Adsorption and polymerization of aniline on Cu(II)-montmorillonite: Vibrational spectroscopy and ab initio calculation

Batch adsorption experiments in the presence of oxygen were performed to st udy the interlayer reactions of aniline on Cu(II)-montmorillonite in aqueou s solutions. At concentrations below a critical value of C-G = 2.6 mmol dm( -3) only a colored Cu(II)-aniline complex is for med, characterized by a st ability constant of log(K-assoc/dm(3) mol(-1)) = 1.5. At concentrations bey ond C-c aniline polymerizes yielding a dark brown product, which is identif ied by two vibrational spectroscopy techniques, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy and FT-Raman spectrosco py. Vibrational assignment of aniline, polyaniline, and the interlayer reac tion products of aniline at Cu(II)-montmorillonite was achieved by a compar ison of the band position and intensity observed in the ATR and FT-Raman sp ectra with wavenumbers and intensities (IR and Raman) from ab initio quantu m-mechanical calculations. Density functional theory (DFT, B3LYP/6-31G*) ha s been used to calculate the geometry, frequencies, and intensities (LR and Raman) of aniline. The geometry and vibrational calculations of a four-rin g unit (emeraldine base, EB) are believed to be a good representation of th e polyaniline at Cu(II)-montmorillonite. The geometry of EB was fully optim ized at the Hartree-Fock level of theory. The data of polyaniline presented suggest that IR and Raman data calculated ab initio on relatively short ol igomers (quantum-mechanical oligomer approach) may provide valuable informa tion regarding the interpretation of vibrational spectra of polymers. From the comparison of experiments and calculations, it is concluded that a cata lytic/intercalation polymerization of aniline to polyaniline took place ins ide the interlayer of the Cu(II)-montmorillonite clay mineral.