Conjugation system

Salem L 1966 Molecular Orbital Theory of Conjugated Systems (Reading, MA Benjamin)... 

Soret band of the macrocycle, a ti to tt transition, is excited instead. It has been found that the vinyl groups do not participate in the conjugated system [4]. This is based on the fact that the vinyl C=C stretch does not... 

The treatment of conjugated systems in terms of electron systems that extend smoothly over all atoms allows the treatment of a variety of structural phenomena, as may be explained with a spedes that shows hindered rotation and with the nitro group. 

Then the Huckel matrix for the conjugated i -system is constructed. The a-values of the Huckel matrix of each atom i of the conjugated system are adjusted to the <7-chaige distribution by Eq. (13). 

The initial values, a, , are derived by correlations with dipole moments of a series of conjugated systems. The exchange integrals are taken from Abraham and Hudson [38] and are considered as being independent of charge. The r-charges are then calculated from the orbital coefficients, c,j, of the HMO theory according to Eq. (14). 

The quality of the r-charge values thus obtained has been demonstrated by the calculation of dipole moments of a series of 80 conjugated systems [39],... 

The dimension of the matrix is the number of atoms in the n conjugated system. Let us take the three-carbon system allyl as our next step. Concentrate on the end... 

Salem, L., 1966. The Molecular Orbital Theory of Conjugated Systems. Benjamin, New York. Saunders, M., 1987. 7. Amer. Chem. Soc. 109, 3150. 

Tertiary amines capable of eliminating a secondary amine to form a conjugated system can react with hydrogen cyanide to form y-keto nitriles by amine replacement. Thus (I) yields p-benzoylpropionitrile (IV) ... 

The former exhibits absorption tjrpical of an isolated keto group, whereas the latter shows a high intensity -band associated with the conjugated system HO—C=C—C=0. The proportions of the two forms under various conditions are readily determined from the ultraviolet spectra. The ultraviolet spectra in various solvents are shown in Fig. A, 7, 2. Since the absorption of the keto form is negligible, the percentage of enol present is 100(em/e ), where e is the observed extinction at 245 mp. and that of the pure enol. It was shown that in alcoholic solution is 1900 and the percentage of enol is 12. Thus e is ca. 16000, and use of this value permits the approximate evaluation of the enol content in different solvents. The results are collected in Table XII. 

The condensation of aldehydes or ketones with secondary amines leads to "encunines via N-hemiacetals and immonium hydroxides, when the water is removed. In these conjugated systems electron density and nudeophilicity are largely transferred from the nitrogen to the a-carbon atom, and thus enamines are useful electroneutral d -reagents (G.A. Cook, 1969 S.F. Dyke, 1973). A bulky heterocyclic substituent supports regio- and stereoselective reactions. 

The pyridine-like nitrogen of the 2H-pyrrol-2-yiidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

However, benzylidene derivatives show a strong bathochromic shift in comparison with alkylidene derivatives. Thus absorption is a result of the whole conjugated system that is comparable to that of the quinoid dyes. The color of this type of compound is sensitive to acids and bases. 

Conjugare is a Latin verb meaning to link or yoke together and allylic carbocations allylic free radicals and conjugated dienes are all examples of conjugated systems In this chapter we 11 see how conjugation permits two functional units within a molecule to display a kind of reactivity that is qualitatively different from that of either unit alone... 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

Allylic carbocations and allylic radicals are conjugated systems involved as reactive intermediates m chemical reactions The third type of conjugated system that we will examine conjugated dienes, consists of stable molecules... 

Huckel realized that his molecular orbital analysis of conjugated systems could be extended beyond neutral hydrocarbons He pointed out that cycloheptatrienyl cation also called tropyhum ion contained a completely conjugated closed shell six tt electron sys tern analogous to that of benzene... 

The carbocation is aromatic the hydrocarbon is not Although cycloheptatriene has six TT electrons m a conjugated system the ends of the triene system are separated by an sp hybridized carbon which prevents continuous tt electron delocalization... 

Huckel s rule is now taken to apply to planar monocyclic completely conjugated systems generally not just to neutral hydrocarbons... 

This electron delocalization stabilizes a conjugated system Under conditions cho sen to bring about their mterconversion the equilibrium between a p 7 unsaturated ketone and an a p unsaturated analog favors the conjugated isomer... 

Carotenoids are natural pigments characterized by a tail to tail linkage between two C20 units and an extended conjugated system of double bonds They are the most widely dis tributed of the substances that give color to our world and occur m flowers fruits plants insects and animals It has been estimated that biosynthesis from acetate produces approximately a hundred million tons of carotenoids per year The most familiar carotenoids are lycopene and (3 carotene pigments found m numerous plants and easily isolable from npe tomatoes and carrots respectively... 

Ultraviolet visible (UV VIS) spectroscopy (Section 13 21) An alytical method based on transitions between electronic en ergy states in molecules Useful in studying conjugated systems such as polyenes... 

It is apparent from the size of the conjugated system here that numerous resonance possibilities exist in this species in both the radical and the molecular form. Styrene also has resonance structures in both forms. On the principle that these effects are larger for radicals than monomers, we conclude that the difference ep. - ej > 0 for both hemin and styrene. On the principle that greater resonance effects result from greater delocalization, we expect the difference to be larger for hemin than for styrene. According to Eq. (7.23), r j oc > 1. According to Eq. (7.24), i2 < 1. 

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