Shell aromaticity

The crystal structure of 864(118207)2 90, 91) has shown 864 to be square planar with an 8e-8e bond distance of 2.283(4) A, significantly less than that of 2.34(2) A found in the 8eg molecule (92), indicating some degree of multiple bonding. 8uch a result is consistent with a valence bond description of the molecule involving four structures of type VII. Alternatively the structure can be understood in terms of molecule orbital theory. The circle in structure VIII denotes a closed-shell (aromatic ) six-w-electron system. Of the four tt molecular orbitals,... 

Shell Cydo Sol. [Shell] Aromatic hydro-carbm solvent for dry cleaning, coatings, automotive and chonical q >ecial-ties. 

Elegant chemical syntheses at UCLA have resulted in the unambiguous structural characterization of monocyclic all-carbon molecules. Cyclo(18) earbon and other species have been prepared and characterized. Neutral cycles satisfying the general formula 4n - - 2 atoms are predicted to be elosed-shell aromatic molecules with special stability. This new class of monocyclic molecules, which might well... 

Due to its crucial role in combustion chemistry and biology, the phenoxyl radical has been the subject of intense experimental and theoretical studies for many years (for recent work, see Ref. 127 and references cited therein). For these reasons we present it here as an example of UV spectroscopy of (neutral) radicals (for the application of DFT to the spectra of aromatic radical cations, see, e.g.. Ref. 128 and for results of MRCI computations, see Ref. 129). The phenoxyl radical is well known as a difficult case for electronic structure methods because it requires a sophisticated treatment of electron correlation similar to closed-shell aromatic hydrocarbons. 

The Hiickel description of aromaticity was based in part on benzene, a cyclic fully conjugated hydrocarbon having (4n -l- 2) -electrons (ff = I) in the closed shell (ring). 

Wax Cracking. One or more wax-cracked a-olefin plants were operated from 1962 to 1985 Chevron had two such plants at Richmond, California, and Shell had three in Europe. The wax-cracked olefins were of limited commercial value because they contained internal olefins, branched olefins, diolefins, aromatics, and paraffins. These were satisfactory for feed to alkyl benzene plants and for certain markets, but unsatisfactory for polyethylene comonomers and several other markets. Typical distributions were C 33% C q, 7% 25% and 35%. Since both odd and... 

A few free radicals are indefinitely stable. Entries 1, 4, and 6 in Scheme 12.1 are examples. These molecules are just as stable under ordinary conditions of temperature and atmosphere as typical closed-shell molecules. Entry 2 is somewhat less stable to oxygen, although it can exist indefinitely in the absence of oxygen. The structures shown in entries 1, 2, and 4 all permit extensive delocalization of the unpaired electron into aromatic rings. These highly delocalized radicals show no tendency toward dimerization or disproportionation. Radicals that have long lifetimes and are resistant to dimerization or other routes for bimolecular self-annihilation are called stable free radicals. The term inert free radical has been suggested for species such as entry 4, which is unreactive under ordinary conditions and is thermally stable even at 300°C. ... 

The thiophene sulfur atom shows very few of the reactions expected of a sulfide. The oxidation to a sulfone is difficult to achieve, but is of special interest, as knowledge of its aromatic character or lack of it would give information about the ability of sulfur to expand its valence shell beyond eight electrons. 

Active Figure 15.11 Energy levels of the five cyclopentadienyl molecular orbitals. Only the six-7r-electron cyclopentadienyl anion has a filled-shell configuration leading to aromaticity. Sign in at www.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Melinite. Fr expl used in shells. The original Melinite was composed of 70ps PA mixed with 3Ops NC, disd in acet or eth-alc (Ref 1). Later Melinites were either 100% PA or PA to which small quantities of other aromatic nitro-compds were added in order to reduce the mp of PA (120—122°), thus facilitating casting operations (see under Picric Acid)... 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

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