Benzene, absorption spectrum dimers

Dissociation constants of 2 have been reported to be higher for dimers with electron donating substituents in the para positions of the phenyl rings [ 15]. We found that these groups in ortho positions resulted qualitatively in very different properties. Any ortho substituent in Ar decreased dissociation. Thus, a benzene solution of the stable dimer of 2-(2-chlorophenyl)-4,5-diphenylimidazolyl radical 2d exhibited no radical absorption spectrum below 80°, whereas the 4-chloro compound 2e was partially dissociated in solution at room temperature. Ortho substituents in Ar and Ar" affected the equilibrium quite modestly but appeared to have the opposite effect, that is, increased dissociation. Qualitatively, meta substituents had little effect on the equilibrium of dissociation. 

Roovers and Bywater 76 examined the temperature dependents of the electronic spectrum of poly(isoprenyi)iithium and were able to calculate an equilibrium constant for the dissociation event. On the basis that the process involved was tetramers dimers, the dissociation enthalpy was determined to be 12.3 kcal/mole in n-octane while a value of 9.0 kcal/mole was found in benzene solution. The latter value was thought to be due to weak solvations of the active centers by benzene. The approach used by Roovers and Bywater 76) is predicated on the assumption that the 272 and 320 nm absorptions represent species differing in their association state. 

When dissolved in benzene or carbon tetrachloride, salicylic acid forms a dimer and the infra red spectrum shows no absorption at die normal OH frequencies. The structure of the dimer must therefore be... 

In the continuation of our work a study has also been made of the system benzene/silica gel. When irradiating this system at 77 °K. it was found that the silica gel could stabilize both monomeric and dimeric cation radicals of benzene (6). Furthermore, the high resolution of the electron spin resonance lines indicated a high degree of mobility for the benzene molecules in the adsorbed layer. No spectrum from trapped electrons could be observed although this could very well be hidden behind the strong cation absorption. However, ethylene, and isobutylene in the adsorbed state at low temperature gave spectra from shortlived species identified as trapped electrons (7). 

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