Probing the acid strength of Bronsted acidic zeolites with acetonitrile:: Quantum chemical calculation of 1H, 15N, and 13C NMR shift parameters

Calculations have been carried out of H-1, N-15, and C-13 NMR chemical shift parameters, which are used to characterize the acid strength and accessibility of Bronsted acid sites in seven high-silica zeolites of structure types CHA, FAU, FER, MFI, MOR, MTW, and TON. The acid sites had previously been selected by a systematic minimization procedure, and an acetonitrile molecule inserted using a Monte Carlo method. The main interactions between acetonitrile and the zeolite are an (NH)-H-...-O type hydrogen bond to the Bronsted proton and van der Waals' interactions with the framework. Calculations using GIAO (with B3LYP/TZV) were performed on optimized clusters, which included the acid site and sufficient atoms to represent the surrounding pore topology. The isotropic shielding values were obtained, as well as the principal components of the shielding tensors and the chemical shift parameters. Both the isotropic shifts of the acid proton, delta(iso)(H-1), and delta(iso)(N-15)) correlate well with the hydroxy bond length, r(OH), and hence to acid strength. In contrast, delta(iso)(C-13) is strongly dependent on the orientation of the acetonitrile molecule, as influenced by the local zeolite geometry. The principal tensor components, delta(11), delta(22), and delta(33), exhibit recognizable trends only for the nitrogen atoms. To describe the powder pattern of the shift in a more illustrative way, we use the span, Omega, the skew, kappa, the anisotropy delta(aniso), and the asymmetry, eta. The symmetry of the powder pattern is related to the local symmetry of the nuclei but not at all to the Bronsted acidity.