jt-centers between the points of attachment to the nitrogen which are arranged by increasing number and put in brackets, e.g. [3.3.3]cyclazine (1) and [2.3.4]cyclazine (2). 

STRATEGY For the electron arrangement, draw the Fewis structure and then use the VSEPR model to decide how the bonding pairs and lone pairs are arranged around the central (nitrogen) atom (consult Fig. 3.2 if necessary). Identify the molecular shape from the layout of atoms, as in Fig. 3.1. 

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