H-BONDING AND INTERACTION ENERGY OF ACETONITRILE NEUTRAL AND PYRIDINE ION-PAIR SURFACE COMPLEXES IN ZEOLITES OF VARIOUS ACIDITY - FTIR AND AB-INITIO STUDY

The relationship between zeolite acidobasic properties and hydrogen bonding in the neutral b ... HO(zeolite) and ion-pair BH+...O--(zeolite) surface complexes of acetonitrile-d(3) and pyridine, respectively, was studied using FTIR spectroscopy and at the ab initio Hartree-Fock (HF) and MP2 theory levels. Attention was also paid to the effects of coverage of zeolite-bridging DH groups. The ion-pair and neutral types df hydrogen bonds were distinguished according to their characteristic spectral behavior observed upon varying the zeolite composition and structure. In the spectra of the CD3CN ... H-O-zeolite complex, the larger acid strength of bridging hydroxyls (along the HNaX < HY < HM congruent to HZSM-5 zeolite sequence) resulted:in larger red and blue shifts of frequencies of stretching nu(OH) and bending delta(OH), gamma(OH) vibrations, respectively. This finding reflects the increase in the dissociation energy Of the neutral H-bonded complex with increasing OH acid strength. In contrast, the frequency shifts of the nu(NH) stretching vibration in the pyridine-H+...O---zeolite complex increase with the basicity of the skeletal oxygens, i.e. in the opposite zeolite sequence than for the CD3CN ... H-O-zeolite complex. This observation provides evidence for the formation of ion-pair H-bonds in pyridine/zeolite surface complexes. This conclusion is also supported by the energetics of the proton-transfer reaction calculated at the HF and MP2/DZP theoretical levels. The interaction energies (enthalpies) obtained for both the neutral and ion-pair structures from spectral data and abinitio calculations are compared with the adsorption heats available in the literature. Of spectroscopic interest is also our observation of the bands of in-plane (delta) and out-of plane (gamma) fundamental deformation vibrations of bridging OH groups in the IR spectra of acetonitrile surface complexes. Independently, the frequencies of these modes were calculated within the Fermi resonance theory from the parameters of the AB and ABC spectral patterns, which is characteristic of medium and strongly hydrogen bonded complexes. The agreement between observed and calculated frequencies justifies the use bf Fermi resonance theories in spectral studies of surface H-bonded complexes.