Tr-systems

The orbital mixing rule was recently applied to the prediction and design of cyclopentadienes having substituents of tr-system at 5-positions (Scheme 8) [13,14],... 

In the proper system, under appropriate conditions, a new intermediate which is simultaneously a carbene and a carbonium ion (i.e., a carbenonium ion) may be envisioned. A particularly attractive system would be 263 and 264, as promotion of an electron from the tr system in vinyl cation 263 a onto the... 

The HO-energy of a ir-system can be changed, e.g. by occupied orbitals of hetero-atoms which are relative donors or acceptors for the HO s. This can also be true of interactions of the LUMO s of the tr-system with vacant AO s of heteroatoms. The consequences e.g. for the preference of certain conformers relative to the type and position of perturbations in tricarbonyl-chromium benzene complexes (organo-metallic example) are described in Figure 2 of Scheme 2.1-4 together with the consequences for the reactivities in benzene derivatives (example of organic chemistry) due to rektive donor- or acceptor-perturbations (see also Scheme 2.1-2 Fig. 2). 

In Scheme 2.3-3 some examples of the influence of DO/ACC-perturbations in and at the catalytically reacting tr-systems are given. It reveals the influence of CH3-or other alkyl-groups af the tr-system and that of the nitrogen atoms in the rr-system for the Ni-catalyzed 2 1 co-cyclotrimerization of butadiene with these substrates. 

DO- or ACC-perturbations in one of two rr-systems can change the molecular ratio in the co-oligomerization of these tr-systems. 

Review Section 2.2. The MO theory focuses attention on the interacting p AO s of the delocalized tr systems, such as conjugated polyenes. The theory states that the number of interacting p AO s is the same as the number of rr molecular orbitals formed. The molecular orbitals are considered to be stationary waves and their relative energies increase as the numbers of nodal points in the corresponding waves increase. Nodes may appear at a C atom, as indicated with a 0 rather than a +... 

Once the structure of the tr-booded molecule has been determined, tt bonds may be added as necessary to complete the molecule. In carbon dioxide, lbepr and pv orbitals on the carbon atom were unused by the tr system and are available for the formation of tr bonds. A complete structure for carbon dioxide would thus be as shown in Fig. 6.1a. 

One of the simplest cases of v bonding in octahedral complexes is found in fCoF. Its tr system will be similar to that in Fig. 11.20. The orbitals of the metal... 

The three nitrogen atoms and the Ruoro-substituted phosphorus atoms are coplanar (within 2.5 pm), but the phenyl-substituted phosphorus atom lies 20.5 pm above this plane. The explanation offered is that the more electropositive phenyl groups cause an expansion of the phosphorus d orbitals, less efficient overlap with the p orbitals of the nitrogen atom, and a weakening of the tr system at that point. This allows27 the ring to deform and the Ph2P moiety (o bend out of the plane. 

FIGURE 7. (a) Definition of the w-orbital axis vector (POAV1). The angles 6, 02 and between n-orbital and [Pg.367]

Electrocyclic reactions, considered in the direction of ring opening, occur in rings composed of two saturated centers joined by a single a bond and by a tr system. There are two general types, depending on whether the tt system has an even or an odd number of atoms the former are neutral closed shell molecules, whereas the latter are cations, anions, or radicals. 

Stilbene is planar in crystalline form, in gas phases, and presumably in solution, although the phenyl group may be rotated as much as 32° to reduce nonbonded repulsions between hydrogen atoms (Waldek 1991 Meier 1992). This modest twisting still allows sufficient 17-overlap for a continuous, conjugated -Tr-system between the phenyl groups and the central double bond. 

The first and, possibly, the most intuitively satisfying among these concerns the nature of the lone pair and the fact that its participation into the tr system bears unmistakable dependence on its mobility as controlled by the effective electronegativity of its heteroatomic host, the unit s suitability as a double-bond substitute decreasing in the order N" > NH NAlkyl > S > NAcyl > O. It is further noted that this heteroatom-electronegativity effect is most spectacularly manifested in the unrestricted members of the An + 2)w series such as the nine-, thirteen-, and seventeen-membered rings where a change between the... 

M. A. Fox, F. A. Matsen, J. Chem. Educ. 62, All (1985). Electronic Structure in Tr-Systems. Part II. The Unification of Hiickel and Valence Bond Theories. 

A great number of chiral TTFs have been prepared [31,32], and some of their charge transfer salts have been crystallised. The 7r-donor 1 can be elec-trocrystallised to give a salt 12 PF6, which has a conductivity of 5 2 1 cm 1 at room temperature and metallic behaviour when cooled down [33]. While the structure of the crystals is chiral, the structure of this salt and other related ones [34] has an essentially achiral stack of donor molecules, which are pseudo-centrosymmetric [35]. The methyl groups at the periphery of the molecule are apparently not sufficient to cause a truly chiral stack of donors, which prefer to stack in parallel arrangements with partial overlap of the tr-systems in the solid state, a situation which is true for the majority of the efforts to prepare salts of this type (even if the salts are metallic). 

In the previous chapters, Bu3SnH has been used as a typical and useful radical reagent in a benzene solvent. Generally, radical reactions with Bu3SnH initiated by AIBN, proceed effectively in benzene, which bears a conjugated Tr-system. Probably, the formed radicals are somewhat stabilized through the SOMO-LUMO or SOMO-HOMO interaction between the radicals and benzene. 

For instance, out of many studies carried out with crystalline olefins that possess slightly different interbond distances and orientations one would only answer Yes or No to questions of reactivity and selectivity. An oversimplified and pictorial representation of this analysis is illustrated in Fig. 2 with a reaction coordinate for the excited state dimerization of a hypothetical set of crystalline alkenes. The reaction coordinate in this case would be given primarily by the displacement between the two Tr-systems which, for simplicity, may be assumed to have the required parallel arrangement. In the figure, different crystals can be though of as positioning the prospective reactants at different distances. Each crystal represents a point along the reaction coordinate [50]. Some molecules may... 

Annelation on to a benzene ring increases considerably the complexity of the spectra, and indole has absorptions at 216 (4.54), 266 sh (3.76), 270 (3.77), 276 (3.76), 278 (3.76) and 287 (3.68) nm in ethanol solution. Because of the widespread occurrence of the indole ring system in nature and the sensitivity of absorption band position and intensity to substitution type, considerable use has been made of electronic spectroscopy in the past for structure identification. An extensive tabulation of data, primarily for monosubstituted derivatives, is available (71PMH 3)67,p. 94). As expected, whereas the effects of alkylation are comparatively slight, introduction of groups capable of mesomeric interaction with the indole tr-system may cause profound changes in the appearance of the spectrum representative examples are given in Table 24. 

Scheme XIII. Stereochemical alternatives in the 5-aminolevulinate synthetase reaction (end-on view down the C-a-N bond the dashed line represents the plane of the tr system of the Schiffs base and pyridine ring).

Heavy elements have many core electrons of high potential energy that are easy to polarize. Hence, high refractive indices (n") and therefore high linear polarizabilities, a, are associated with their presence in molecules. In comparable structures, an increased linear polarizability may be intuitively suspected to lead to an increased non-linear polarizability. The homologous series of haloforms shows this to be the case (Table 1) (Kama and Dupuis, 1990 Samoc et al., 1992 Kohler et al., 1993). Unfortunately, the maximum values of /3 achievable with tr systems even of very high molar refractivities are too low for practical applications. In addition, the incorporation of rr-bonded heavy elements tends to impart chemical (thermal, photochemical) instability on the molecule. 

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