Molecule conjugated

In the Hiickel theory, the 7r-electron energy of a conjugated molecule can be expressed by the following equation ... 

Frosts circle (Section 11 19) A mnemonic that gives the Huckel TT MOs for cyclic conjugated molecules and 10ns Functional class nomenclature (Section 4 2) Type of lUPAC nomenclature in which compounds are named according to functional group families The last word in the name... 

The new orbital of the TT-electron in the 7T -state is larger than it was in the unexcited state. The two adjacent atoms with which the electron was associated in the ground state may be partially held by the electron in its expanded 7r -orbital. The atoms, in adjusting to the new binding condition, must move farther apart. They absorb the energy necessary to do this from the electron in its 7r -orbital. An additional vibrational ampHtude is attained by the two atoms as a system. Some added energy is transmitted to other atoms of the conjugated molecule. These atomic vibrational adjustments take place very quickly, in 10 to 10 s. 

The UV spectrum of a complex conjugated molecule is usually observed to consist of a few broad band systems, often with fine structure, which may be sharpened up in non-polar solvents. Such a spectrum can often be shown to be more complex than it superficially appears, by investigation of the magnetic circular dichroism (MCD) spectrum, or by introduction of dissymmetry and running the optical rotatory dispersion (ORD) or circular dichroism (CD) spectrum. These techniques will frequently separate and distinguish overlapping bands of different symmetry properties <71PMH(3)397). 

Frost s circle (Section 11.19) A mnemonic that gives the Hiickel -tr MOs for cyclic conjugated molecules and ions. 

One of the earliest models for treating conjugated molecules is afforded by the Hiickel rr-electron model. This dates from the work of E. Hiickel in 1931. The ideas are simple and appealing, and the model enjoyed many years of successful application to individual molecules, molecular clusters and solids. 

Although I used the example of ethene, where n =2, the same consideration, apply to ZDO calculations on all conjugated molecules. All overlap matrices are real symmetric, positive definite and so have eigenvalues > 0. 

Many early studies in ESR spectroscopy were concerned with anions, cations and triplet states derived from conjugated molecules. The unpaired electron(s)... 

In contrast to the alkaloids mentioned so far, heterocyclic mesomeric betaines are defined as neutral conjugated molecules which can be represented only by dipolar structures in which both the negative and the positive charges are delocalized within the rr-electron system (38JCS824, 85T2239). The first heterocyclic mesomeric betaine was prepared unknowingly by Emil Fischer (1882LA316). The real structure remained unknown... 

The wavelength necessary to effect the tt — tt transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated tt electron system present in a molecule. 

Other kinds of conjugated systems, such as conjugated enones and aromatic rings, also have characteristic UV absorptions that are useful in structure determination. The UV absorption maxima of some representative conjugated molecules are given in Table 14.2. 

Table 14.2. Ultraviolet Absorptions ofSome Conjugated Molecules Name Structure...

Recall the requirements fox aromaticity—a planar, cyclic, conjugated molecule with 4n + 2 7r electrons—and see how these requirements apply to thiophene. 

Benzene is described by molecular orbital theory as a planar, cyclic, conjugated molecule with six it electrons. According to the Htickel rule, a molecule must have 4n + 2 77 electrons, where n - 0, 1, 2, 3, and so on, to be aromatic. Pianar, cyclic, conjugated molecules with other numbers of tt electrons are antiaromatic. 

Nitriles show an intense and easily recognizable C=N bond absorption near 2250 cm-1 for saturated compounds and 2230 cm-1 for aromatic and conjugated molecules. Since few other functional groups absorb in this region, IR spectroscopy is highly diagnostic for nitriles. 

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. 

Aromaticity (Chapter 15 introduction) The special characteristics of cyclic conjugated molecules. These characteristics include unusual stability, the presence of a ring current in the 1H NMR spectrum, and a tendency to undergo substitution reactions rather than addition reactions on treatment with electrophiles. Aromatic molecules are planar, cyclic, conjugated species that have An + 2 7T electrons. 

Huckel s rule (Section 15.3) A rule stating that monocyclic conjugated molecules having 4n + 2 tt electrons n = an integer) are aromatic. 

Figure 9-3. Conventional multilayer light emission device (LED) indium tin oxide (ITO) electrode on a substrate, active layers A (hole transport), B (emitter), C (electron transport), and a niclat electrode. A possible encapsulation layer has been omitted, which would prevent the conjugated molecules from photo-oxidation.

While there is much to discuss about order in films of different conjugated molecules, a comprehensive survey of the structural properties of various conjugated polymers can be found in Ref. [9]. This section focuses on the relation between microscopic order and macroscopic properties, and on structure-property relations. 

In order to summarize this section it must be noted that the possibility to evaporate conjugated oligomers in such a way that their chain axes point predominantly in one direction allows us to determine the anisotropic electronic properties of conjugated molecules. From these studies it becomes evident that in order to obtain polarized absorption or emission of light as required in polarization Filters or LCD-backlights, the application of conjugated molecules is one of the most attractive areas of application for these materials. 

Figure 9-2. Based on ihe conjugated backbone of a /H/ra-phenylcne the most common principle structures of conjugated molecules am described (a) the pure backbone (b) a backbone with attached side-groups R and R (e) a ladder type-eonjugaled backbone (d) and (c) represent different types of copolymers. where (d) represents a side cliain-gnill copolymer and (c) a single eliain-bloek copolymer.

Manning, P. P., Proc. Roy. Soc. London) A225, 136, 244, The molecular orbital theory of chemical valency. XVII. Higher approximations. XVIII. Localized orbitals in conjugated molecules."... 

The delocalization of molecular orbitals lies at the heart of modern chemistry. The concept that the tt orbitals of benzene or naphthalene cover the entire carbon skeleton promoted the successful understanding of conjugated molecules. The work of R. Hoffmann and others has proven that in saturated molecules [Pg.1]

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