Conformation coplanar

Some PCB congeners have coplanar structures (see, e.g., 3,4,3, 4 -tetrachloro-biphenyl in Figure 6.1). The coplanar conformation is taken up when there is no chlorine substitution in ortho positions. If there is substitution of chlorine in only one ortho position, the molecule may still be close to coplanarity, because of only limited interaction between Cl and H on adjoining rings. Substitution of chlorines in... 

Molecular models suggest that the aldehyde (68, X = H) can assume a coplanar conformation with the naphthyl group, whereas such a coplanar conformation is not possible for any of the other compounds. This situation is reflected in the infrared carbonyl stretching frequencies. The ap forms of these compounds absorb generally at higher frequencies than do their sp counterparts. 

Stereochemical considerations in the E2 reactions The E2 follows a concerted mechanism, where removal of the proton and formation of the double bond occur at the same time. The partial 7t bond in the transition state requires the parallel alignment or coplanar arrangement of the p orbitals. When the hydrogen and leaving group eclipse each other (0°), this is known as the 5 yn-coplanar conformation. 

Behaviour of this type is typified by the co-ordination of a ligand such as 2,2 -bipyri-dine. The equilibrium solution conformation of the ligand is trans-coplanar, although there is free rotation about the interannular C-C bond, whereas in its metal complexes it usually behaves as a chelating didentate species, and, thus, adopts the m-coplanar conformation (Fig. 2-4). 

As the last example of coalescence of signals of diastereotopic nuclei due to site exchange in solution we present the case of the Schiff base of a-naphthyl isopropyl ketone and benzylamine (106, Fig. 49)" . This molecule possesses a prochiral axis by virtue of the fact that coplanarity of the C=N double bond with the naphthalene ring leads to strong steric interactions the coplanar conformation... 

Thiourea is known to form a series of complexes HgCl2Lrt (n = 1 — 4, L = thiourea). Structural studies on the complexes n = 2 or 4 have now been reported. The crystal structure of HgCl2L2187 shows the mercury to be co-ordinated in a distorted trigonal coplanar conformation by two equivalent thiourea sulphur atoms (Hg—S = 2-42 A) and a chlorine atom (Fig—Cl = 2.57 A). The other ionic chlorine atom lies normal to this plane, between and equidistant from a pair of mercury atoms (Hg—Cl = 3.22 A). 

In hot, dilute dichlorobenzene solutions (>50 °C) broad absorption at Amax 480 nm is apparent in the UV-visible spectrum of PQT-12 (Fig. 4.8a), but the absorption is slightly red-shifted with the concomitant appearance of weak absorption at A max 610 nm (Fig. 4.8b) when the solution is cooled to room temperature. This is obviously because of the migration of PQT-12 molecules from the twisted disordered conformation in hot solution to an ordered coplanar conformation at lower temperatures. Because the HOMO level of PQT-12 in the solid state, esti-... 

---