Cationic state

Phenylpteridine and its 4- and 7-monomethyl, 4,7- and 6,7-dimethyl and 4,6,7-trimethyl derivatives (69JCS(C)1408), as well as the coijresponding 4-phenylpteridine series and its 2- and 7-methyl, 2,7- and 6,7-dimethyl and 2,6,7-trimethyl derivatives (69JCS(C)1883), exist as neutral molecules in aqueous solution, essentially as unhydrated species. In acid solution 2- and 4-phenylpteridine and its 4- and 2-methyl derivatives favour the 5,6,7,8-dihydrated cation state, while 7-mono- or 6,7-disubstitution shifts the equilibrium mixture towards the 3,4-monohydrates. 

Both CSs and CSs were also successfully generated by the fragmentation of ionized 4,5-dioxo-2-thioxo-l,3-dithione (65) and 2-thioxo-l,3-dithiole (66) (90JA3750). Tire three sulfur atoms in the anion and cation radicals were chemically equivalent, suggesting that they take the D h (or C2u) form (67 or 68). On the other hand, under similar conditions, 3-thioxo-1,2-dithiole (69) yielded two isomeric cation radicals the (or 2 ) form and the carbon disulfide 5-sulfide form (70). Ab initio calculations on three electronic states of CS3 at the 6-31G -l-ZPVE level indicated that the C21, form (68) was more stable than the carbon disulfide 5-sulfide form (70) in the neutral (both singlet and triplet states) and the anion radical states, but 68 was less stable than 70 in the radical cation state. 

The three highest occupied orbitals of sulfoxides are the lone pairs ns and n0, as well as the 7iso bond210. The 1,3-dithietane 1-oxide adds a lone-pair ionization and destabilizes the n0 and nso radical-cation states compared with thietane oxide. According to a hyperconjugative MO model, the ns+ combination in 1,3-dithietane is destabilized by about leV relative to the basis orbital energy a(ns) due to the combination with the... 

The smaller cluster ions 83", 84" and 85 + have been examined by Zakrzewski and von Niessen at the HF/6-3H-G level [82]. The lowest cationic states are predicted to be 82, and A" for 83 (Cyv), 84 (I>4h) and (Cs), respectively. The ionisation processes may result in significant structural relaxation leading to the sequence of states different from that of the vertical states. The calculated lowest adiabatic ionisation energies, using the GI method with a very large ANO basis set, are 9.53, 8.05, and 8.20 eV for 83, 84 and 85 , respectively. 

Applications of electron propagator methods with a single-determinant reference state seldom have been attempted for biradicals such as ozone, for operator space partitionings and perturbative corrections therein assume the dominance of a lone configuration in the reference state. Assignments of the three lowest cationic states were inferred from asymmetry parameters measured with Ne I, He I and He II radiation sources [43]. 

As already mentioned above, the oxidation of halogenated TTFs to the radical cation state is found to activate the halogen atom for entering into a halo-... 

On the other hand, resonance assignments for CP of threonine and serine, and C and Cy of hydroxy proline, were difficult to make, because of their proximity to carbohydrate carbon resonances. In most cases then, the resonances were assigned on the basis of the effects of pH on the chemical shifts of those resonances. It was shown that the chemical shifts for the carbohydrate carbon resonances were virtually unaffected (AS 0.4 p.p.m.) when going from the cationic state (pH 2) to the anionic state (pH 11) of the amino acid residues. The chemical shifts of C and CP of the amino acid residues, however, shifted considerably (up to 3.1 and 6.6 p.p.m. for C" and CP, respectively see Table VI). 

In the ZEKE-PFI technique, the ionizing laser frequency is scanned through the manifold of cation states (Figure 1). A ZEKE-PFI signal is detected only when the... 

Figure 6. ZEKE-PFI spectra of phenylsilane (Do) taken with (Oi fixed on three S1-S0 bands as indicated and 2 scanned through the low-energy manifold of cation states.

In principle, refined and relatively reliable quantum-theoretical methods are available for the calculation of the energy change associated with the process of equation 2. They take into account the changes in geometry, in electron distribution and in electron correlation which accompany the transition M(1 fio) — M+ (2 P/-), and also vibronic interactions between the radical cation states. Such sophisticated treatments yield not only reliable predictions for the different ionization energies 7 , 77 or 7 , but also rather precise Franck-Condon envelopes for the individual bands in the PE spectrum. However, the computational expenditure of these methods still limits their application to smaller molecules. We shall mention them later in connection with examples where such treatments are required. 

FIGURE 19. Correlation diagram of the jr-ionization energies /( and of cyclohexa-1,4-diene, bridged in positions 3,6 by a polymethylene chain — CCfE ), —, as a function of the dihedral angle radical cation states 2A and 2B2 are those obtained by electron ejection from the -orbitals i and 1)2 > respectively... 

Di(l-azulenyl)(6-azulenyl)methyl cation (24+) represented in Figure 17 exemplifies the cyanine-cyanine hybrid (20). Di(l-azulenyl)methylium unit in 24+ acts as a cyanine terminal group. The tropylium substructure stabilizes the cationic state (24+). Reduction of 24+ should afford the neutral radical 24, which is stabilized by capto-dative substitution effect, because 24 is substituted with azulenes in the donor and acceptor positions. The anionic state (24") is also stabilized by contribution of the cyclopentadienide substructure, which should exhibit the third color change in this system. 

From the constructions of Figs. 3.2 and 3.3, it is clear that a large electrolyte stability window Eg requires not only a large energy difference m — i, but also the absence of any cationic states above the top of the bonding anion-p band. It follows that most practical electrolytes are generally confined to fluorides, oxides and chlorides of the main-group... 

The ratio R defined by Equation (27) lies between zero and unity. We classify localized states as anionic or cationic according to whether R is greater or smaller than An electron in an anionic state is concentrated more on the foreign atom than on the crystal for a cationic state the reverse is true. The occurrence of anionic and cationic localized states is shown in Fig. 7. This is a superposition of Figs. 2 and 6 with the extra information on the ionic character of the states. Alower energy if (P states are bonding (/3 < 0). 

These structures may be viewed as distorted from the Bj-type geometries via a second-order JT-type mechanism or, alternatively, as Aj-type with the substituents at the wrong carbon atom. The calculations suggest that the radical cation state preference can be fine-tuned by appropriate substituents and predict substantial differences in spin-density distributions. These predictions should be verifiable by an appropriate spectroscopic technique (ESR or CIDNP) and might be probed via the chemical reactivity of the radical cations (vide infra). 

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