Density Functional Calculations of the ²⁹Si and ²⁷Al MAS NMR Spectra of the Zeolite Mazzite: Analysis of Geometrical and Electronic Effects

Abstract

The 29Si and 29Al NMR chemical shifts of the two crystallographic sites of the zeolite mazzite have been evaluated from the NMR shielding tensors calculated using the SOS-DFPT method. The calculations were carried out on one-site and two-site models, including from one to three coordination shells around each site. The effects of the cluster size, basis set extension, and choice of the exchange and correlation functional have been analyzed. The 1T models including two coordination shells around the site are sufficient for a good description of the NMR peak separation, whereas the absolute values of the 29Si and 29Al NMR chemical shifts are obtained using a 2T model including the four-membered ring between the two sites. The effect of geometrical and electronic factors on the NMR chemical shifts have been analyzed, showing their dependence on the cluster size and also on the shape of the zeolitic system.