Reactive intermediates time-resolved characterization methods

Protonated polymethylbenzenes281 and the chlorohexamethylbenzenium cation,282 intermediates in aromatic electrophilic substitutions known as Wheland intermediates, have been isolated as crystalline salts, allowing investigators to obtain their X-ray crystal structure. Nitrosoarenium a complexes of various arenes were directly observed by transient absorption spectroscopy.283 Kochi presented a method combining appropriate instrumental techniques (X-ray crystallography, NMR, time-resolved UV-vis spectroscopy) for the observation, identification, and structural characterization of reactive intermediates fa and n complexes) in electrophilic aromatic substitution.284... 

Pioneering work of Dowd on trimethylene methane [142], 1,8-naphthoquinodi-methane diradicals were among the first examples of non-Kekule 7r,7r-diradicals that were found to be persistent in their triplet ground state at cryogenic temperatures and were thoroughly characterized by various spectroscopic methods [143], Time resolved studies of diradicals have provided lifetimes and reactivities of numerous diradical intermediates in solution [144], y-irradiation of CAN, 68, in haloalkane glasses at 77 K yielded radical cation 68 + its electronic absorption spectrum has been obtained and represents first report of absorption spectrum of an ionized diradical [145]. Irradiation at >640 nm converted it into an isomer identical with that formed by ionization of 1,4-dihydro-l,4-ethanonaphtho[l,8-rfe][l,2]diazepine 70, and subsequent irradiation of the radical cation at A > 540 nm. Results identified the photo isomer of 68 + to be 69 + (Scheme 4). 

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