Evidence for the Planarity

The presence of the NH2 group in position 2 of the pyrimidine ring allows the formation of weak intermolecular hydrogen bonds between the amine hydrogens 

Although other 2,3-bis(N-pyrazolyl)quinoxaline, pyrazine or pyridazine heterocycles cannot be found in the Cambridge Crystallographic Data Centre (CCDC), the analogous 2,3-bis(2-pyridyl)quinoxalines [21] and pyrazines [22] do not show coplanarity between the heterocycles. However, as one would expect, coplanar rings were found in the 3,6-bis(2-pyridyl)pyridazine derivative [23]. 

Another remarkable difference that can be predicted for these grids is a consequence of the existence of a spacer carbon atom between the N-donor atoms of the pyrimidine or triazine rings that is absent in the pyridazine heterocycle. This leads to a greater distance between the metal centers in the former systems and, consequently, the cavity in the grid should be bigger. All of these envisaged differences are demonstrated in Section 3.3.2, in which X-ray structures are discussed. 

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The analogy between the trivalent boron compounds and car-bonium ions extends to the geometry. Although our arguments for a preferred planar structure in carbonium ions are indirect, there is electron diffraction evidence for the planar structure of boron trimethyl and the boron trihalides.298 Like carbonium ions, the boron and aluminum analogs readily form a fourth covalent bond to atoms having the requisite non-bonding electrons. Examples are the compounds with ammonia, ether, and fluoride ion.297... 

C(8) to be parallel with the nitrogen orbitals, thus providing evidence for the planarity of flavin semiquinones 2. More recent X-ray crystallographic data on the Cl. MP flavodoxin provide confirmatory data on the planarity of the iso-alloxazine ring in its one-electron reduced form. 

A vibrational analysis of the ED intensity data provided firm evidence for the planarity of this molecule [2],... 

In the gold(lll) halides (except the fluoride) there is evidence for the formation of double molecules, AujXg (cf. chlorides of iron(III) and aluminium) so that the coordination is brought up to four, but with a planar structure ... 

Four-coordinate complexes provide good examples of the early use of preparative methods for establishing stereochemistry. For complexes of the type [Ma2b2], where a and b are unidentate ligands, a tetrahedral structure cannot produce isomerism whereas a planar structure leads to cis and trans isomers (see below). The preparation of 2 isomers of [PtCl2(NH3)2], for instance, was taken as good evidence for their planarity. ... 

Further evidence for the unusual nature of benzene is that all its carbon-carbon bonds have the same length—139 pm—intermediate between typical single (154 pm) and double (134 pm) bonds. In addition, an electrostatic potential map shows that the electron density in all six carbon-carbon bonds is identical. Thus, benzene is a planar molecule with the shape of a regular hexagon. All C-C—C bond angles are 120°, all six carbon atoms are sp2-hybridized. and each carbon has a p orbital perpendicular to the plane of the six-membered ring. 

Like the kinetic evidence, the stereochemical evidence for the SnI mechanism is less clear-cut than it is for the Sn2 mechanism. If there is a free carbocation, it is planar (p. 224), and the nucleophile should attack with equal facility from either side of the plane, resulting in complete racemization. Although many first-order substitutions do give complete racemization, many others do not. Typically there is 5-20% inversion, though in a few cases, a small amount of retention of configuration has been found. These and other results have led to the conclusion that in many SnI reactions at least some of the products are not formed from free carbocations but rather from ion pairs. According to this concept," SnI reactions proceed in this manner ... 

Further evidence for the importance of mesomeric ion, 166b, comes from a careful examination of the solvolytic behavior of the cyclic bromodienes, 167 (153). In these systems, the planarity of the diene double bonds precludes... 

In the examples given, there is good evidence for the formation of an unstable epoxide intermediate in the production of monohydroxymetabolites. However, there is an ongoing debate about the possible operation of other mechanisms of primary oxidative attack that do not involve epoxide formation, for example, in the production of 2 OH 3,3, 4,4 -TCB (Figure 6.3). As mentioned earlier, P450s of gene family 1 (CYP 1) tend to be specific for planar substrates, including coplanar PCBs they do not appear to be involved in the metabolism of nonplanar PCBs. On the other hand. 

Figure 6. STM images of hexaether 41 adsorbed on Cu(111) and heated for 10-15 min to temperatures of (a) 570 K, (b) 620 K), (c) 670 K, and (d) 720 K. The line profiles on the right were taken along the white lines indicated in the images. The decrease of the height with increasing temperatures is strong evidence for the stepwise planarization of 41 and formation of the fully fused HBC structure.

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