Conjugated pi-electron systems

Lack of response to water and carbon disulfide allows these to be used as solvents without interference with the analysis Good for electronegative compounds such as those containing extensively conjugated pi-electron systems, nitro- and highly conjugated aromatic compounds and compounds which can be readily derivatized to respond. " Response depends also upon carrier gas flow rate... 

Recently, a series of chemical substances called functional dyes have attracted considerable attention. Because such dyes have long conjugated pi-electron systems and in many cases possess intramolecular charge transfer structures, functional dyes are expected to show interesting optical and electronic properties. Among functional dyes, phthalocyanine compounds have been extensively investigated because of their excellent physical, chemical and coloristic properties, as mentioned above. For example, about 1000 related US patents, published from 1990 onwards, are retrievable from the World Patent Index data base of Derwent, and more than 30 % of these are classified in such non-colorant applications as electrophotography, catalysis, and infrared radiation absorption. 

Phthalocyanine compounds contain macrocyclic conjugated pi-electron systems and metal ions in the central void. These phthalocyanine structures are very similar to porphyrins, one of the most important compounds in biochemistry. Accordingly, phthalocyanine compounds are expected to facilitate interesting catalysis and multielectron processes. Almost all metal and semi-metal ions can be incorporated in the central void of the phthalocyanine molecule, and the characteristics of such phthalocyanine compounds can be modified by the metal ions. About seventy metals are known to incorporate in the phthalocyanine molecule. 

Since the discovery of doped polyacetylene, a range of polymer-intense semiconductor devices have been studied including normal transistors and field-effect transistors (FETs), and photodiodes and light-emitting diodes (LEDs). Like conductive polymers, these materials obtain their properties due to their electronic nature, specifically the presence of conjugated pi-bonding systems. 

The spin density (pi) at the unsubstituted end of the radical can be calculated directly from the proton hyperfine coupling constants, using the relationship, A = 2i-lp (McConnell and Chesnut, 1958 Fessenden and Schuler, 1963). The spin density (ps) at the substituted end of the radicals can be calculated if it is assumed that the total spin density in the conjugated 77-electron system of the radical is unaffected by substitution. Then ... 

The existence of a pi-electron system of this type does not imply that similarities are to be expected between polyphosphazenes and conjugated (or non-conjugated) organic polymers that possess an unsaturated skeleton. 

Molecular fluorescence involves the emission of radiation as excited electrons return to the ground state. The wavelengths of the radiation emitted are different from those absorbed and are useful in the identification of a molecule. The intensity of the emitted radiation can be used in quantitative methods and the wavelength of maximum emission can be used qualitatively. A considerable number of compounds demonstrate fluorescence and it provides the basis of a very sensitive method of quantitation. Fluorescent compounds often contain multiple conjugated bond systems with the associated delocalized pi electrons, and the presence of electron-donating groups, such as amine and hydroxyl, increase the possibility of fluorescence. Most molecules that fluoresce have rigid, planar structures. 

Inner electrons Sigma electrons n and pi electrons Conjugated systems Highly conjugated systems Vibrational and rotational Rotational... 

It is apparent that the molecular orbital theory is a very useful method of classifying the ground and excited states of small molecules. The transition metal complexes occupy a special place here, and the last chapter is devoted entirely to this subject. We believe that modem inorganic chemists should be acquainted with the methods of the theory, and that they will find approximate one-electron calculations as helpful as the organic chemists have found simple Hiickel calculations. For this reason, we have included a calculation of the permanganate ion in Chapter 8. On the other hand, we have not considered conjugated pi systems because they are excellently discussed in a number of books. 

Pattern of molecular orbitals in a cyclic conjugated system. In a cyclic conjugated system, the lowest-lying MO is filled with two electrons. Each of the additional shells consists of two degenerate MOs, with space for four electrons. If a molecule has AN+2) pi electrons, it will have a filled shell. If it has (47V) electrons, there will be two unpaired electrons in two degenerate orbitals. 

Since the degenerate pair must be completely filled or completely empty in order for the molecule to be aromatic, stable arrangements arise when 2, 6, 10,. .. or 4n + 2 pi electrons are in the cyclic conjugated pi system. Unstable half-filled degenerate shells will occur with 4, 8, 12,. .. or 4n pi electrons. The 4n systems tend to distort from planarity to diminish pi overlap and this destabilizing antiaromatic conjugation. 

After understanding the usefulness of unsaturated compound, or conjugated system, we hope to explore the unique structure of aromatic compounds, including why benzene should not be called 1,3,5-hexatriene because it is more stable than a typical triene, and seemingly unreactive. Called aromatic initially because of its fragrance, aromaticity now refers to the stability of compounds that are considered aromatic, not only benzene. Any cyclic compound with 4n+2 pi electrons in the system is aromatic. The stability of aromatic compounds arises because all bonding orbitals are filled and low in energy. 

Nitrogen 1 is sp hybridized and its unshared pair of electrons is in an sp nonbonding AO. Nitrogen 2 is also sp hybridized, but its unshared pair of electrons is in a p orbital and is part of the conjugated pi system. Nitrogen 3 is sp hybridized and its unshared pair of electrons is in an sp nonbonding AO. 

The electrons on the N with the H are part of the conjugated pi system and are not very basic. The electrons on the N without the H are in an AO that is perpendicular to the pi system. These electrons are not involved in resonance and are much more basic. 

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