Trans radical cations

The 9,10-dicyanoanthracene sensitized irradiation of c/i-stilbene results in nearly quantitative isomerization (>98%) to the trans isomer with quantum yields greater than unity. Therefore, the isomerization was formulated as a free radical cation chain mechanism with two key features (1) rearrangement of the c/i-stilbene radical cation and (2) electron transfer from the unreacted cis-olefin to the rearranged (trans-) radical cation. 

Irradiation of particles of semiconductors like Ti02 or CdS suspended in a solution of stilbene led to isomerization of its cis- to trans-isomer (29-30). The reaction seems to be initiated by electron transfer from cis-stilbene to the positive hole generated in the semiconductor by irradiation. The resulting cis radical cation (c ) is several kcal mol" higher in energy than a trans radical cation (t ) over the ground state trans isomer (31). Accordingly, the isomerization from c " to t " is exothermic. 

The chain reaction can proceed by two mechanisms. The first is electron transfer between the trans-radical cation produced by the unimolecular cis" - - trans isomerization and the neutral cis isomer to give trans isomer and cis - (Scheme 14) [139]. The second is addition - elimination between radical cation and neutral cis isomer to induce cis -+ trans isomerization (Scheme 15) [140,145,155]. 

Under optimum conditions electron transfer can produce excited states efficiently. Triplet fluoranthrene was reported to be formed in nearly quantitative yield from reaction of fluoranthrene radical anion with the 10-phenylphenothia2ine radical cation (171), and an 80% triplet yield was indicated for electrochemiluminescence of fluoranthrene by measuring triplet sensiti2ed isomeri2ation of trans- to i j -stilbene (172). 

For a reaction initiated by iminium salts, see Lopez, L. Mele, G. Mazzeo, C. J. Chem. Soc., Perkin Trans. 1,1994,779. For reactions initiated by radical cations, see de Sanabia,... 

It may be noted that competitive deprotonation of 29 at C-l gives rise to 2-deoxyribonolactone (27) with the concomitant release of free 5-methylcy-tosine as minor processes. Interestingly, competitive hydration of 5-MedCyd radical cations (29) occurs exclusively at C-6 as inferred from labeling experiments with 1802 (36) [61]. Thus, mass spectrometry analysis of the four cis and trans diastereomers of 5-MedCyd glycols 36 showed that incorporation of 1802 takes place exclusively at C-5 of 6-hydroxy-5,6-dihydro-2 -deoxycy-tyd-5-yl radicals (34). 

The 327-670 GHz EPR spectra of canthaxanthin radical cation were resolved into two principal components of the g-tensor (Konovalova et al. 1999). Spectral simulations indicated this to be the result of g-anisotropy where gn=2.0032 and gi=2.0023. This type of g-tensor is consistent with the theory for polyacene rc-radical cations (Stone 1964), which states that the difference gxx gyy decreases with increasing chain length. When gxx-gyy approaches zero, the g-tensor becomes cylindrically symmetrical with gxx=gyy=g and gzz=gn. The cylindrical symmetry for the all-trans carotenoids is not surprising because these molecules are long straight chain polyenes. This also demonstrates that the symmetrical unresolved EPR line at 9 GHz is due to a carotenoid Jt-radical cation with electron density distributed throughout the whole chain of double bonds as predicted by RHF-INDO/SP molecular orbital calculations. The lack of temperature... 

Piekara-Sady, L., A. S. Jeevarajan et al. (1995). ENDOR study of the (7,7 -dicyano)- and (7 -phenyl)-7 -apo-carotene radical cations formed by UV photolysis of carotenoids adsorbed on silica gel. J. Chem. Soc. Faraday Trans. 91 2881-2884. 

Chemically inert triplet quenchers e.g. trans-stilbene, anthracene, or pyrene, suppress the characteristic chemiluminescence of radical-ion recombination. When these quenchers are capable of fluorescence, as are anthracene and pyrene, the energy of the radical-ion recombination reaction is used for the excitation of the quencher fluorescence 15°). Trans-stilbene is a chemically inert 162> triplet quencher which is especially efficient where the energy of the first excited triplet state of a primary product is about 0.2 eV above that of trans-stilbene 163>. This condition is realized, for example, in the energy-deficient chemiluminescent system 10-methyl-phenothiazian radical cation and fluoranthene radical anion 164>. 

The asymmetric unit contains one [Cun(hfac)2(TTF—CH=CH—py)2]+ radical cation and one PFfi anion both in special positions, as well as one dichloromethane solvent molecule in the general position. The Cu11 ion lies in a distorted octahedral coordination, two oxygen from two hfac ions and two nitrogen from TTF CH=CH py are trans to each other and form the square plane (Cu-N bond length of 2.016(5) A) the two reminiscent oxygen atoms from the two hfac ions occupy the apical positions. 

