Jt-electrons

The simplest approximation to the Schrodinger equation is an independent-electron approximation, such as the Hiickel method for Jt-electron systems, developed by E. Hiickel. Later, others, principally Roald Hoffmann of Cornell University, extended the Hiickel approximations to arbitrary systems having both n and a electrons—the Extended Hiickel Theory (EHT) approximation. This chapter describes some of the basics of molecular orbital theory with a view to later explaining the specifics of HyperChem EHT calculations. 

Although CNTs showed similar EELS pattern in plasmon-loss and core-loss regions to graphite, SWCNT and fine MWCNT with a diameter less than 5 nm had different features. Furthermore, it has been found out that the angular-dependent EELS along the direction normal to the longitudinal axis of CNT shows stronger contribution from Jt electrons than [Pg.38]

TT-Electron materials, which are defined as those having extended Jt-electron clouds in the solid state, have various peculiar properties such as high electron mobility and chemical/biological activities. We have developed a set of techniques for synthesizing carbonaceous K-electron materials, especially crystalline graphite and carbon nanotubes, at temperatures below 1000°C. We have also revealed new types of physical or chemical interactions between Jt-electron materials and various other materials. The unique interactions found in various Jt-electron materials, especially carbon nanotubes, will lay the foundation for developing novel functional, electronic devices in the next generation. 

Sigmatropic rearrangement (Section 24.13) Migration of a cr bond from one end of a conjugated jt electron system to the other. The Claisen rearrangement is an example. 

The next step came in the 1950s, with more serious attempts to include formally the effect of electron repulsion between the valence electrons. First came the jT-electron models associated with the name of Pople, and with Pariser and Parr. You might like to read the synopses of their first papers. 

It is also interesting that the hypothetical helicene 255 has been incorporated in a study of the jt -electron spectra of a series of infinite helicenes composed of various structural units (77BAU2532). A compound 256, closely resembling an indolo-[2,3-c]carbazole but possessing an additional six-membered ring between the... 

Ohm, Y., "Quantum Mechanical Studies of Eletronic Spectra of Atomic and Jt-Electron Systems" Acta Universitatis Upsaliensis 1966, 68. 

In general compounds with heteroatoms (N, O, S and P) are more amenable to fluorescence reactions" than pure hydrocarbons. Under the influence of the catalytic sorbents substances rich in Jt-electrons are formed, that conjugate to rigid reaction products that are fluorescent when appropriately excited. The formation of fluorescent derivatives is frequently encouraged by gassing with nitrogen or carbon dioxide. 

Numerous colorless organic compounds with extended jt-electron systems can be converted to colored cations or anions with polymethyne chromophore by protonation or deprotonation. The intense coloration of the corresponding salts is usually attributable to the fact that the lone pairs of electrons of the heteroatoms participate in the mesomerism of the conjugated n-electron systems [4]. 

Naphthalene undergoes electrophihc substitutions at the a rather than p position. The Hueckel molecular orbital calculations show that all the carbons have the same jt electron density 1.0. This is not in agreement with the theory of organic reactions based on the Coulombic interaction that electrophilic attack occurs on the most negatively charged atom. Fukui [7] proposed the frontier orbital theory for the discrepancy between the theory and the experimental observation. The importance of... 

Stable localized singlet 1,3-diradicals are bnilt on 2,4-disilacyclobntane-l,3-diyls with electron-withdrawing o-bonds on the silicon atoms and jt-electron... 

In the 1930 s HiickeP proposed, on the basis of molecular-orbital calculations, a theoretical criterion for aromaticity of cyclic polyenes, known as Hiickers rule, which states that cyclic polyenes should be aromatic if, and only if, they contain 4n- -2 Jt-electrons. At that time only two of such cyclic polyenes were known benzene and cyclo-pentadienyl anion, each having six rc-electrons and satisfying Huckel s rule. Since then, the validity of Hiickel s rule had not been challenged... 

In this case the excited molecules produced on interaction with radiation undergo spin reversal to yield a triplet state with a much longer lifetime than that of the singlet excited state One or more Jt-bonds are broken in the triplet state since one of the Jt-electrons affected is in an antibonding n molecular orbital. This means that the a-bond is free to rotate and cis and trans isomers can be formed next to each other on recombination of the double bond. 

Betalains are N-heterocyclic water-soluble pigments deposited in vacuoles. Their common precursor is betalamic acid consisting of a 1,7-diazaheptamethin system, an extended Jt-electron system exhibiting a canary yellow color. Betalamic acid may condense with cyc/o-dopa to yield betanidin, the common precursor aglycon of the red betacyanins. Betanidin in turn may be glycosylated and/or acylated, yielding 29... 

The oxidation and/or reduction reactions yield polymeric systems having an extended Jt-electron system along the chain. Doping to the conducting state, in the instance of polyacetylene by exposnre to iodine vapor (p-doping, oxidizing). 

Figure 9.23. Enol (a) - keto (b) tautomerism in o-hydroxyphenyl-(l,3,5)-triazine, indicating the number of Jt-electrons in the ground state for each ring A and B (adapted from reference 16).

Ethyne has two jt bonding orbitals at right angles to each other and a resultant jt electron density that is cylindrically symmetric with respect to the internuclear axis. Complexes of ethyne with HF [133], HC1 [134], HBr [135], C1F [66], CI2 [47], BrCl [50], Br2 [92] and IC1 [95] have been characterised by... 

A ten Jt electron heterocycle, imidazo [7,2-a] Pyridine was studied by Paudler and Blewitt 115>. The protonation occurred at Ni, which was calculated to have a total n electron density less than N4 (Fig. 

Since divalent sulfur is isoelectronic with the carbon-carbon double bond and can participate in Jt-electron delocalization, the anions derived from 1,2- (143), 1,3- (144), and 1,4-dithiepins (145) may be regarded as 1 Orc-electron aromatic systems. According to the prediction by Zahradnik and Parkanyi851 based on Hiickel s molecular... 

Fernando and Senadeera [40] used different natural pigments incorporating anthocyanins that were extracted from various tropical flowers. The extract was acidified to p H < 1, assuming a deeper coloration, and was used for the preparation of the photovoltaic apparatus. The generation at pH <1 of the oxonium ion allows the wide conjugation of the Jt-electron density throughout the aglycone portion of the... 

Phthalocyanine (Pc) molecules consisting of four interconnected isoindole units are synthetic analogues of porphyrins. They represent planar aromatic macrocycles with an 18 Jt-electron aromatic system, which is delocalized over an arrangement of alternated carbon and nitrogen atoms. For years, phthalocyanines have... 

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