Radical cations configurations

In complex organic molecules calculations of the geometry of excited states and hence predictions of chemiluminescent reactions are very difficult however, as is well known, in polycyclic aromatic hydrocarbons there are relatively small differences in the configurations of the ground state and the excited state. Moreover, the chemiluminescence produced by the reaction of aromatic hydrocarbon radical anions and radical cations is due to simple one-electron transfer reactions, especially in cases where both radical ions are derived from the same aromatic hydrocarbon, as in the reaction between 9.10-diphenyl anthracene radical cation and anion. More complex are radical ion chemiluminescence reactions involving radical ions of different parent compounds, such as the couple naphthalene radical anion/Wurster s blue (see Section VIII. B.). 

The next approximation is to assume that exactly the same CMOs j of the radical cation M+, by simply removing one of the two electrons occupying the CMO closed-shell molecule M. This yields... 

FIGURE 3. Graphical representation of the electron configuration o of a closed-shell molecule M and of the configurations 20 of its radical cation M+ as approximations to the states and 2j (J = 1,2,3). The arrows in the representations of 22 and 23 indicate that these configurations correspond to electronic excitations of M+, relative to its ground-state configuration 2 i... 

Finally, before turning to a brief review of methodological and theoretical aspects, we mention that one of the distinguishing features of planar conjugated polyene radical cations (cf Section II.D) is that their EA spectra reveal a breakdown of the single-configuration picture for ionic excited states which had been used so successfully in interpreting their PE spectra (cf. Section II). 

FIGURE 27. Different types of configurations of radical cations (the numbering does not imply an energetic ordering). A-type are Koopmans, B- and C-type are non-Koopmans configurations... 

Although in many cases, particularly in PE spectroscopy, single configurations or Slater determinants 2d> (M+ ) were shown to yield heuristically useful descriptions of the corresponding spectroscopic states 2 f i(M+ ), this is not generally true because the independent particle approximation (which implies that a many-electron wavefunction can be approximated by a single product of one-electron wavefunctions, i.e. MOs 4>, as represented by a Slater determinant 2 j) may break down in some cases. As this becomes particularly evident in polyene radical cations, it seems appropriate to briefly elaborate on methods which allow one to overcome the limitations of single-determinant models. 

In a recent comprehensive study at the CASSCF level of ab initio theory, Cave and lohnson have carried out calculations for all six rotamers of the hexatriene radical cation. In agreement with experiment they found that the first excited state is hardly affected by the additional interactions which prevail in partially cA-configurated rotamers, whereas the energy of the second excited states decreases as the number of those cA-interactions increases. On this basis, they were able to confirm some of the original assignments of the observed spectra305 but proposed revisions for some of the others. 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

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... 

Figure 4. One-dimensional MNDO isomerization pathways for tetra-methyl tetrahedrane and its radical cation. For the definition of the coordinate a cf. ( ) Cl denotes calculations including limited configuration interaction (see text).

On the other hand, the CIDNP approach and recent ab initio calculations have provided what we consider unambiguous evidence for the nature of the vinylcyclo-propane radical cation as well as for some simple derivatives. We have carried out ab initio calculations on the prototype 19 and several simple derivatives, and probed by CIDNP the structures of three simple vinylcyclopropane radical cations in which the two functionalities are locked in a syn configuration. 

The nature of vinylcyclopropane radical cations was elucidated via the electron transfer induced photochemistry of a simple vinylcyclopropane system, in which the two functionalities are locked in the anri-configuration, viz., 4-methylene-l-isopropylbicyclo[3.1.0]hexane (sabinene, 39). Substrates, 39 and 47 are related, except for the orientation of the olefinic group relative to the cyclopropane function trans for 39 versus cis for 47. The product distribution and stereochemistry obtained from 39 elucidate various facets of the mechanism and reveal details of the reactivity and structure of the vinylcyclopropane radical cation 19 . 

Electron transfer-induced nucleophilic addition to several otho cyclopropane compounds was also studied. The nucleophilic addition of methanol to quadricy-clane radical cation 8 produces the two methanol adducts 53 and 54. The stereochemistry of the methoxy groups in these structures identifies the preferred direction of nucleophilic attack upon the intermediate radical cations 8. Detailed NOE experiments delineate the structure of 53 and establish conclusively that the norbomene derivative 54 contains a 7-fl ri-methoxy group. The stereochemistry of both is compatible with stereospecific nucleophilic attack exclusively firom the exo-position. 7-Methylenequadricyclane also is attacked exclusively from the exo-face.These results can be explained via backside attack with inversion of configuration. 

The UV-visible (vis) spectra of many organic radical ions show significant bath-ochromic shifts relative to their precursors. The open-shell configurations of singly occupied bonding (radical cations) or antibonding orbitals (radical anions) introduce new electronic transitions of lower energies, in the visible or near-IR (cf. Fig. 6.5). ... 

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