Experimental and ab initio spectra of the persistent carbocation generated upon adsorption of vinylanisole in acid zeolites

Upon adsorption of vinylanisole on acid zeolites, a new species visually characterized by its intense purple color is generated. Recent product studies and optical spectroscopy by Cozens et al. (J. Phys. Chem. B, 1997, 101, 6921) had previously suggested a structure of 1,3-dianisyl-2-buten-1-ylium ion (2) for this species. Herein, we have complemented the reported spectroscopic characterization of the purple species by recording its IR spectrum in a zeolite matrix. In addition, species 2 has also been generated in solution and its most characteristic low-field signals in C-13-NMR measured. Theoretical calculations at the Hartree-Fock and B3LYP levels have been carried out to predict the UV/visible, IR and C-13-NMR spectra for the two more stable stereoisomers of a species such as 2. The good match between the experimental and calculated spectra reinforces the structural assignment of all E-2 as the actual purple species upon incorporation of vinylanisole in acid zeolites. The degree of accuracy of the predicted spectra has been corroborated by performing concurrently the same calculations for two structurally related cations such as 1,3-diphenylpropenylium and 4-methoxyphenylethylium, whose experimental spectroscopic data have already been reported in the literature.