Vibrational Spectra of Arenium Ion Salts

The vibrational spectra of arenium ion salts and respective hydrocarbons show great differences at 600-1600 cm" as well. This is not surprising since the addition of an aromatic molmile changes its atom symmetry and, hence, the vibrations active in the IR and the Raman spectra. Thus, when passing from benzene to the benzenium ion the symmetry d xeases from to C2,. In the IR spectrum of benzenium ion salts an intensive absorption band at 1595 cm > is appeared which is due to the skeleton C—C bond vibrations inactive in the IR spectrum of benzene but manifesting themselves in that of its derivatives. Other examples can be found in... 

The vibrational spectra of arenium ion salts with different anions have not been studied yet. Evidence on the vibration frequencies of the other anions, if necessary, can be found in papers for BF GaBr 9A99.5od AlCl BrT / >, AllT Aljl," SbClg" AsFg" as well as in books... 

Vibrational spectra of arenium ions with heteroatomic substituents are very scarce. The protonation of hexamethylcyclohexa-2,5-dienone 71) and -2,4-dienone forming the complex (77) HCl - 2 AlClj in CHjClj shifts the stretching vibration band of the carbon-oxygen bond from 1615 cm (C=0)to 1510 cm (see also The band of the OH group at 3450cm and the before mentioned data are in good agreement with those of the PMR, i.e. the complex is a salt of the 4-hydroxy-1,1,2,3,5,6-hexamethylbenzenium ion (57 a)... 

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