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The IR spectra of increasing doses of CH3CN interacting with H-ZSM-5, H-MOR, and H-NAFION are investigated and compared. In all cases the formation of neutral hydrogen-bonded adducts is observed and complete vibrational assignment is given. The basic IR spectroscopy of these complexes is discussed in the framework of the Evans approach on the Fermi resonance between the narrow 2δ and 2γ levels with the continuum distribution of levels associated with the ν(OH···B) mode coupled with the low-frequency ν(O···B) vibrations and with the external (thermal) librations. At low dosages the interaction of H2O with H-ZSM-5 and H-MOR gives neutral hydrogen-bonded adducts. At higher dosages these species are transformed into H+(H2O) n (n average = 3−5) ionic species. This process can be only partially reversed by decreasing the relative pressure of H2O. At low dosage the concentration of ionic species is higher on H-MOR than on H-ZSM-5. The interaction of H2O with the superacidic H-NAFION membrane readily gives directly the H+(H2O) n products without formation of the neutral intermediates. The decrease of the relative pressure of H2O is not accompanied by a back-transfer of the proton to the membrane. This differentiates the H-ZSM-5 and H-MOR solid acids from the superacidic H-NAFION membrane. The comparison of the IR spectra obtained on H-ZSM-5 and H-MOR on one side and on H-NAFION on the other side allows a detailed assignment of the spectroscopic manifestations of the neutral and protonated species.

The interaction of pyridine with acidic H-ZSM5, H-β, and H-MORD zeolites and a perfluorosulfonic superacidic membrane (H-Nafion) has allowed us to study the IR spectrum of PyH+ hydrogen bonded to the negative zeolite skeleton (Z-) or to CF2SO3 - groups of the membrane. In the presence of excess Py the formation of PyH+···Py is also detected. On H-MORD (a zeolite with higher Al content) the adsorption of Py at room temperature is limited to the external surfaces of the crystallite, owing to the blockage of channel mouths caused by the PyH+ formed in high concentration. The IR spectra of hydrogen-bonded PyH+ in the ν(NH···) stretching region are strongly influenced by Fermi resonance effects with the overtones and combinations of low-frequency modes. In the H-Nafion/Py system the modification induced by the proton transfer on the internal modes of the −SO3H groups (with formation of SO3 - species) gives additional information on the proton transfer process. To complete the assignment of all the observed surface species (including the weakly adsorbed ones) the interaction of Py with silicalite (a purely siliceous zeolite with a pentasilic structure) has also been investigated.

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