Planar molecules with delocalized

Criterion Three Use planar molecules with delocalized tc molecular orbitals... 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

The chemistry and properties of heteronins 221 (Scheme 82) have been reviewed.264-266 These compounds are thermally stable and possess delocalized planar molecules with strong diamagnetic ring currents.2673 Schleyer and co-workers calculated by ab initio and density functional methods aromatic stabilization energies as well as other properties of heteronins, and found that only 221 anions with X = N and P are aromatic and planar.2676... 

Diazole (imidazole) is a planar molecule with substantial delocalization (resonance) energy. 

The stability of a molecule based on its filled and empty MOs, and its delocalization energy or resonance energy, based on a comparison of its total n-energy with that of a reference system. The pattern of filled and empty MOs led to Hiickel s rule (the An I 2 rule) which says that planar molecules with completely conjugated p orbitals containing An I 2 electrons should be aromatic. 

Characteristics Planar molecule with hexagonal C framework trigonal-planar geometry and sp hybridization at C atoms a framework has C-C and C-H single bonds 2p-orbitals (one from each C atom) overlap side-by-side form three rr-bonds six 7t-electrons) delocalized over the entire six C atom ring... 

Fig. 14.9 (a) cyclooctatetraene, drawn as a planar molecule with four localized double bonds as might be appropriate for it acting as s symmetrically bonded ligand. (Actually, the molecule is non-planar if it were planar the double bonds would be delocalized.) (b) And (c) are two complexes containing cyclooctatetraene as a ligand. 

The six tt electrons of benzene are shared by all six carbons. Thus, benzene is a planar molecule with six delocalized tt electrons. 

Antiaromatic (Section 15.3) Referring to a planar, conjugated molecule with 4n tt electrons. Delocalization of the t7 electrons leads to an increase in energy. 

One of the most popular of the semi-empirical LCAO methods is that of Hfickel. It is applicable to planar molecules which have n -electron systems. The delocalization of these systems, as treated by this method, has particular chemical significance. The traditional application is to the benzene molecule. Historically, different structures of this molecule were suggested by K6kuI6+ and by Dewar that are described in virtually all textbooks of organic chemistry. These structures represented the first efforts to represent the delocalization of the 3r orbitals in such systems. In the present context the delocalization can be better specified with the use of the method of Hilckel. 

Equation (4.4) stresses that the surface plasma wave propagates within the aluminium substrate whose frequency is modified by the dielectric response of the molecular adlayer. From the measurements reported, with hcog = 8.5 eV and hcob = 15 eV, one obtains e = 2.1 for CuPc, a value in agreement with those measured for other planar organic molecules, with an extended delocalization of 7T-electrons (Alonso et al, 2003). 

The only monocyclic 1,2,3-triazine of which the structure has been determined by X-ray crystallographic analysis is 4,5,6-tris(4-methoxyphenyl)-l,2,3-triazine (72CB3704). The results show that the 1,2,3-triazine ring is planar, as one expects for a molecule with some degree of electron delocalization. The experimental bond distances and angles are given in Table 1. They are in reasonable agreement with calculated values. 

The X-ray crystallographic analysis of 3,6-diphenyl-l,2,4,5-tetrazine (51) was published in 1972 (72AX(B)739). The observed bond distances and angles are in the same range as were found for the parent compound (38 Figure 2). In both cases the heterocyclic ring is planar as expected for a molecule with some degree of electron delocalization. 

The N-N distance in planar N2O4 is 175 pm, with a rotation barrier of about 9.6 kJ mol-1. Although the N-N bond is of the a type, it is lengthened because the bonding electron pair is delocalized over the entire N2O4 molecule with a large repulsion between the doubly occupied MOs on the two N atoms. 

Planar molecules D or A with delocalized n molecular orbitals are best. Molecular components should be of appropriate or compatible size. Onedimensional metals need segregated stacks of radicals, and not mixed stacks. 

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