1,3,5-hexatriene radical cation

Similar results are also obtained for the next higher homologue of the reaction, the ring closing reaction of the 1,3,5-hexatriene radical cation 6 + to give the 1,3-cyclohexadiene radical cation 7 +. Even though 6 + is spectroscopically well... 

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. 

The third mechanism of isomerization, photoinduced rearrangements of radical cations, has been pursued in a variety of systems. Matrix isolated radical cations have been noted to undergo some rigorous reorganizations as well as subtle ones. For example, the ring opening of cyclohexadiene to hexatriene radical cation and the interconversion of its different rotamers have been achieved by irradiation with UV or visible light [173-174]. 

However, the ring closure of substituted hexatriene radical cations has been reported (a) Barkow, A. and Griitzmacher, H.-F. (1994). Inti, J. Mass Spectrom Ion Proc. 142,195 It should also be noted that the ring closure of a hexatriene radical anion has also been reported (b) Fox, M.A. and Hurst, J.R. (1984). J. Am. Chem. Soc. 106, 7626... 

In their first study on triene radical cations, Shida and coworkers found that irradiation of ionized hexatriene in frozen glasses yields a multitude of species with similar EA spectra, which could be selectively interconverted by narrow-band photolysis. They assigned the observed band systems to four different rotamers of the hexatriene radical cation. Allan and Maier found that (3 )- and (3Z)-hexatriene gave distinct gas-phase emission spectra in agreement with the earlier PE findings . However, the two isomers could not be distinguished by photodissociation spectroscopy. ... 

In subsequent argon matrix isolation studies, similar bands were found when hexatriene or cyclohexadiene are ionized , and eventually, five of the six possible rotamers of hexatriene radical cation were identified by selective, wavelength-specific in ter conversions . Similar results were later obtained for octatetraene , where six of the twenty possible rotamers are formed on ionization in argon (Figure 31) which could be interconverted and identified by selective photolysis . Interestingly, in the case of the butadiene radical cation the s-cis rotamer could not be detected, even if the diene radical cation was formed from the cyclobutene radical cation . In contrast, in a recent resonance Raman study, some weak bands were detected and assigned to the s-cA-butadiene radical cation which might have escaped detection in the earlier ESR and EA experiments . ... 

The reaction can be analyzed by assuming that a homolytic cleavage in a [1,2] shift results in the production of a vinyl radical cation and in a [1,6] shift results in the production of a hexatriene radical cation. In case of a [1,2] shift, the HOMO of the rr-framework of the transition state is and in the [1,6]-shift it is W3. In either case the HOMO has mirror symmetry, and, therefore, under thermal conditions, the [1,2] or [1,6] sigmatropic shift occurs via a suprafacial pathway without inversion at the migrating center (Figure 3.18). 

The 7C-framework during the transition state of [1, 2]-shift is vinyl radical cation (C==C—) and dining [1, 6]-shift is hexatriene radical cation, (C=C—C=C—C=C). During [1, 2]-shift HOMO of Ji-framework of transition state is n and during [1, 6]-shift it is 11/3. In both cases HOMO has m-symmetry therefore, suprafacial shift without inversion at the migrating centre is thermally allowed. 

DPB as well as other DPP molecules (t-stilbene, diphenyl-hexatriene) with relatively low ionization potential (7.4-7.8 eV) and low vapor pressure was successfully incorporated in the straight channel of acidic ZSM-5 zeolite. DPP lies in the intersection of straight channel and zigzag channel in the vicinity of proton in close proximity of Al framework atom. The mere exposure of DPP powder to Bronsted acidic ZSM-5 crystallites under dry and inert atmosphere induced a sequence of reactions that takes place during more than 1 year to reach a stable system which is characterized by the molecule in its neutral form adsorbed in the channel zeolite. Spontaneous ionization that is first observed is followed by the radical cation recombination according to two paths. The characterization of this phenomenon shows that the ejected electron is localized near the Al framework atom. The reversibility of the spontaneous ionization is highlighted by the recombination of the radical cation or the electron-hole pair. The availability of the ejected electron shows that ionization does not proceed as a simple oxidation but stands for a real charge separated state. 

FIGURE 23. Gas-phase photodissociation (PD) spectra of the radical cation of hexatriene (a) at low resolution, (b) expanded scan of the first absorption band at high resolution219,220... 

Various substituted 1,3-cyclohexadienes and their open-chain isomers, the respective 1,3,5-hexatrienes, have been studied by El mass spectrometry with special regard to the stereospecificity of the mutual pericyclic interconversion. A brief discussion including the parent systems, ionized 1,3-cyclohexadiene and 1,3,5-hexatriene has been provided by Dass in his review on pericyclic reactions of radical cations. McLafferty and coworkers have shown that the two parent isomers are (almost) indistinguishable... 

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