Infrared Spectrum of an Acidic Zeolite OH with Adsorbed Acetonitrile

The effect of acetonitrile adsorption on the infrared spectrum of an acidic hydroxyl group of a zeolite was studied using quantum-chemical calculations. The hydroxyl and its surroundings in the zeolite were modeled by a cluster molecule. Potential energy and dipole surfaces of the model were computed with density functional theory applying the Becke3LYP functional. A potential energy surface has been constructed as a function of the stretch, in-plane bending, and out-of-plane bending coordinates of both the hydrogen and the oxygen atom of the hydroxyl group, as well as the center-of-mass stretch coordinate of acetonitrile. Taking into full account anharmonicities, we computed the vibrational wave functions and infrared absorption intensities using a variational approach. To facilitate their interpretation, the computed spectra were decomposed with respect to the different vibrational coordinates. It was found that the use of center of mass conserving coordinates for the hydroxyl group is insufficient to obtain accurate hydroxyl stretch frequencies, and that oxygen coordinates need to be included in the calculation. The inclusion of oxygen coordinates furthermore improves the computed Fermi resonance splitting. A new explanation for the width of the A,B spectra is proposed.