Jt electron system

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. 

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. 

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

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]. 

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). 

The film composed of compounds with highly conjugated Jt-electron systems showed larger pyroelectricity. 

A basic property of all conducting polymers is the conjugation of the chain-linked electroactive monomeric units, that is, the monomers interact via a jt-electron system. In this respect, they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated and the interaction between the isolated redox counters is weak. Consequently, redox polymers are nonconductors [17]. They will not be discussed in this context. 

The electron-donating effect originates from the lone pair electrons on oxygen, with overlap into the Jt electron system. This electron donation will stabilize the non-ionized acid via electron delocalization, but would destabilize the conjugate base by creating a double charge in the carboxylate system. The net result is lower acidity. 

Although the nitrogen atom in these systems carries a lone pair of electrons, these electrons are not able to accept a proton in the same way as a simple amine. The dramatic differences in basicity are a consequence of the Jt electron systems, to which the nitrogen contributes (see Section 2.9.6). 

Pyridine, like benzene, is an aromatic system with six jt electrons (see Section 11.3). The ring is planar, and the lone pair is held in an sp orbital. The increased s character of this orbital, compared with the sp orbital in piperidine, means that the lone pair electrons are held closer to the nitrogen and, consequently, are less available for protonation. This hybridization effect explains the lower basicity of pyridine compared with piperidine. Pyrrole is also aromatic, but there is a significant difference, in that both of the lone pair electrons are contributing to the six-jr-electron system. As part of the delocalized Jt electron system, the lone pairs are consequently not available for bonding to... 

Benzene conforms to Hiickel s ruie, which predicts that planar cyclic polyenes containing 4 -I- 2 7t electrons show enhanced stability associated with aromaticity (see Section 2.9.3). Pyridine is also aromatic nitrogen contributes one electron in a orbital to the Jt electron system, and its lone pair is located in an sp orbital that is in the plane of the ring and perpendicular to the n electron system. It also conforms to HtickeTs rule, in that we still have an aromatic sextet of Jt electrons. 

Nitrogen is more electronegative than carbon, and this influences the electron distribution in the Jt-electron system in pyridine through inductive effects, such that nitrogen is electron rich. In addition, the... 

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-... 

Upon photoexcitation, lumophores, especially those with groups held outside but close to the Jt-electron system of the lumophore, show an increased susceptibility to redox reactions. Knowledge of the redox potentials of these reactions can be used... 

Ultraviolet Spectral Properties of Polysilanes Containing jt-Electron Systems... 

The R group possesses Jt-electrons or lone pairs of electrons that can interact with the rest of the Jt-electron system. The most important electron donor is the amino group. Triarylmethine dyes are usually divided into mono-, di-, and triami-notriarylmethine dyes. In some di- and triarylmethine dyes, the ring carbon atoms ortho to the central methine carbon atom are bonded via a heteroatom to form a heterocyclic six-membered ring. These include the acridine, xanthene, and thiox-anthene dyes. 

The chromophores of optical brighteners are Jt electron systems in which Jt-Jt transitions occur. The chromophores must be rigid [5], and their conformations should differ only slightly in the electronic ground state and in the first excited state. 

The electron is not localized, but is part of the extended jt-electron system 7). A third type of reaction can be defined, which cannot be classified under the two previous ones, but is important in catalysis ... 

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