Emission spectra have been recorded for electron injection into Au and Ag spherical electrodes and hole injection into Au(lll) planar electrodes. These processes were brought about in solutions of acetonitrile containing tetrabutylammonium hexafluoro-phosphate (TBAHP), using the trans-stilbene radical anion as the electron injector and the thianthrene radical cation as hole injector. The spectrum for the hole injection process into planar Au(lll) electrodes has been resolved into the P S-polarised components of the emitted light. A comparison of the spectral distribution of emitted light for the above electron injection process, occurring at both Au and Ag... 

Zappey, H.W. Ingemann, S. Nibbering, N.M.M. Isomerization and Fragmentation of Aliphatic Thioether Radical Cations in the Gas Phase Ion-Neutral Complexes in the Reactions of Metastable Ethyl Propyl Thioether Ions. J. Chem. Soc., Perkin Trans. 2 1991, 1887-1892. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

Scheme 6.4. Energy level diagrams for reaclions of aromatic radical-cations with their sub.stratc S (a) Substrates having electron donating nielhoxy substituents, (b) No electron donating substituents present. The wavy lines indicate a single electron tran.sfcrprocess.

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 . 

Class (1) reactions were observed in all four cycloalkanes. The highest rate constants were observed for reactions of cyclohexane hole with low-IP aromatic solutes, (3-4.5) x 10" sec at 25°C [75]. In these irreversible reactions, a solute radical cation is generated. Class (2) reactions were observed for reactants 1,1-dimethylcyclo-pentane, trans-l, 2-dimethylcyclopentane, and 2,3-dimethyl-pentane in cyclohexane [74], trans-dtcaXm, bicyclohexyl, and Ao-propylcyclohexane in methylcyclohexane [69], and benzene in cis-... 

Class (3) reactions include proton-transfer reactions of solvent holes in cyclohexane and methylcyclohexane [71,74,75]. The corresponding rate constants are 10-30% of the fastest class (1) reactions. Class (4) reactions include proton-transfer reactions in trans-decalin and cis-trans decalin mixtures [77]. Proton transfer from the decalin hole to aliphatic alcohol results in the formation of a C-centered decalyl radical. The proton affinity of this radical is comparable to that of a single alcohol molecule. However, it is less than the proton affinity of an alcohol dimer. Consequently, a complex of the radical cation and alcohol monomer is relatively stable toward proton transfer when such a complex encounters a second alcohol molecule, the radical cation rapidly deprotonates. Metastable complexes with natural lifetimes between 24 nsec (2-propanol) and 90 nsec (tert-butanol) were observed in liquid cis- and tra 5-decalins at 25°C [77]. The rate of the complexation is one-half of that for class (1) reactions the overall decay rate is limited by slow proton transfer in the 1 1 complex. The rate constant of unimolecular decay is (5-10) x 10 sec for primary alcohols, bimolecular decay via proton transfer to the alcohol dimer prevails. Only for secondary and ternary alcohols is the equilibrium reached sufficiently slowly that it can be observed at 25 °C on a time scale of > 10 nsec. There is a striking similarity between the formation of alcohol complexes with the solvent holes (in decalins) and solvent anions (in sc CO2). 

Cyclobutanes disubstituted in the 1,2-positions should favor strucmre-type C or a related distonic structure with one broken C—C bond. Calculations [QCISD-(T)/ 6-31G //UMP2/6-31G ] suggest a trapezoidal structure for frawi-1,2-dimethyl-cyclobutane radical cation.This expectation is born out by experimental results such as the ET induced cis-trans-isomerization of 1,2-diaryloxycyclobutane (Ar = aryl), leading to IS " ", and likely involving the distonic radical cation (14 +) formed via a type C ion. ... 

In some cases, a stepwise mechanism is indicated by randomization of the dienophile stereochemistry. For example, addition of cw-anethole radical cation (100 +) to cyclopentadiene produces comparable yields of four possible diastereoi-someric adducts (102) clearly supporting a distonic radical cation intermediate (lOl ). Only products supporting the stepwise mechanim, that is, trans,endo-and trans,exo-lQ2, are shown. " ... 

Although site effects are not as prevalent in UV-vis absorption as they are in IR spectra, they do exist and manifest themselves sometimes very clearly in band systems that comprise sharp peaks. An example is the radical cation of all-trans-octatetraene whose first absorption band consists of multiple peaks that can be selectively bleached by highly monochromatic light. The site stmcture can become more evident in laser-induced fluorescence, where excitation of individual sites is possible down to the level of single molecules in favorable cases, but a discussion of this fascinating phenomenon is beyond the scope of this chapter. 

While geometrical isomers usually give very similar MS, this is not always the case. An interesting example is that of cis- and trans- 2,4-dimethyloxetane (71ACS763). The principal fragment ion from the trans isomer (16) was the propene radical cation (m/e 42), while that from the cis isomer (17) was protonated acetaldehyde (m/e 45), apparently a hydrogen-rearrangement product. 

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