Tt electron system

Katritzky and Topsom have reviewed the information available, largely from infrared and n.m.r. studies, concerning the distortion of the tt-electron system in the benzene ring brought about in the ground state by substituents. Of particular interest is the observation that both n.m.r. studies (of m- F and chemical shifts) and infrared investigations (of the intensities of bands due to certain skeletal vibrations) suggest that the value of Taft s [Pg.226]

The main application of UV VIS spectroscopy which depends on transitions between electronic energy levels is in identifying conjugated tt electron systems... 

Section 13 21 Transitions between electronic energy levels involving electromagnetic radiation m the 200-800 nm range form the basis of UV VIS spec troscopy The absorption peaks tend to be broad but are often useful m indicating the presence of particular tt electron systems within a mole cule... 

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 tt electron systems where an electron may be associated with several carbon atoms... 

Sigmatropic rearrangement (Section 24 13) Migration of a a bond from one end of a conjugated tt electron system to the other The Claisen rearrangement is an example... 

A detailed comparison of spectral data concluded that bora2iae has a delocali2ed TT-electron system like that of ben2ene (112) other workers, however, have concluded from spia-coupled calculations that bora2iae has Htde aromatic stabili2ation as compared to ben2ene (113). 

Charge-Transfer Compounds. Similat to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the iatense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a TT-electron system as ia the case of aromatic compounds or from lone-pairs of electrons as ia ethers and amines. 

Acceptor donor CT occurs, eg, in the solution of iodine in ben2ene, where an electron can transfer from the Tt-electron system in ben2ene to the I2 molecule. Organic dyes containing both donor and acceptor groups can also be approached from this viewpoint. 

The TT-electron systems of anisole, mesitylene, thiophene and 2,4,6-trimethylpyridine attack the carbon atoms of 5-alkylthiiranium salts (Scheme 98) (81IZV1929). 

The TT-electron system of the addition intermediate is isoelectronic with that of a pentadienyl anion. 

Additional evidence for electron delocalization in 1,3-butadiene can be obtained by considering its conformations. Overlap of the two tt electron systems is optimal when the four carbon atoms are coplanar-. Two conformations allow this coplanarity they are called the 5-cis and 5-trans conformations. 

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

The structural unit associated with an electronic transition in UV-VIS spectroscopy is called a chromophore. Chemists often refer to model compounds to help interpret UV-VIS spectra. An appropriate model is a simple compound of known structure that incorporates the chromophore suspected of being present in the sfflnple. Because remote substituents do not affect of the chromophore, a strong similarity between the spectrum of the model compound and that of the urrknown can serve to identify the kind of tt electron system present in the sfflnple. There is a substantial body of data concerning the UV-VIS spectra of a great many chromophores, as well as empirical conelations of substituent effects on Such data are helpful when using UV-VIS spectroscopy as a tool for structure determination. 

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

The [S3N3]" cation is of interest as an example of an antiaromatic eight TT-electron system (Section 4.4). Ab initio molecular orbital calculations indicate that a triplet cation, with a planar ring, is more stable than the singlet cation. The [S3N3]" cation has been obtained as the norbomene adduct 5.16, but salts of the free cation have not been isolated. ... 

Mass spectrometry Molecular size and formula IR spectroscopy Functional groups present NMR spectroscopy Carbon-hydrogen framework UV spectroscopy Nature of conjugated tt electron system... 

The wavelength necessary to effect the tt — tt transition in a conjugated molecule depends on the energy gap between HOMO and LUMO, which in turn depends on the nature of the conjugated system. Thus, by measuring the UV spectrum of an unknown, we can derive structural information about the nature of any conjugated tt electron system present in a molecule. 

As noted previously, arylamines are generally less basic than alkylamines. Anilinium ion has pKa = 4.63, for instance, whereas methylammonium ion has pfCa = 10.64. Arylamines are less basic than alkylamines because the nitrogen lone-pair electrons are delocalized by interaction with the aromatic ring tt electron system and are less available for bonding to H+. In resonance terms, aryl-amines are stabilized relative to alkylamines because of their five resonance forms. 

Azo-coupled products are widely used as dyes for textiles because their extended conjugated tt 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. 

Antarafacial (Section 30.6) A pericyclic reaction that takes place on opposite faces of the two ends of a tt electron system. 

Delocalization (Section 10.5) A spreading out of electron density over a conjugated tt electron system. For example, allylic cations and allylic anions are delocalized because their charges are spread out over the entire 77 electron system. 

Sigmatropic reaction (Section 30.8) A pericyclic reaction that involves the migration of a group from one end of a tt electron system to the other. 

The TT-electron system-substituted organodisilanes such as aryl-, alkenyl-, and alkynyldisilanes are photoactive under ultraviolet irradiation, and their photochemical behavior has been extensively studied (1). However, much less interest has been shown in the photochemistry of polymers bearing TT-electron substituted disilanyl units (2-4). In this paper, we report the synthesis and photochemical behavior of polysiloxanes involving phenyl(trimethylsilyl)-siloxy units and silicon polymers in which the alternate arrangement of a disilanylene unit and a phenylene group is found regularly in the polymer backbone. We also describe lithographic applications of a double-layer system of the latter polymers. 

S. Yamaguchi and K. Tamao, A key role of orbital interaction in the main group element-containing TT-electron systems, Chem. Lett., 34 2-7, 2005. 

---