Cations dications

Spectral data recorded in this table have been selected from the literature in the preferred order of the pure species (0, neutral, +, cation ++, dication anion —, dianion) in aqueous solution at specific pH (or // ) values, and solutions in cyclohexane (CjH, j), alcohols, and other solvents. Published (nm) and log e values are quoted, except that some values have been estimated from the spectral curves. The letters cv in the table indicate the existence of a published spectral curve. Values in italics refer to shoulders and inflections. 

The further oxidation of [Fe( -C5Me5)2]+ by electrolysis in an acidic AlCl3-M-butylpyridinium chloride melt gives [Fe(f/-C5Me5)2]2+, which is stable under argon the ° value for the cation-dication couple is 1.6 V. The ferrocenium ion is also oxidizable in this medium, albeit irreversibly... 

The porphyrin Tr-system can donate a number of electrons to form the radical cation, dication, etc. Pulse radiolysis has been used to oxidize porphyrins to the TT-radical cations both in aqueous solutions and in organic solvents. In aqueous solutions, the - OH radical is not a good oxidant for porphyrins because it adds to the double bonds. However, oxidizing radicals such as Cl2 , Br2- N3-, CCI3O2 -, and CH2CHO are suitable for this purpose. For example, oxidation by Br2 " takes place with rate constants in the range of 10 L mol ... 

Chemical and electro-oxidation of this spiro molecule were studied using electrochemical methods and UV-visible absorption spectroscopy [11,36]. The hep-tamer segments were oxidized sequentially by using FeCls to produce the mono radical cation, the bis-(radical cation), the radical cation/dication and the bis-(dication) respectively (Scheme 14.13). 

Reactions with Parting of Radicals. The one-electron oxidation of cationic dyes yields a corresponding radical dication. The stabihty of the radicals depends on the molecular stmcture and concentration of the radical particles. They are susceptible to radical—radical dimerization at unsubstituted, even-membered methine carbon atoms (77) (Fig. 6). 

Substituents on the methine chain can stabilize the dye radical cation if the substituent (like methyl) is located on the high electron density carbons. However, no significant stabilization occurs when alkyl groups are on the alternate positions (like 9, 11 for the dication in Fig. 9). Current results for several dyes including die arbo cyanines and carbocyanines indicate that electronic stabilization of the dication radical lengthens the radical lifetime and also enhances the reversibiUty of the dimerization process (37). 

An allowed alternative to the -ylium suffixes is citing the term cation (or dication , etc.) after the name of the corresponding radical. 

The calculated barrier to dissociation of the [S3N2] dication into [SN]" and [S2N] in the gas phase is 10.9 kcal mof . However, lattice-stabilization effects allow the isolation of [MEg] salts (M = As, Sb) of this six r-electron system in the solid state from the cycloaddition of [SN] and [S2N] cations in SO2 (Eq. 5.11)."° The S-S and S-N bond distances in the planar, monomeric dication are shorter than those in the... 

Figure 15.39 Structures of (a) the planar radical cation S3N2, (b) its dimer SeN4 + and (c) the corresponding planar diamagnetic dication S3N2 +.

The hydrated cation of quinazoline in dilute acid solution becomes dehydrated when the acidity of the solution is progressively increased. At Ho —4.3, the solution consists predominantly of the anhydrous cation with some anhydrous dication ( 7%). The ultraviolet spectrum of the anhydrous cation is similar to that of the neutral molecule (there is a small bathochromic shift) and it is also similar to that of quinazoline in anhydrous dichloroacetic acid. When the acid strength is further increased to Ho —9.4, the quinazoline dication is formed (pKa —5.5). 

When equimolar quantities of 80a and its dication 110 are combined in acetonitrile, single electron transfer occurs and the coproportionation product was obtained (95TL2741).Tliis deeply red-colored, air-sensitive radical cation 111 showed a strong ESR signal (g = 2.0034). On the other hand, the excellent electron donor 80a could be prepared by electrolytic reduction starting from 110. It was necessary to carry out the reduction with scrupulous exclusion of oxygen. Tlius, the electrolysis of 110 at -1.10 V initially gave rise to an intense red color, which was presumably due to the formation of 111. Upon further reduction, the red color faded and the tetraaza-fulvalene 80a was isolated at a 62% yield (Scheme 45). 

Figure 2. Radical cations (polarons) and dications (bipolarons) obtained by oxidation of the neutral chain. The rotated angles and counter-ions needed to retain electroneutrality are not shown.

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