CT-interactions

Clark [15] reported relaxation of the strain in the three-membered rings of fluorosnbstitnted phosphireninm 17 and silirene 19 by the Jt-ct interaction in comparison with the componnds 18 and 7. 

The authors [33] have elucidated the linear dependence of Ao0 (z-dep) on E for the polyanions by a quantum chemical consideration. A model Hamiltonian approach to the charge transfer (CT) interaction between a polyanion and solvents has been made on the basis of the Mulliken s CT complex theory [34]. 

In the previous paper [33], the effect of the ionic charge z on AW was investigated for the CT interaction between a polyanion and water. The effect will come out through and IToi-... 

When the ion is much larger than the solvent, it can be assumed that the number (N) of solvent molecules adjacent to the ion is proportional to the surface area of the ion N = An p (where p is the number of solvent molecules per unit surface area of the ion). Accordingly, the contribution of the CT interaction to the ion solvation energy AG is given by... 

Accordingly, the contribution of the CT interaction to the ion-transfer potential [being given by Eq. (7)] can be expressed as... 

In this manner one can see that the terminal monomers of the chain acquire net ionic character (because they can participate in CT only in a single direction), whereas interior monomers remain relatively electro neutral (due to there being equal numbers of CT interactions in and out ). 

Owing to the 7tCc-ocrc CT interactions, the usual equivalence between the two Kekule structures of benzene is broken and the ring distorts strongly to D3h symmetry, with pronounced alternation (by 0.02 A) of C=C bond lengths. Complexes of this type are evidently closely related to the metallocene sandwich compounds discussed elsewhere in this book (Section 4.9.5), with the benzene molecule described as a tridentate ligand in the language of metal coordination chemistry. 

Figure 4. Schematic energy level diagram for a catenane based on charge-transfer (CT) interactions and for its separated components. The wavy lines indicate nonradiative decay paths of the electronic excited states.

In macrocycle 3,1201 the two electron-acceptor units, a bipyridinium and a bis(pyridinium)ethy-lene, are different and therefore are reduced at different potentials, as expected on the basis of their electron-acceptor ability (Figure 5). When this macrocycle is interlocked with 1/5DN38C10 in the cate nane 4, the bipyridinium unit occupies the inside position and therefore it becomes more difficult to reduce compared with the bis(pyridinium)ethylene one since it experiences a stronger CT interaction. As a consequence, the first reduction of the bis(pyri-dinium)ethylene unit becomes the first reduction process of the whole system and therefore is displaced toward less negative potentials with respect to same process in the free macrocycle 3, in which such a process follows the first reduction of the other unit. 

In this kind of pseudorotaxanes, rotaxanes, and catenanes, the stability of a specific (supra)molecu-lar structure is a result, at least in part, of the CT interaction. In order to cause mechanical move ments, such a CT interaction has to be destroyed. 

This requirement can be fulfilled by reduction of the electron-acceptor unit(s) or by oxidation of the electron-donor unit(s) by chemical, electrochemical, or photochemical redox processes. In most cases, the CT interaction can be restored by an opposite redox process, which thus promotes a reverse mechanical movement leading to the original structure. 

The mn ionizations of tertiary aliphatic amines (Table 5) are mostly lower than 9 eV and thus small enough for an electron donor in charge-transfer (CT) interactions. Mutai and collaborators64 have studied intramolecular CT interaction in a series of 1 -( >-dimethylaminoalkyl)-4-nitrobenzenes. 

However, the first ionization bands in the PE spectra of these compounds (vertical IPs are n = 1 8.67 eV, n = 2 8.61 eV, n = 3 8.50 eV) show no apparent anomaly which might be ascribed to intramolecular n /it type CT interaction. This fact suggests that the CT interaction found by UV spectroscopy is weak and that the molecules are present in their open-chain forms under the experimental conditions employed for recording the PE spectra. 

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