Calculation of²⁹Si and²⁷Al MAS NMR Chemical Shifts in Zeolite-β Using Density Functional Theory:  Correlation with Lattice Structure

Abstract

29 Si and 27 Al NMR chemical shifts have been evaluated from the NMR shielding tensors obtained using the SOS-DFPT technique. The 29 Si NMR chemical shifts have been calculated for the nine crystallographically distinct Si sites of the zeolite-β lattice. The calculations were carried out on cluster models Si(OSiH 3 ) 4 and R 3 SiOSiR 3 (R ) OSiH 3 ) representing one (1T) and two (2T) sites in the zeolite lattice, respectively. Using the 1T models, the nine signals of the experimental spectrum have been assigned with a relative error of less than 1 ppm, the absolute error being estimated at about 5 ppm. The use of a "fragmentaveraging" technique based on the results obtained with the 2T models leads to calculated absolute NMR shifts with an accuracy of (1 ppm. The 27 Al NMR shifts of AlH(OSiH 3 ) 4 models have also been calculated, after relaxation of the local geometry. It was found that the relative positions of the 27 Al chemical shifts are in agreement with the experimental spectrum.

Aminolysis of epoxides catalyzed by three-dimensional, mesoporous titanosilicates, Ti-SBA-12 and Ti-SBA-16

“…This deshielding effect on the position of 29 Si MAS NMR signals is possibly due to differences in the structures of Ti-SBA-12 and Ti-SBA-16. In the past, experimental and theoretical studies on zeolites showed correlations between chemical shifts and Si-O-Si bond angles [37][38][39]. While the intensity of Q 3 signal is higher than that of Q 4 in SBA-12 materials, the reverse is the case for SBA-16 materials (Table 2).…”

Calculated magnetic properties for the characterization of zeolite active sites

“…20,22,23,28 Experimental geometries are known only for fully siliceous zeolites, which implies that NMR calculations for models including other elements in the framework must be performed using optimized geometries.…”

High sensitivity of O17 NMR to p-d hybridization in transition metal perovskites: First principles calculations of large anisotropic chemical shielding

“…Linear response methods have long been used to calculateσ for isolated molecules and clusters. 7,8,9,10,11,12,13 In extended systems complications arise due to the use of periodic boundary conditions (PBC), 14,15,16 and implementations have been limited to planewave-based methods. Recently, an alternative to the linear response method has been proposed based on calculations of the orbital magnetization.…”