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 calculation s. The hydroxyl and its surroundings in the zeolite were modeled by a clust er molecule. Potential energy and dipole surfaces of the model were compute d with density functional theory applying the Becke3LYP functional. A poten tial energy surface has been constructed as a function;of the stretch, in-p lane 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 stret ch coordinate of acetonitrile. Taking into full account anharmonicities, we computed the vibrational wave functions and infrared absorption intensitie s using a variational approach. To facilitate their interpretation, the com puted spectra were decomposed with respect to the different vibrational coo rdinates. It was found that the use of center of mass conserving coordinate s for the hydroxyl group is insufficient to obtain accurate hydroxyl stretc h frequencies, and that oxygen coordinates need to be included in the calcu lation. The inclusion of oxygen coordinates furthermore improves the comput ed Fermi resonance splitting. A new explanation for the width of the A,B sp ectra is proposed.