Aromatic molecules, cation

Noncovalent interactions in metal complexes of biomolecules may play an important role in the creation of supramolecular structures around the metal center. For instance, extensive three-dimensional hydrogen-bonded stmcmres grow around metal complexes of barbiturates, recognized as the most widely used drugs for the treatment of epilepsy.Electrostatic interactions between a cation and the Trring of an aromatic molecule (cation-tt interactions) are common motifs in protein structures. Little is known about alkali and alkali-earth cation-tt inter-... 

Radiation techniques have been used extensively with non-aqueous systems as well. Much work has been done on aromatic molecule cations and anions and on electron transfer processes involving these species (see the review by Dorfman, 1970). These and other studies on radical ions, on excited states, and on charge-transfer complexes have been reviewed by Fendler and Fendler (1970). 

Organic aromatic molecules are usually sweet, bitter, a combination of these, or tasteless, probably owing to lack of water solubiUty. Most characteristic taste substances, especially salty and sweet, are nonvolatile compounds. Many different types of molecules produce the bitter taste, eg, divalent cations, alkaloids, some amino acids, and denatoirium (14,15). 

Figure 11 shows schematically how such sites may be formed on Mo/alumlna or CoMo/alumlna catalysts. The pair site contains a reduced metal next to an acidic metal cation, either Co or Mo. Prestimably this dual site can remove S, leaving an olefin or aromatic molecule attached to the cation. This Is then hydrogenated by hydrogen from the reduced metal component of the site. 

Petrenko, A., K. Redding et al. (2005). The influence of the structure of the radical cation dimer pair of aromatic molecules on the principal values of a g-tensor DFT predictions. Chem. Phys. Lett. 406 327-331. 

The weak interactions that may exist between group 2 cations and anionic hydrocabyl ligands are demonstrated in the metal-in-a-box compounds such as [M(THF)6][Me3Si(fluorenyl)]2 (M = Ca 159 or Mg), which are formed by the addition of THF to solutions of the bis(fluorenyl) complexes in non-polar solvents. The box may be completed by the presence of aromatic molecules, as in 159 (Figure 84). The disruption of the metal-carbon bonds is thought to stem from a combination of robust M-THF interaction, the stability of the free [Me3Si(fluorcnyl)] ion, and the formation of numerous C-H- -7r interactions between THF and the anions. These and related examples are reviewed elsewhere. 

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

Rate constants of bimolecular, micelle-assisted, reactions typically go through maxima with increasing concentration of inert surfactant (Section 3). But a second rate maximum is observed in very dilute cationic surfactant for aromatic nucleophilic substitution on hydrophobic substrates. This maximum seems to be related to interactions between planar aromatic molecules and monomeric surfactant or submicellar aggregates. These second maxima are not observed with nonplanar substrates, even such hydrophobic compounds as p-nitrophenyl diphenyl phosphate (Bacaloglu, R. 1986, unpublished results). 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

In some cases, the ground state is not a singlet state, e.g. dioxygen, anion and cation radicals of aromatic molecules. 

The strong affinity of ionic liquids for aromatics has been attributed to the formation of liquid clathrates (90-92). Liquid clathrates (93) are semi-ordered liquids containing complex salt hosts. They are formed by associative interactions between aromatic molecules and salt ions, which separate cation-anion-packing interactions to a sufficient degree that localized cage structures are formed. Although the aromatic compounds are highly soluble in the ionic liquid phase, the... 

For ESR studies, cation radicals of aromatic molecules have most generally been formed by dissolution of the parent compound in concentrated sulfuric acid.19 Neither this nor any of the several new chemical methods of generating these species in solution26-35 provides a particularly suitable medium for subsequent energetic chemical reduction. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

An alternative approach to the experimental estimation of REs utilizes equilibrium (protonation) data rather than thermochemical data, the idea being that comparisons of the basicities of pyrrole and its benzo fused analogues with those of non-aromatic systems which form cations of 7r-electron structure similar to the aromatic compounds should furnish a measure of the loss of RE accompanying protonation of the aromatic system (76T1767, 72CI(L)335). Thus, for the a-protonation of N-methylpyrrole, the model non-aromatic system was chosen as (20). Combination of pKa values for the protonation of the aromatic and non-aromatic molecules, taking into account the intrinsic resonance stabilization of the... 

The compounds M(NH3)2Ni(CN)4 (M = Zn or Cd), which consist of two-dimensional polymeric sheets of tetracyanonickelate ions bridged by coordinating diamminemetal(II) cations, function as host lattices for clathration of small aromatic molecules such as thiophene, furan, pyrrole or pyridine IR studies indicate the presence of hydrogen bonding between the host lattice ammonia and the aromatic guest molecules.132,133 A crystal structure determination of the related clathrate Cd(en)Ni(CN)4(pyrrole)2 has been reported.134 Similarly, the complex Cd(py)2Ni(CN)4 consists of polymeric [Cd—Ni(CN)4] layers held together by Cd-bound pyridine.135... 

In practice, extrapolations of p fR in water have usually used the older acidity function based method, for example, for trityl,61,62 benzhydryl,63 or cyclopropenyl (6) cations.66,67 These older data include studies of protonation of aromatic molecules, such as pKSi = —1.70 for the azulenium ion 3,59 and Kresge s extensive measurements of the protonation of hydroxy- and methoxy-substituted benzenes.68 Some of these data have been replotted as p fR or pKa against XQ with only minor changes in values.25,52 However, for more unstable carbocations such as 2,4,6-trimethylbenzyl, there is a long extrapolation from concentrated acid solutions to water and the discrepancy is greater use of an acidity function in this case gives pA 2° = —17.5,61 compared with —16.3 (and m = 1.8) based on X0. Indeed because of limitations to the acidity of concentrated solutions of perchloric or sulfuric acid pICs of more weakly nucleophilic carbocations are not accessible from equilibrium measurements in these media. 

Evaluation of pATR from measurements of rate and equilibrium constants for the protonation of carbon-carbon double bonds of alkenes suggests the possibility of a similar approach for aromatic double bonds. Protonated aromatic molecules are the parent structures of the arenonium ion intermediates of electrophilic aromatic substitution. For these cations the equilibrium constant Kk refers to equilibria with the corresponding aromatic hydrates, as is illustrated in Scheme 5 for the benzenonium ion (cyclohexadienyl cation) 9 for which the hydrate is cyclohexadienol 10. 

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