Conjugated it electron system

Ultraviolet spectroscopy Is a conjugated it electron system present ... 

The aromaticity of naphthalene is explained by the orbital picture in Figure 15.12. Naphthalene has a cyclic, conjugated it electron system, with p orbital overlap both around the ten-carbon periphery of the molecule and across the central bond. Since ten 77 electrons is a Hiickel number, there is tt electron delocalization and consequent aromaticity in naphthalene. 

Ultraviolet-visible (UV-VIS) spectroscopy, which probes the electron distribution, especially in molecules that have conjugated it electron systems. 

FIGURE 18.6 Acrolein (H2C=CHCH=0) is a planar molecule. Oxygen and each carbon is sp2-hybridized, and each contributes one electron to a conjugated it electron system analogous to that of 1,3-butadiene. 

Delocalization (Section 1.9) Association of an electron with more than one atom. The simplest example is the shared electron pair (covalent) bond. Delocalization is important in conjugated it electron systems, where an electron may be associated with several carbon atoms. 

Sigmatropic rearrangement (Section 24.13) Migration of a electron system to the other. The Claisen rearrangement is an example. 

Fig. 12.5. Type interaction between one end of a conjugated 7T-electron system and one end of another conjugated it-electron system influence of the orbital occupancy of the it MOs on the electronic energy. 

TABLE 20.2 Absorption of Light by Molecules with Conjugated it Electron Systems... 

As an example of a conjugated it electron system involving the thiocarbonyl group we consider the cyclic trithiocarbonate l,3-dithiole-2-thione (iS). 38 and its selenium analogues 39-43 comprise an ideal series for the study of the valeqce... 

One of the spectacular achievements in the syntheses of conjugated it electron systems is that of l,6-methano[10]annulene/ The material is aromatic as evidenced by chemical reactivity and by NMR chemical shifts which are independent of temperature Interestingly, the annulene was prepared by synthesis of its valence... 

Azo-coupled products are widely used as dyes for textiles because their extended conjugated it electron system causes them to absorb in the visible region of the electromagnetic spectrum (Section 14.9). />-(Dimethylamino)-azobenzene, for instance, is a bright yellow compound that was at one time used as a coloring agent in margarine. 

Ultraviolet visible (UV VIS) spectroscopy, which probes the electron distribution especially m molecules that have conjugated n electron systems Mass spectrometry (MS), which gives the molecular weight and formula both of the molecule itself and various structural units within it... 

Readily measurable fluorescence intensities are found for molecules having aromatic and heteroaromatic rings, in particular when annulated rings are present, and in the case of conjugated 7x-electron systems. If the polymer molecules contain such fluorescence-active subunits they can be characterized by this technique, either directly via their fluorescence spectrum or via fluorescence quenching experiments (for polymers with appropriate quencher groups). It is... 

Their isolation by flash chromatography on silica gel was comparatively easy. The CD spectra of related pairs of diastereomers whose addition pattern represent pairs of enantiomers, reveal pronounced Cotton effects and mirror image behavior. It is the chiral arrangement of the conjugated Jt-electron system within the fullerene core that predominantly determines the chiroptical properties. Adducts with a C2-... 

Aromaticity has been long recognized as one of the most useful theoretical concepts in organic chemistry. It is essential in understanding the reactivity, structure and many physico-chemical characteristics of heterocyclic compounds. Aromaticity can be defined as a measure of the basic state of cyclic conjugated TT-electron systems, which is manifested in increased thermodynamic stability, planar geometry with non-localized cyclic bonds, and the ability to sustain an induced ring current. In contrast to aromatic compounds there exist nonaromatic and antiaromatic systems. Thus, pyrazine (69)... 

The ir ir transition in the UV spectrum of trans-stilbene (frans-C6H5CH=CHC6H5) appears at 295 nm compared with 283 nm for the cis stereoisomer. The extinction coefficient emax is approximately twice as great for trans-stilbene as for c/.s-stilbene. Both facts are normally interpreted in terms of more effective conjugation of the it electron system in trans-stilbene. Construct a molecular model of each stereoisomer, and identify the reason for the decreased effectiveness of conjugation in cw-stilbene. 

Polycyclic aromatic hydrocarbons contain extended conjugated rc-electron systems and are electroactive within the cathodic potential window of many organic sol-vent/tetraalkylammonium electrolyte combinations. For many of them reduction potentials were measured and reported. It is important to note that the reduction of polycyclic aromatics involves consecutive electron-transfers and protonations. The nature of the product depends on the number of electrons and protons consumed and higher numbers result in more hydrogenated, less conjugated and thus less electroactive products. A simplified reduction scheme for polycyclic aromatics is shown below. The products are divided into three groups (A) are reduced within the potential window and (B) require drastic reduction conditions as those described for the substrates in Chapt. 3. 

Figure 17.21 (part I) also shows the most stable conformer each of (DHQ)2-PHAL and (DHQD)2-PHAL. Its conformation first reflects the propensity of the oxygen atoms at the central heterocycle to conjugate with its -electron system leading to an -hybridization of these oxygen atoms. Second, the bulky sec-alkyl substituents are in search for the least hindered position. The result is that they try to avoid/>eri-interaction with the labeled H atoms of the central heterocycle (for the term />eri-interaction see the discussion of Figure 5.8), but do not tolerate a U-conformation in the non-hydrogen substituent-Csec-0-C=N substructure. 

Reductive silylation of pteridine (1) with potassium and trimethylsilyl chloride in THF leads in low yield to 5,8-bis-trimethylsilyl-5,8-dihydropteridine (75), which was characterized by NMR and is a highly reactive substance due to its conjugated 8 -electron system (Equation (5)) <89JOM(362)37>. 

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 ... 

Trimethylenecyclopropane, [3]radialene (169), which is one of the benzene isomers and the smallest member of the radialene family, is characterized by a cyclic arrangement of cross-conjugated Ti-electron systems. The synthesis and properties of radialenes have been reviewed recently by Hopf and Maas The C—C and C=C bond lengths in 169,1.453 (3) and 1.343 (3) A, determined by are similar to those in its hexamethyl derivative... 

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