Vibrational spectra of hydrogen-bonded complexes on zeolite surfaces as a benchmark for evaluating performance of ab initio methods. Complex with the pyridinium ion

Abstract

Vibrational spectroscopy of molecules hydrogen-bonded to bridging hydroxyls in zeolites is suggested as a promising tool for the direct comparison with gas-phase ab initio results. As an illustration, the FT-IR spectra of pyridine adsorbed on H-Y and H-mordenite zeolites which differ in their acid strength while having the same local structure of the interacting site, are compared with HF6–31G calculations of the (zeol)OH⋯pyridine complexes. For comparison purposes, adsorption on weakly acidic silicagel was also studied. Within the double minimum potential for the proton transfer, HF6–31G spectra of both the ‘ion-pair’ and ‘neutral’ hydrogen bonded complexes were calculated. HF6–31G calculations strongly overestimate observed frequency shifts, but this discrepancy can be successfully corrected through few empirical scale factors. It was shown that within the each stationary state, effects of varying zeolite acid strength on the vibrational spectra can be reproduced by modifying these scale factors. For pyridine adsorbed at HY zeolite, a coexistence of the ‘neutral’ and ‘ion-pair’ types of hydrogen bonds in the sample was observed.