Planar it systems

Most MO methods find a bond alternation pattern in the minimum-energy structure, but calculations that include electron correlation lead to a delocalized minimum-energy structure. Thus, although the n system in 1 is not completely planar, it appears to be sufficiently close to provide a delocalized 10-electron Ji system. A resonance energy of 17.2 kcal has been obtained on the basis of an experimental heat of hydrogenation. ... 

Interestingly, tetrazetine 78 and tetracyclophosphene 79 are 6e it systems, but p-systems, but not planar, and saturated N and P atoms are pyramidalized. The lone pair must be pyramidahzed to have high s character for the a-relaxation. 

The benzene molecule in its equilibrium configuration is planar. Its symmetry is described by the point group as shown in Fig. 8-l(c). The delocalized n system is represented there by dotted lines. The six pz orbitals contribute to tbe jt system, as simply described by the Htickel approximation. The reduction r, = B2g Hjg E2u can be found as in the previous examples. 

The extended Huckel method [13] is an extension of the traditional Huckel method [14] expanding its range of applicability beyond planar conjugate systems. From a mathematical point of view, it consists simply in solving the matricial equation 1, where H is the hamiltonian matrix, C are... 

All the template models produced have the same common features of a basic nitrogen atom at a distance of 5-7 A from the site of metabolism which is in general on or near a planar aromatic system. It is currently believed that aspartic acid residue 301 provides the carboxylate residues which binds the basic nitrogen of the substrates. 

Electroosmosis refers to the movement of the liquid adjacent to a charged snrface, in contact with a polar liquid, under the influence of an electric field applied parallel to the solid-liquid interface. The bulk fluid of liquid originated by this electrokinetic process is termed electroosmotic flow. It may be prodnced either in open or in packed or in monolithic capillary columns, as well as in planar electrophoretic systems employing a variety of snpports, such as paper or hydrophilic polymers. The origin of electroosmosis is the electrical donble layer generated at the plane of share between the snrface of either the planar support or the inner wall of the capillary tube and the surronnding solntion, as a consequence of the nneven distribntion of ions within the solid/liquid interface. 

Whether this condition can be fulfiUed depends on the electron count of the metal, and the stereochemistry of the elimination. For instance, in m-elimination from octahedral d , or square planar d , systems, metal ndipP -y ) acts as acceptor, and this should be a facile process ( e Fip. 1, 2). For /rans-elimination, on tiie other hand, the lowest empty orbital of correct symmetry is (n + l)p. Such elimination Kerns energetically less Ukely, unless a non-concerted pathway (such as successive anionic and cationic loss) is available. The same arguments apply, of course, to oxidative additions. It foUows that the many known cases of traits oxidative addition to square planar t/ systems are unlikely to take place by a concerted mechanism, and this conclusion is now generally accepted There are special complexities in reductive elimination from trigonal systems, and these are discussed furdier in Part III. 

Similar studies on the dibenzostibole system were less successful. Although optical activity was identified in a limited number of dibenzostiboles, these materials had only limited optical stability and although these authors provided strong evidence that the dibenzostibole system is planar, it was not conclusively proved. These optically active heterocycles are listed in Table 2. 

Homoaromaticity as a chemical concept is based on the concepts of aromaticity and homoconjugation. In its simplest form aromaticity is an electron counting concept. Thus if there are 4q + 2 7t-electrons in a planar (or nearly planar) cyclic system, then this is considered to be aromatic. ... 

With these definitions, useful descriptions of non-planar rc-systems and potentially homoconjugated systems have been developed. The descriptions are particularly attractive to experimentalists because orbital overlap is accepted as a major contributing factor to bonding and it is easy to visualize. Although Haddon did not formally define homoaromaticity in his work63 one can use his threshold value of S to define homoaromaticity in the following way ... 

---