Color charge

The dipole moment varies according to the solvent it is ca 5.14 x 10 ° Cm (ca 1.55 D) when pure and ca 6.0 x 10 ° Cm (ca 1.8 D) in a nonpolar solvent, such as benzene or cyclohexane (14,15). In solvents to which it can hydrogen bond, the dipole moment may be much higher. The dipole is directed toward the ring from a positive nitrogen atom, whereas the saturated nonaromatic analogue pyrroHdine [123-75-1] has a dipole moment of 5.24 X 10 ° C-m (1.57 D) and is oppositely directed. Pyrrole and its alkyl derivatives are TT-electron rich and form colored charge-transfer complexes with acceptor molecules, eg, iodine and tetracyanoethylene (16). 

Equilibrium constants for complex formation (A") have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

Benzpinacols (or their trimethylsilyl ethers) are effective electron donors and readily form vividly colored charge-transfer complexes with common electron acceptors such as chloranil (CA), dichlorodicyanobenzoquinone (DDQ), tetracyanobenzene (TCNB), methyl viologen (MV2+), and nitrosonium (NO+) cation.191-194 For example, the exposure of a silylated benzpinacol to chloranil... 

Bicumene readily forms a highly colored charge-transfer complex with nitro-sonium (NO+) cation that can be isolated as a crystalline salt.194 Thus, a solution of bicumene and NO+SbClcharge-transfer bands with A = 338 and 420 nm that are indistinguishable from those of the nitrosonium complexes with other methylbenzenes (equation 62). 

A variety of other highly-strained electron-rich donors also form colored complexes (similar to homobenzvalene) with various electron acceptors, which readily undergo thermal cycloadditions (with concomitant bleaching of the color).209 For example, Tsuji et al.210 reported that dispiro[2.2.2.2]deca-4,9-diene (DDD), with an unusually low ionization potential of 7.5 eV,211 readily forms a colored charge-transfer complex with tetracyanoquinodimethane (TCNQ). The [DDD, TCNQ] charge-transfer complex undergoes a thermal cycloaddition to [3,3]paracyclophane in excellent yield, i.e.,... 

A. Nitropyridinium cations. The spontaneous formation of vividly colored charge-transfer (CT) complexes occurs upon exposure of jV-nitropyridinium (PyNO ) cation to various aromatic donors,235 i.e.,... 

B. Tetranitromethane. Tetranitromethane forms colored charge-transfer (CT) complexes with a variety of organic donors such as substituted benzenes, naphthalenes, anthracenes, enol silyl ethers, olefins, etc. For example, an orange solution is instantaneously obtained upon exposure of a colorless solution of methoxytoluene (MT) to tetranitromethane (TNM),237 i.e.,... 

Donor-acceptor association. It is experimentally well established that nitrosonium cation forms vividly colored charge-transfer complexes with a wide variety of aromatic donors243 (equation 85). 

If a macroscopic chunk of quark matter is created in heavy ion collisions or exists inside the compact stars, it must be in color singlet. So in the following discussions, color charge neutrality condition is always satisfied. 

The formation of unusually light colored charge-transfer complexes of 8 with both picric acid and trinitrobenzene may be attributed to the decrease of the interaction between the donor and acceptor molecules due to the curvature of the former. ... 

Extensive help is displayed automatically for every menu operation. For example, if you COLOR Charge, you are told how and where to calculate the charge or pi of your protein on-line. If you SELECT Nucleic, there is a button to help you distinguish DNA from RNA. 

For nucleon-nucleon strong interactions within nuclei, pions (= two-quark particles see below) may be the mediating particles Gluons are probably not involved directly, since the nucleons have no "color charge." The inter-nucleon potential goes to zero beyond 1.7fm = 1.7 x 10-15m. 

The NS2+ cation is an important reagent in S-N chemistry, especially in thermally allowed cycloaddition reactions with organic nitriles and alkynes that give quantitative yields of heterocyclic cations (see Scheme 3). The dominant orbital interaction in these cycloadditions is between the LUMO of S2N+ and the HOMO of the alkyne or nitrile (see Figure 2). Cycloaddition reactions also occur with alkenes, and colored charge-transfer complexes are formed with arenes. 

Tetraalkylborates are mild and selective alkylation reagents [186, 187], and they are commonly considered as sources of nucleophilic alkyl groups (R ) just as borohy-drides are depicted as hydride (H ) sources. However, since organoborates represent excellent electron donors (see Table 5, Section 2.2.1), the question arises as to what role electron donor-acceptor interactions play in the nucleophilic alkyl transfer. Phenyl- and alkyl-substituted borate ions form highly colored charge-transfer salts with a variety of cationic pyridinium acceptors [65], which represent ideal substrates to probe the methyl-transfer mechanisms. Most pyridinium borate salts are quite stable in crystalline form (see for example Figure 5C), but decompose rapidly when dissolved in tetrahydrofuran to yield methylated hydropyridines (Eq. 65). 

According to Nishida ° the [27t-l-27r] cycloadditions proceed via dipolar intermediates, whereas the [2n + 2(7] cycloadditions start with a single-electron-transfer process. In [27i-l-27t] additions a deeply colored charge-transfer complex is initially formed and the reaction is favored by polar solvents (as is usually the case with [2jt-l-2jt] additions of TCNE to electron-rich alkenes). Vinylcyclopropanes with a very low ionization potential afford the [2 t-f 2(t] product they do not form a charge-transfer complex with TCNE and the reactions have to be performed under oxygen-free conditions (Table 5). 

Perimidine forms deeply colored charge transfer complexes with 7r-acids, including weak acids such as 4-nitrobenzaldehyde. The complexes usually have 1 1 composition in the crystalline state. In terms of the 7r-donor capacity, perimidines are superior to well-known 7r-donors such as pheno-thiazine. 2,3-Dihydroperimidine exhibits the same 7r-donor capacity as perimidines. Aceperi-midylene, perimidinones, thioperimidinones, and 2-imino-2,3-dihydroperimidines are inferior to perimidines as 7t-donors <81RCR816>. 

TCNQ complex formation with electron-rich partners has been used for quantitative spectrophotometric analysis. Thus various pharmaceuticals like benzothiadiazines, benzenesulfonamides, Terfenadine, penicillins, antihistamines, some MAO inhibitors and procainamide hydrochloride have been determined via their colored charge-transfer complexes. In contrast to these papers a critical evaluation has been published, which pointed out problems occurring in the use of TCNQ for quantitative spectrophotometric analysis. This led the authors to the conclusion that the use of TCNQ should be restricted to qualitative applications. 

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